diff --git a/docs/tutorial/LangImpl01.rst b/docs/tutorial/LangImpl01.rst index f7fbd150ef1..1ff4dc8af44 100644 --- a/docs/tutorial/LangImpl01.rst +++ b/docs/tutorial/LangImpl01.rst @@ -1,293 +1,7 @@ -================================================= -Kaleidoscope: Tutorial Introduction and the Lexer -================================================= +:orphan: -.. contents:: - :local: - -Tutorial Introduction +===================== +Kaleidoscope Tutorial ===================== -Welcome to the "Implementing a language with LLVM" tutorial. This -tutorial runs through the implementation of a simple language, showing -how fun and easy it can be. This tutorial will get you up and started as -well as help to build a framework you can extend to other languages. The -code in this tutorial can also be used as a playground to hack on other -LLVM specific things. - -The goal of this tutorial is to progressively unveil our language, -describing how it is built up over time. This will let us cover a fairly -broad range of language design and LLVM-specific usage issues, showing -and explaining the code for it all along the way, without overwhelming -you with tons of details up front. - -It is useful to point out ahead of time that this tutorial is really -about teaching compiler techniques and LLVM specifically, *not* about -teaching modern and sane software engineering principles. In practice, -this means that we'll take a number of shortcuts to simplify the -exposition. For example, the code uses global variables -all over the place, doesn't use nice design patterns like -`visitors `_, etc... but -it is very simple. If you dig in and use the code as a basis for future -projects, fixing these deficiencies shouldn't be hard. - -I've tried to put this tutorial together in a way that makes chapters -easy to skip over if you are already familiar with or are uninterested -in the various pieces. The structure of the tutorial is: - -- `Chapter #1 <#language>`_: Introduction to the Kaleidoscope - language, and the definition of its Lexer - This shows where we are - going and the basic functionality that we want it to do. In order to - make this tutorial maximally understandable and hackable, we choose - to implement everything in C++ instead of using lexer and parser - generators. LLVM obviously works just fine with such tools, feel free - to use one if you prefer. -- `Chapter #2 `_: Implementing a Parser and AST - - With the lexer in place, we can talk about parsing techniques and - basic AST construction. This tutorial describes recursive descent - parsing and operator precedence parsing. Nothing in Chapters 1 or 2 - is LLVM-specific, the code doesn't even link in LLVM at this point. - :) -- `Chapter #3 `_: Code generation to LLVM IR - With - the AST ready, we can show off how easy generation of LLVM IR really - is. -- `Chapter #4 `_: Adding JIT and Optimizer Support - - Because a lot of people are interested in using LLVM as a JIT, - we'll dive right into it and show you the 3 lines it takes to add JIT - support. LLVM is also useful in many other ways, but this is one - simple and "sexy" way to show off its power. :) -- `Chapter #5 `_: Extending the Language: Control - Flow - With the language up and running, we show how to extend it - with control flow operations (if/then/else and a 'for' loop). This - gives us a chance to talk about simple SSA construction and control - flow. -- `Chapter #6 `_: Extending the Language: - User-defined Operators - This is a silly but fun chapter that talks - about extending the language to let the user program define their own - arbitrary unary and binary operators (with assignable precedence!). - This lets us build a significant piece of the "language" as library - routines. -- `Chapter #7 `_: Extending the Language: Mutable - Variables - This chapter talks about adding user-defined local - variables along with an assignment operator. The interesting part - about this is how easy and trivial it is to construct SSA form in - LLVM: no, LLVM does *not* require your front-end to construct SSA - form! -- `Chapter #8 `_: Compiling to Object Files - This - chapter explains how to take LLVM IR and compile it down to object - files. -- `Chapter #9 `_: Extending the Language: Debug - Information - Having built a decent little programming language with - control flow, functions and mutable variables, we consider what it - takes to add debug information to standalone executables. This debug - information will allow you to set breakpoints in Kaleidoscope - functions, print out argument variables, and call functions - all - from within the debugger! -- `Chapter #10 `_: Conclusion and other useful LLVM - tidbits - This chapter wraps up the series by talking about - potential ways to extend the language, but also includes a bunch of - pointers to info about "special topics" like adding garbage - collection support, exceptions, debugging, support for "spaghetti - stacks", and a bunch of other tips and tricks. - -By the end of the tutorial, we'll have written a bit less than 1000 lines -of non-comment, non-blank, lines of code. With this small amount of -code, we'll have built up a very reasonable compiler for a non-trivial -language including a hand-written lexer, parser, AST, as well as code -generation support with a JIT compiler. While other systems may have -interesting "hello world" tutorials, I think the breadth of this -tutorial is a great testament to the strengths of LLVM and why you -should consider it if you're interested in language or compiler design. - -A note about this tutorial: we expect you to extend the language and -play with it on your own. Take the code and go crazy hacking away at it, -compilers don't need to be scary creatures - it can be a lot of fun to -play with languages! - -The Basic Language -================== - -This tutorial will be illustrated with a toy language that we'll call -"`Kaleidoscope `_" (derived -from "meaning beautiful, form, and view"). Kaleidoscope is a procedural -language that allows you to define functions, use conditionals, math, -etc. Over the course of the tutorial, we'll extend Kaleidoscope to -support the if/then/else construct, a for loop, user defined operators, -JIT compilation with a simple command line interface, etc. - -Because we want to keep things simple, the only datatype in Kaleidoscope -is a 64-bit floating point type (aka 'double' in C parlance). As such, -all values are implicitly double precision and the language doesn't -require type declarations. This gives the language a very nice and -simple syntax. For example, the following simple example computes -`Fibonacci numbers: `_ - -:: - - # Compute the x'th fibonacci number. - def fib(x) - if x < 3 then - 1 - else - fib(x-1)+fib(x-2) - - # This expression will compute the 40th number. - fib(40) - -We also allow Kaleidoscope to call into standard library functions (the -LLVM JIT makes this completely trivial). This means that you can use the -'extern' keyword to define a function before you use it (this is also -useful for mutually recursive functions). For example: - -:: - - extern sin(arg); - extern cos(arg); - extern atan2(arg1 arg2); - - atan2(sin(.4), cos(42)) - -A more interesting example is included in Chapter 6 where we write a -little Kaleidoscope application that `displays a Mandelbrot -Set `_ at various levels of magnification. - -Lets dive into the implementation of this language! - -The Lexer -========= - -When it comes to implementing a language, the first thing needed is the -ability to process a text file and recognize what it says. The -traditional way to do this is to use a -"`lexer `_" (aka -'scanner') to break the input up into "tokens". Each token returned by -the lexer includes a token code and potentially some metadata (e.g. the -numeric value of a number). First, we define the possibilities: - -.. code-block:: c++ - - // The lexer returns tokens [0-255] if it is an unknown character, otherwise one - // of these for known things. - enum Token { - tok_eof = -1, - - // commands - tok_def = -2, - tok_extern = -3, - - // primary - tok_identifier = -4, - tok_number = -5, - }; - - static std::string IdentifierStr; // Filled in if tok_identifier - static double NumVal; // Filled in if tok_number - -Each token returned by our lexer will either be one of the Token enum -values or it will be an 'unknown' character like '+', which is returned -as its ASCII value. If the current token is an identifier, the -``IdentifierStr`` global variable holds the name of the identifier. If -the current token is a numeric literal (like 1.0), ``NumVal`` holds its -value. Note that we use global variables for simplicity, this is not the -best choice for a real language implementation :). - -The actual implementation of the lexer is a single function named -``gettok``. The ``gettok`` function is called to return the next token -from standard input. Its definition starts as: - -.. code-block:: c++ - - /// gettok - Return the next token from standard input. - static int gettok() { - static int LastChar = ' '; - - // Skip any whitespace. - while (isspace(LastChar)) - LastChar = getchar(); - -``gettok`` works by calling the C ``getchar()`` function to read -characters one at a time from standard input. It eats them as it -recognizes them and stores the last character read, but not processed, -in LastChar. The first thing that it has to do is ignore whitespace -between tokens. This is accomplished with the loop above. - -The next thing ``gettok`` needs to do is recognize identifiers and -specific keywords like "def". Kaleidoscope does this with this simple -loop: - -.. code-block:: c++ - - if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]* - IdentifierStr = LastChar; - while (isalnum((LastChar = getchar()))) - IdentifierStr += LastChar; - - if (IdentifierStr == "def") - return tok_def; - if (IdentifierStr == "extern") - return tok_extern; - return tok_identifier; - } - -Note that this code sets the '``IdentifierStr``' global whenever it -lexes an identifier. Also, since language keywords are matched by the -same loop, we handle them here inline. Numeric values are similar: - -.. code-block:: c++ - - if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+ - std::string NumStr; - do { - NumStr += LastChar; - LastChar = getchar(); - } while (isdigit(LastChar) || LastChar == '.'); - - NumVal = strtod(NumStr.c_str(), 0); - return tok_number; - } - -This is all pretty straight-forward code for processing input. When -reading a numeric value from input, we use the C ``strtod`` function to -convert it to a numeric value that we store in ``NumVal``. Note that -this isn't doing sufficient error checking: it will incorrectly read -"1.23.45.67" and handle it as if you typed in "1.23". Feel free to -extend it :). Next we handle comments: - -.. code-block:: c++ - - if (LastChar == '#') { - // Comment until end of line. - do - LastChar = getchar(); - while (LastChar != EOF && LastChar != '\n' && LastChar != '\r'); - - if (LastChar != EOF) - return gettok(); - } - -We handle comments by skipping to the end of the line and then return -the next token. Finally, if the input doesn't match one of the above -cases, it is either an operator character like '+' or the end of the -file. These are handled with this code: - -.. code-block:: c++ - - // Check for end of file. Don't eat the EOF. - if (LastChar == EOF) - return tok_eof; - - // Otherwise, just return the character as its ascii value. - int ThisChar = LastChar; - LastChar = getchar(); - return ThisChar; - } - -With this, we have the complete lexer for the basic Kaleidoscope -language (the `full code listing `_ for the Lexer -is available in the `next chapter `_ of the tutorial). -Next we'll `build a simple parser that uses this to build an Abstract -Syntax Tree `_. When we have that, we'll include a -driver so that you can use the lexer and parser together. - -`Next: Implementing a Parser and AST `_ - +The Kaleidoscope Tutorial has `moved to another location `_ . diff --git a/docs/tutorial/LangImpl02.rst b/docs/tutorial/LangImpl02.rst index 6982e969c8a..1ff4dc8af44 100644 --- a/docs/tutorial/LangImpl02.rst +++ b/docs/tutorial/LangImpl02.rst @@ -1,737 +1,7 @@ -=========================================== -Kaleidoscope: Implementing a Parser and AST -=========================================== +:orphan: -.. contents:: - :local: - -Chapter 2 Introduction -====================== - -Welcome to Chapter 2 of the "`Implementing a language with -LLVM `_" tutorial. This chapter shows you how to use the -lexer, built in `Chapter 1 `_, to build a full -`parser `_ for our Kaleidoscope -language. Once we have a parser, we'll define and build an `Abstract -Syntax Tree `_ (AST). - -The parser we will build uses a combination of `Recursive Descent -Parsing `_ and -`Operator-Precedence -Parsing `_ to -parse the Kaleidoscope language (the latter for binary expressions and -the former for everything else). Before we get to parsing though, let's -talk about the output of the parser: the Abstract Syntax Tree. - -The Abstract Syntax Tree (AST) -============================== - -The AST for a program captures its behavior in such a way that it is -easy for later stages of the compiler (e.g. code generation) to -interpret. We basically want one object for each construct in the -language, and the AST should closely model the language. In -Kaleidoscope, we have expressions, a prototype, and a function object. -We'll start with expressions first: - -.. code-block:: c++ - - /// ExprAST - Base class for all expression nodes. - class ExprAST { - public: - virtual ~ExprAST() {} - }; - - /// NumberExprAST - Expression class for numeric literals like "1.0". - class NumberExprAST : public ExprAST { - double Val; - - public: - NumberExprAST(double Val) : Val(Val) {} - }; - -The code above shows the definition of the base ExprAST class and one -subclass which we use for numeric literals. The important thing to note -about this code is that the NumberExprAST class captures the numeric -value of the literal as an instance variable. This allows later phases -of the compiler to know what the stored numeric value is. - -Right now we only create the AST, so there are no useful accessor -methods on them. It would be very easy to add a virtual method to pretty -print the code, for example. Here are the other expression AST node -definitions that we'll use in the basic form of the Kaleidoscope -language: - -.. code-block:: c++ - - /// VariableExprAST - Expression class for referencing a variable, like "a". - class VariableExprAST : public ExprAST { - std::string Name; - - public: - VariableExprAST(const std::string &Name) : Name(Name) {} - }; - - /// BinaryExprAST - Expression class for a binary operator. - class BinaryExprAST : public ExprAST { - char Op; - std::unique_ptr LHS, RHS; - - public: - BinaryExprAST(char op, std::unique_ptr LHS, - std::unique_ptr RHS) - : Op(op), LHS(std::move(LHS)), RHS(std::move(RHS)) {} - }; - - /// CallExprAST - Expression class for function calls. - class CallExprAST : public ExprAST { - std::string Callee; - std::vector> Args; - - public: - CallExprAST(const std::string &Callee, - std::vector> Args) - : Callee(Callee), Args(std::move(Args)) {} - }; - -This is all (intentionally) rather straight-forward: variables capture -the variable name, binary operators capture their opcode (e.g. '+'), and -calls capture a function name as well as a list of any argument -expressions. One thing that is nice about our AST is that it captures -the language features without talking about the syntax of the language. -Note that there is no discussion about precedence of binary operators, -lexical structure, etc. - -For our basic language, these are all of the expression nodes we'll -define. Because it doesn't have conditional control flow, it isn't -Turing-complete; we'll fix that in a later installment. The two things -we need next are a way to talk about the interface to a function, and a -way to talk about functions themselves: - -.. code-block:: c++ - - /// PrototypeAST - This class represents the "prototype" for a function, - /// which captures its name, and its argument names (thus implicitly the number - /// of arguments the function takes). - class PrototypeAST { - std::string Name; - std::vector Args; - - public: - PrototypeAST(const std::string &name, std::vector Args) - : Name(name), Args(std::move(Args)) {} - - const std::string &getName() const { return Name; } - }; - - /// FunctionAST - This class represents a function definition itself. - class FunctionAST { - std::unique_ptr Proto; - std::unique_ptr Body; - - public: - FunctionAST(std::unique_ptr Proto, - std::unique_ptr Body) - : Proto(std::move(Proto)), Body(std::move(Body)) {} - }; - -In Kaleidoscope, functions are typed with just a count of their -arguments. Since all values are double precision floating point, the -type of each argument doesn't need to be stored anywhere. In a more -aggressive and realistic language, the "ExprAST" class would probably -have a type field. - -With this scaffolding, we can now talk about parsing expressions and -function bodies in Kaleidoscope. - -Parser Basics -============= - -Now that we have an AST to build, we need to define the parser code to -build it. The idea here is that we want to parse something like "x+y" -(which is returned as three tokens by the lexer) into an AST that could -be generated with calls like this: - -.. code-block:: c++ - - auto LHS = llvm::make_unique("x"); - auto RHS = llvm::make_unique("y"); - auto Result = std::make_unique('+', std::move(LHS), - std::move(RHS)); - -In order to do this, we'll start by defining some basic helper routines: - -.. code-block:: c++ - - /// CurTok/getNextToken - Provide a simple token buffer. CurTok is the current - /// token the parser is looking at. getNextToken reads another token from the - /// lexer and updates CurTok with its results. - static int CurTok; - static int getNextToken() { - return CurTok = gettok(); - } - -This implements a simple token buffer around the lexer. This allows us -to look one token ahead at what the lexer is returning. Every function -in our parser will assume that CurTok is the current token that needs to -be parsed. - -.. code-block:: c++ - - - /// LogError* - These are little helper functions for error handling. - std::unique_ptr LogError(const char *Str) { - fprintf(stderr, "LogError: %s\n", Str); - return nullptr; - } - std::unique_ptr LogErrorP(const char *Str) { - LogError(Str); - return nullptr; - } - -The ``LogError`` routines are simple helper routines that our parser will -use to handle errors. The error recovery in our parser will not be the -best and is not particular user-friendly, but it will be enough for our -tutorial. These routines make it easier to handle errors in routines -that have various return types: they always return null. - -With these basic helper functions, we can implement the first piece of -our grammar: numeric literals. - -Basic Expression Parsing -======================== - -We start with numeric literals, because they are the simplest to -process. For each production in our grammar, we'll define a function -which parses that production. For numeric literals, we have: - -.. code-block:: c++ - - /// numberexpr ::= number - static std::unique_ptr ParseNumberExpr() { - auto Result = llvm::make_unique(NumVal); - getNextToken(); // consume the number - return std::move(Result); - } - -This routine is very simple: it expects to be called when the current -token is a ``tok_number`` token. It takes the current number value, -creates a ``NumberExprAST`` node, advances the lexer to the next token, -and finally returns. - -There are some interesting aspects to this. The most important one is -that this routine eats all of the tokens that correspond to the -production and returns the lexer buffer with the next token (which is -not part of the grammar production) ready to go. This is a fairly -standard way to go for recursive descent parsers. For a better example, -the parenthesis operator is defined like this: - -.. code-block:: c++ - - /// parenexpr ::= '(' expression ')' - static std::unique_ptr ParseParenExpr() { - getNextToken(); // eat (. - auto V = ParseExpression(); - if (!V) - return nullptr; - - if (CurTok != ')') - return LogError("expected ')'"); - getNextToken(); // eat ). - return V; - } - -This function illustrates a number of interesting things about the -parser: - -1) It shows how we use the LogError routines. When called, this function -expects that the current token is a '(' token, but after parsing the -subexpression, it is possible that there is no ')' waiting. For example, -if the user types in "(4 x" instead of "(4)", the parser should emit an -error. Because errors can occur, the parser needs a way to indicate that -they happened: in our parser, we return null on an error. - -2) Another interesting aspect of this function is that it uses recursion -by calling ``ParseExpression`` (we will soon see that -``ParseExpression`` can call ``ParseParenExpr``). This is powerful -because it allows us to handle recursive grammars, and keeps each -production very simple. Note that parentheses do not cause construction -of AST nodes themselves. While we could do it this way, the most -important role of parentheses are to guide the parser and provide -grouping. Once the parser constructs the AST, parentheses are not -needed. - -The next simple production is for handling variable references and -function calls: - -.. code-block:: c++ - - /// identifierexpr - /// ::= identifier - /// ::= identifier '(' expression* ')' - static std::unique_ptr ParseIdentifierExpr() { - std::string IdName = IdentifierStr; - - getNextToken(); // eat identifier. - - if (CurTok != '(') // Simple variable ref. - return llvm::make_unique(IdName); - - // Call. - getNextToken(); // eat ( - std::vector> Args; - if (CurTok != ')') { - while (1) { - if (auto Arg = ParseExpression()) - Args.push_back(std::move(Arg)); - else - return nullptr; - - if (CurTok == ')') - break; - - if (CurTok != ',') - return LogError("Expected ')' or ',' in argument list"); - getNextToken(); - } - } - - // Eat the ')'. - getNextToken(); - - return llvm::make_unique(IdName, std::move(Args)); - } - -This routine follows the same style as the other routines. (It expects -to be called if the current token is a ``tok_identifier`` token). It -also has recursion and error handling. One interesting aspect of this is -that it uses *look-ahead* to determine if the current identifier is a -stand alone variable reference or if it is a function call expression. -It handles this by checking to see if the token after the identifier is -a '(' token, constructing either a ``VariableExprAST`` or -``CallExprAST`` node as appropriate. - -Now that we have all of our simple expression-parsing logic in place, we -can define a helper function to wrap it together into one entry point. -We call this class of expressions "primary" expressions, for reasons -that will become more clear `later in the -tutorial `_. In order to parse an arbitrary -primary expression, we need to determine what sort of expression it is: - -.. code-block:: c++ - - /// primary - /// ::= identifierexpr - /// ::= numberexpr - /// ::= parenexpr - static std::unique_ptr ParsePrimary() { - switch (CurTok) { - default: - return LogError("unknown token when expecting an expression"); - case tok_identifier: - return ParseIdentifierExpr(); - case tok_number: - return ParseNumberExpr(); - case '(': - return ParseParenExpr(); - } - } - -Now that you see the definition of this function, it is more obvious why -we can assume the state of CurTok in the various functions. This uses -look-ahead to determine which sort of expression is being inspected, and -then parses it with a function call. - -Now that basic expressions are handled, we need to handle binary -expressions. They are a bit more complex. - -Binary Expression Parsing -========================= - -Binary expressions are significantly harder to parse because they are -often ambiguous. For example, when given the string "x+y\*z", the parser -can choose to parse it as either "(x+y)\*z" or "x+(y\*z)". With common -definitions from mathematics, we expect the later parse, because "\*" -(multiplication) has higher *precedence* than "+" (addition). - -There are many ways to handle this, but an elegant and efficient way is -to use `Operator-Precedence -Parsing `_. -This parsing technique uses the precedence of binary operators to guide -recursion. To start with, we need a table of precedences: - -.. code-block:: c++ - - /// BinopPrecedence - This holds the precedence for each binary operator that is - /// defined. - static std::map BinopPrecedence; - - /// GetTokPrecedence - Get the precedence of the pending binary operator token. - static int GetTokPrecedence() { - if (!isascii(CurTok)) - return -1; - - // Make sure it's a declared binop. - int TokPrec = BinopPrecedence[CurTok]; - if (TokPrec <= 0) return -1; - return TokPrec; - } - - int main() { - // Install standard binary operators. - // 1 is lowest precedence. - BinopPrecedence['<'] = 10; - BinopPrecedence['+'] = 20; - BinopPrecedence['-'] = 20; - BinopPrecedence['*'] = 40; // highest. - ... - } - -For the basic form of Kaleidoscope, we will only support 4 binary -operators (this can obviously be extended by you, our brave and intrepid -reader). The ``GetTokPrecedence`` function returns the precedence for -the current token, or -1 if the token is not a binary operator. Having a -map makes it easy to add new operators and makes it clear that the -algorithm doesn't depend on the specific operators involved, but it -would be easy enough to eliminate the map and do the comparisons in the -``GetTokPrecedence`` function. (Or just use a fixed-size array). - -With the helper above defined, we can now start parsing binary -expressions. The basic idea of operator precedence parsing is to break -down an expression with potentially ambiguous binary operators into -pieces. Consider, for example, the expression "a+b+(c+d)\*e\*f+g". -Operator precedence parsing considers this as a stream of primary -expressions separated by binary operators. As such, it will first parse -the leading primary expression "a", then it will see the pairs [+, b] -[+, (c+d)] [\*, e] [\*, f] and [+, g]. Note that because parentheses are -primary expressions, the binary expression parser doesn't need to worry -about nested subexpressions like (c+d) at all. - -To start, an expression is a primary expression potentially followed by -a sequence of [binop,primaryexpr] pairs: - -.. code-block:: c++ - - /// expression - /// ::= primary binoprhs - /// - static std::unique_ptr ParseExpression() { - auto LHS = ParsePrimary(); - if (!LHS) - return nullptr; - - return ParseBinOpRHS(0, std::move(LHS)); - } - -``ParseBinOpRHS`` is the function that parses the sequence of pairs for -us. It takes a precedence and a pointer to an expression for the part -that has been parsed so far. Note that "x" is a perfectly valid -expression: As such, "binoprhs" is allowed to be empty, in which case it -returns the expression that is passed into it. In our example above, the -code passes the expression for "a" into ``ParseBinOpRHS`` and the -current token is "+". - -The precedence value passed into ``ParseBinOpRHS`` indicates the -*minimal operator precedence* that the function is allowed to eat. For -example, if the current pair stream is [+, x] and ``ParseBinOpRHS`` is -passed in a precedence of 40, it will not consume any tokens (because -the precedence of '+' is only 20). With this in mind, ``ParseBinOpRHS`` -starts with: - -.. code-block:: c++ - - /// binoprhs - /// ::= ('+' primary)* - static std::unique_ptr ParseBinOpRHS(int ExprPrec, - std::unique_ptr LHS) { - // If this is a binop, find its precedence. - while (1) { - int TokPrec = GetTokPrecedence(); - - // If this is a binop that binds at least as tightly as the current binop, - // consume it, otherwise we are done. - if (TokPrec < ExprPrec) - return LHS; - -This code gets the precedence of the current token and checks to see if -if is too low. Because we defined invalid tokens to have a precedence of --1, this check implicitly knows that the pair-stream ends when the token -stream runs out of binary operators. If this check succeeds, we know -that the token is a binary operator and that it will be included in this -expression: - -.. code-block:: c++ - - // Okay, we know this is a binop. - int BinOp = CurTok; - getNextToken(); // eat binop - - // Parse the primary expression after the binary operator. - auto RHS = ParsePrimary(); - if (!RHS) - return nullptr; - -As such, this code eats (and remembers) the binary operator and then -parses the primary expression that follows. This builds up the whole -pair, the first of which is [+, b] for the running example. - -Now that we parsed the left-hand side of an expression and one pair of -the RHS sequence, we have to decide which way the expression associates. -In particular, we could have "(a+b) binop unparsed" or "a + (b binop -unparsed)". To determine this, we look ahead at "binop" to determine its -precedence and compare it to BinOp's precedence (which is '+' in this -case): - -.. code-block:: c++ - - // If BinOp binds less tightly with RHS than the operator after RHS, let - // the pending operator take RHS as its LHS. - int NextPrec = GetTokPrecedence(); - if (TokPrec < NextPrec) { - -If the precedence of the binop to the right of "RHS" is lower or equal -to the precedence of our current operator, then we know that the -parentheses associate as "(a+b) binop ...". In our example, the current -operator is "+" and the next operator is "+", we know that they have the -same precedence. In this case we'll create the AST node for "a+b", and -then continue parsing: - -.. code-block:: c++ - - ... if body omitted ... - } - - // Merge LHS/RHS. - LHS = llvm::make_unique(BinOp, std::move(LHS), - std::move(RHS)); - } // loop around to the top of the while loop. - } - -In our example above, this will turn "a+b+" into "(a+b)" and execute the -next iteration of the loop, with "+" as the current token. The code -above will eat, remember, and parse "(c+d)" as the primary expression, -which makes the current pair equal to [+, (c+d)]. It will then evaluate -the 'if' conditional above with "\*" as the binop to the right of the -primary. In this case, the precedence of "\*" is higher than the -precedence of "+" so the if condition will be entered. - -The critical question left here is "how can the if condition parse the -right hand side in full"? In particular, to build the AST correctly for -our example, it needs to get all of "(c+d)\*e\*f" as the RHS expression -variable. The code to do this is surprisingly simple (code from the -above two blocks duplicated for context): - -.. code-block:: c++ - - // If BinOp binds less tightly with RHS than the operator after RHS, let - // the pending operator take RHS as its LHS. - int NextPrec = GetTokPrecedence(); - if (TokPrec < NextPrec) { - RHS = ParseBinOpRHS(TokPrec+1, std::move(RHS)); - if (!RHS) - return nullptr; - } - // Merge LHS/RHS. - LHS = llvm::make_unique(BinOp, std::move(LHS), - std::move(RHS)); - } // loop around to the top of the while loop. - } - -At this point, we know that the binary operator to the RHS of our -primary has higher precedence than the binop we are currently parsing. -As such, we know that any sequence of pairs whose operators are all -higher precedence than "+" should be parsed together and returned as -"RHS". To do this, we recursively invoke the ``ParseBinOpRHS`` function -specifying "TokPrec+1" as the minimum precedence required for it to -continue. In our example above, this will cause it to return the AST -node for "(c+d)\*e\*f" as RHS, which is then set as the RHS of the '+' -expression. - -Finally, on the next iteration of the while loop, the "+g" piece is -parsed and added to the AST. With this little bit of code (14 -non-trivial lines), we correctly handle fully general binary expression -parsing in a very elegant way. This was a whirlwind tour of this code, -and it is somewhat subtle. I recommend running through it with a few -tough examples to see how it works. - -This wraps up handling of expressions. At this point, we can point the -parser at an arbitrary token stream and build an expression from it, -stopping at the first token that is not part of the expression. Next up -we need to handle function definitions, etc. - -Parsing the Rest -================ - -The next thing missing is handling of function prototypes. In -Kaleidoscope, these are used both for 'extern' function declarations as -well as function body definitions. The code to do this is -straight-forward and not very interesting (once you've survived -expressions): - -.. code-block:: c++ - - /// prototype - /// ::= id '(' id* ')' - static std::unique_ptr ParsePrototype() { - if (CurTok != tok_identifier) - return LogErrorP("Expected function name in prototype"); - - std::string FnName = IdentifierStr; - getNextToken(); - - if (CurTok != '(') - return LogErrorP("Expected '(' in prototype"); - - // Read the list of argument names. - std::vector ArgNames; - while (getNextToken() == tok_identifier) - ArgNames.push_back(IdentifierStr); - if (CurTok != ')') - return LogErrorP("Expected ')' in prototype"); - - // success. - getNextToken(); // eat ')'. - - return llvm::make_unique(FnName, std::move(ArgNames)); - } - -Given this, a function definition is very simple, just a prototype plus -an expression to implement the body: - -.. code-block:: c++ - - /// definition ::= 'def' prototype expression - static std::unique_ptr ParseDefinition() { - getNextToken(); // eat def. - auto Proto = ParsePrototype(); - if (!Proto) return nullptr; - - if (auto E = ParseExpression()) - return llvm::make_unique(std::move(Proto), std::move(E)); - return nullptr; - } - -In addition, we support 'extern' to declare functions like 'sin' and -'cos' as well as to support forward declaration of user functions. These -'extern's are just prototypes with no body: - -.. code-block:: c++ - - /// external ::= 'extern' prototype - static std::unique_ptr ParseExtern() { - getNextToken(); // eat extern. - return ParsePrototype(); - } - -Finally, we'll also let the user type in arbitrary top-level expressions -and evaluate them on the fly. We will handle this by defining anonymous -nullary (zero argument) functions for them: - -.. code-block:: c++ - - /// toplevelexpr ::= expression - static std::unique_ptr ParseTopLevelExpr() { - if (auto E = ParseExpression()) { - // Make an anonymous proto. - auto Proto = llvm::make_unique("", std::vector()); - return llvm::make_unique(std::move(Proto), std::move(E)); - } - return nullptr; - } - -Now that we have all the pieces, let's build a little driver that will -let us actually *execute* this code we've built! - -The Driver -========== - -The driver for this simply invokes all of the parsing pieces with a -top-level dispatch loop. There isn't much interesting here, so I'll just -include the top-level loop. See `below <#full-code-listing>`_ for full code in the -"Top-Level Parsing" section. - -.. code-block:: c++ - - /// top ::= definition | external | expression | ';' - static void MainLoop() { - while (1) { - fprintf(stderr, "ready> "); - switch (CurTok) { - case tok_eof: - return; - case ';': // ignore top-level semicolons. - getNextToken(); - break; - case tok_def: - HandleDefinition(); - break; - case tok_extern: - HandleExtern(); - break; - default: - HandleTopLevelExpression(); - break; - } - } - } - -The most interesting part of this is that we ignore top-level -semicolons. Why is this, you ask? The basic reason is that if you type -"4 + 5" at the command line, the parser doesn't know whether that is the -end of what you will type or not. For example, on the next line you -could type "def foo..." in which case 4+5 is the end of a top-level -expression. Alternatively you could type "\* 6", which would continue -the expression. Having top-level semicolons allows you to type "4+5;", -and the parser will know you are done. - -Conclusions -=========== - -With just under 400 lines of commented code (240 lines of non-comment, -non-blank code), we fully defined our minimal language, including a -lexer, parser, and AST builder. With this done, the executable will -validate Kaleidoscope code and tell us if it is grammatically invalid. -For example, here is a sample interaction: - -.. code-block:: bash - - $ ./a.out - ready> def foo(x y) x+foo(y, 4.0); - Parsed a function definition. - ready> def foo(x y) x+y y; - Parsed a function definition. - Parsed a top-level expr - ready> def foo(x y) x+y ); - Parsed a function definition. - Error: unknown token when expecting an expression - ready> extern sin(a); - ready> Parsed an extern - ready> ^D - $ - -There is a lot of room for extension here. You can define new AST nodes, -extend the language in many ways, etc. In the `next -installment `_, we will describe how to generate LLVM -Intermediate Representation (IR) from the AST. - -Full Code Listing -================= - -Here is the complete code listing for our running example. Because this -uses the LLVM libraries, we need to link them in. To do this, we use the -`llvm-config `_ tool to inform -our makefile/command line about which options to use: - -.. code-block:: bash - - # Compile - clang++ -g -O3 toy.cpp `llvm-config --cxxflags` - # Run - ./a.out - -Here is the code: - -.. literalinclude:: ../../examples/Kaleidoscope/Chapter2/toy.cpp - :language: c++ - -`Next: Implementing Code Generation to LLVM IR `_ +===================== +Kaleidoscope Tutorial +===================== +The Kaleidoscope Tutorial has `moved to another location `_ . diff --git a/docs/tutorial/LangImpl03.rst b/docs/tutorial/LangImpl03.rst index da465ef7061..1ff4dc8af44 100644 --- a/docs/tutorial/LangImpl03.rst +++ b/docs/tutorial/LangImpl03.rst @@ -1,568 +1,7 @@ -======================================== -Kaleidoscope: Code generation to LLVM IR -======================================== +:orphan: -.. contents:: - :local: - -Chapter 3 Introduction -====================== - -Welcome to Chapter 3 of the "`Implementing a language with -LLVM `_" tutorial. This chapter shows you how to transform -the `Abstract Syntax Tree `_, built in Chapter 2, into -LLVM IR. This will teach you a little bit about how LLVM does things, as -well as demonstrate how easy it is to use. It's much more work to build -a lexer and parser than it is to generate LLVM IR code. :) - -**Please note**: the code in this chapter and later require LLVM 3.7 or -later. LLVM 3.6 and before will not work with it. Also note that you -need to use a version of this tutorial that matches your LLVM release: -If you are using an official LLVM release, use the version of the -documentation included with your release or on the `llvm.org releases -page `_. - -Code Generation Setup +===================== +Kaleidoscope Tutorial ===================== -In order to generate LLVM IR, we want some simple setup to get started. -First we define virtual code generation (codegen) methods in each AST -class: - -.. code-block:: c++ - - /// ExprAST - Base class for all expression nodes. - class ExprAST { - public: - virtual ~ExprAST() {} - virtual Value *codegen() = 0; - }; - - /// NumberExprAST - Expression class for numeric literals like "1.0". - class NumberExprAST : public ExprAST { - double Val; - - public: - NumberExprAST(double Val) : Val(Val) {} - virtual Value *codegen(); - }; - ... - -The codegen() method says to emit IR for that AST node along with all -the things it depends on, and they all return an LLVM Value object. -"Value" is the class used to represent a "`Static Single Assignment -(SSA) `_ -register" or "SSA value" in LLVM. The most distinct aspect of SSA values -is that their value is computed as the related instruction executes, and -it does not get a new value until (and if) the instruction re-executes. -In other words, there is no way to "change" an SSA value. For more -information, please read up on `Static Single -Assignment `_ -- the concepts are really quite natural once you grok them. - -Note that instead of adding virtual methods to the ExprAST class -hierarchy, it could also make sense to use a `visitor -pattern `_ or some other -way to model this. Again, this tutorial won't dwell on good software -engineering practices: for our purposes, adding a virtual method is -simplest. - -The second thing we want is an "LogError" method like we used for the -parser, which will be used to report errors found during code generation -(for example, use of an undeclared parameter): - -.. code-block:: c++ - - static LLVMContext TheContext; - static IRBuilder<> Builder(TheContext); - static std::unique_ptr TheModule; - static std::map NamedValues; - - Value *LogErrorV(const char *Str) { - LogError(Str); - return nullptr; - } - -The static variables will be used during code generation. ``TheContext`` -is an opaque object that owns a lot of core LLVM data structures, such as -the type and constant value tables. We don't need to understand it in -detail, we just need a single instance to pass into APIs that require it. - -The ``Builder`` object is a helper object that makes it easy to generate -LLVM instructions. Instances of the -`IRBuilder `_ -class template keep track of the current place to insert instructions -and has methods to create new instructions. - -``TheModule`` is an LLVM construct that contains functions and global -variables. In many ways, it is the top-level structure that the LLVM IR -uses to contain code. It will own the memory for all of the IR that we -generate, which is why the codegen() method returns a raw Value\*, -rather than a unique_ptr. - -The ``NamedValues`` map keeps track of which values are defined in the -current scope and what their LLVM representation is. (In other words, it -is a symbol table for the code). In this form of Kaleidoscope, the only -things that can be referenced are function parameters. As such, function -parameters will be in this map when generating code for their function -body. - -With these basics in place, we can start talking about how to generate -code for each expression. Note that this assumes that the ``Builder`` -has been set up to generate code *into* something. For now, we'll assume -that this has already been done, and we'll just use it to emit code. - -Expression Code Generation -========================== - -Generating LLVM code for expression nodes is very straightforward: less -than 45 lines of commented code for all four of our expression nodes. -First we'll do numeric literals: - -.. code-block:: c++ - - Value *NumberExprAST::codegen() { - return ConstantFP::get(TheContext, APFloat(Val)); - } - -In the LLVM IR, numeric constants are represented with the -``ConstantFP`` class, which holds the numeric value in an ``APFloat`` -internally (``APFloat`` has the capability of holding floating point -constants of Arbitrary Precision). This code basically just creates -and returns a ``ConstantFP``. Note that in the LLVM IR that constants -are all uniqued together and shared. For this reason, the API uses the -"foo::get(...)" idiom instead of "new foo(..)" or "foo::Create(..)". - -.. code-block:: c++ - - Value *VariableExprAST::codegen() { - // Look this variable up in the function. - Value *V = NamedValues[Name]; - if (!V) - LogErrorV("Unknown variable name"); - return V; - } - -References to variables are also quite simple using LLVM. In the simple -version of Kaleidoscope, we assume that the variable has already been -emitted somewhere and its value is available. In practice, the only -values that can be in the ``NamedValues`` map are function arguments. -This code simply checks to see that the specified name is in the map (if -not, an unknown variable is being referenced) and returns the value for -it. In future chapters, we'll add support for `loop induction -variables `_ in the symbol table, and for `local -variables `_. - -.. code-block:: c++ - - Value *BinaryExprAST::codegen() { - Value *L = LHS->codegen(); - Value *R = RHS->codegen(); - if (!L || !R) - return nullptr; - - switch (Op) { - case '+': - return Builder.CreateFAdd(L, R, "addtmp"); - case '-': - return Builder.CreateFSub(L, R, "subtmp"); - case '*': - return Builder.CreateFMul(L, R, "multmp"); - case '<': - L = Builder.CreateFCmpULT(L, R, "cmptmp"); - // Convert bool 0/1 to double 0.0 or 1.0 - return Builder.CreateUIToFP(L, Type::getDoubleTy(TheContext), - "booltmp"); - default: - return LogErrorV("invalid binary operator"); - } - } - -Binary operators start to get more interesting. The basic idea here is -that we recursively emit code for the left-hand side of the expression, -then the right-hand side, then we compute the result of the binary -expression. In this code, we do a simple switch on the opcode to create -the right LLVM instruction. - -In the example above, the LLVM builder class is starting to show its -value. IRBuilder knows where to insert the newly created instruction, -all you have to do is specify what instruction to create (e.g. with -``CreateFAdd``), which operands to use (``L`` and ``R`` here) and -optionally provide a name for the generated instruction. - -One nice thing about LLVM is that the name is just a hint. For instance, -if the code above emits multiple "addtmp" variables, LLVM will -automatically provide each one with an increasing, unique numeric -suffix. Local value names for instructions are purely optional, but it -makes it much easier to read the IR dumps. - -`LLVM instructions <../LangRef.html#instruction-reference>`_ are constrained by strict -rules: for example, the Left and Right operators of an `add -instruction <../LangRef.html#add-instruction>`_ must have the same type, and the -result type of the add must match the operand types. Because all values -in Kaleidoscope are doubles, this makes for very simple code for add, -sub and mul. - -On the other hand, LLVM specifies that the `fcmp -instruction <../LangRef.html#fcmp-instruction>`_ always returns an 'i1' value (a -one bit integer). The problem with this is that Kaleidoscope wants the -value to be a 0.0 or 1.0 value. In order to get these semantics, we -combine the fcmp instruction with a `uitofp -instruction <../LangRef.html#uitofp-to-instruction>`_. This instruction converts its -input integer into a floating point value by treating the input as an -unsigned value. In contrast, if we used the `sitofp -instruction <../LangRef.html#sitofp-to-instruction>`_, the Kaleidoscope '<' operator -would return 0.0 and -1.0, depending on the input value. - -.. code-block:: c++ - - Value *CallExprAST::codegen() { - // Look up the name in the global module table. - Function *CalleeF = TheModule->getFunction(Callee); - if (!CalleeF) - return LogErrorV("Unknown function referenced"); - - // If argument mismatch error. - if (CalleeF->arg_size() != Args.size()) - return LogErrorV("Incorrect # arguments passed"); - - std::vector ArgsV; - for (unsigned i = 0, e = Args.size(); i != e; ++i) { - ArgsV.push_back(Args[i]->codegen()); - if (!ArgsV.back()) - return nullptr; - } - - return Builder.CreateCall(CalleeF, ArgsV, "calltmp"); - } - -Code generation for function calls is quite straightforward with LLVM. The code -above initially does a function name lookup in the LLVM Module's symbol table. -Recall that the LLVM Module is the container that holds the functions we are -JIT'ing. By giving each function the same name as what the user specifies, we -can use the LLVM symbol table to resolve function names for us. - -Once we have the function to call, we recursively codegen each argument -that is to be passed in, and create an LLVM `call -instruction <../LangRef.html#call-instruction>`_. Note that LLVM uses the native C -calling conventions by default, allowing these calls to also call into -standard library functions like "sin" and "cos", with no additional -effort. - -This wraps up our handling of the four basic expressions that we have so -far in Kaleidoscope. Feel free to go in and add some more. For example, -by browsing the `LLVM language reference <../LangRef.html>`_ you'll find -several other interesting instructions that are really easy to plug into -our basic framework. - -Function Code Generation -======================== - -Code generation for prototypes and functions must handle a number of -details, which make their code less beautiful than expression code -generation, but allows us to illustrate some important points. First, -let's talk about code generation for prototypes: they are used both for -function bodies and external function declarations. The code starts -with: - -.. code-block:: c++ - - Function *PrototypeAST::codegen() { - // Make the function type: double(double,double) etc. - std::vector Doubles(Args.size(), - Type::getDoubleTy(TheContext)); - FunctionType *FT = - FunctionType::get(Type::getDoubleTy(TheContext), Doubles, false); - - Function *F = - Function::Create(FT, Function::ExternalLinkage, Name, TheModule.get()); - -This code packs a lot of power into a few lines. Note first that this -function returns a "Function\*" instead of a "Value\*". Because a -"prototype" really talks about the external interface for a function -(not the value computed by an expression), it makes sense for it to -return the LLVM Function it corresponds to when codegen'd. - -The call to ``FunctionType::get`` creates the ``FunctionType`` that -should be used for a given Prototype. Since all function arguments in -Kaleidoscope are of type double, the first line creates a vector of "N" -LLVM double types. It then uses the ``Functiontype::get`` method to -create a function type that takes "N" doubles as arguments, returns one -double as a result, and that is not vararg (the false parameter -indicates this). Note that Types in LLVM are uniqued just like Constants -are, so you don't "new" a type, you "get" it. - -The final line above actually creates the IR Function corresponding to -the Prototype. This indicates the type, linkage and name to use, as -well as which module to insert into. "`external -linkage <../LangRef.html#linkage>`_" means that the function may be -defined outside the current module and/or that it is callable by -functions outside the module. The Name passed in is the name the user -specified: since "``TheModule``" is specified, this name is registered -in "``TheModule``"s symbol table. - -.. code-block:: c++ - - // Set names for all arguments. - unsigned Idx = 0; - for (auto &Arg : F->args()) - Arg.setName(Args[Idx++]); - - return F; - -Finally, we set the name of each of the function's arguments according to the -names given in the Prototype. This step isn't strictly necessary, but keeping -the names consistent makes the IR more readable, and allows subsequent code to -refer directly to the arguments for their names, rather than having to look up -them up in the Prototype AST. - -At this point we have a function prototype with no body. This is how LLVM IR -represents function declarations. For extern statements in Kaleidoscope, this -is as far as we need to go. For function definitions however, we need to -codegen and attach a function body. - -.. code-block:: c++ - - Function *FunctionAST::codegen() { - // First, check for an existing function from a previous 'extern' declaration. - Function *TheFunction = TheModule->getFunction(Proto->getName()); - - if (!TheFunction) - TheFunction = Proto->codegen(); - - if (!TheFunction) - return nullptr; - - if (!TheFunction->empty()) - return (Function*)LogErrorV("Function cannot be redefined."); - - -For function definitions, we start by searching TheModule's symbol table for an -existing version of this function, in case one has already been created using an -'extern' statement. If Module::getFunction returns null then no previous version -exists, so we'll codegen one from the Prototype. In either case, we want to -assert that the function is empty (i.e. has no body yet) before we start. - -.. code-block:: c++ - - // Create a new basic block to start insertion into. - BasicBlock *BB = BasicBlock::Create(TheContext, "entry", TheFunction); - Builder.SetInsertPoint(BB); - - // Record the function arguments in the NamedValues map. - NamedValues.clear(); - for (auto &Arg : TheFunction->args()) - NamedValues[Arg.getName()] = &Arg; - -Now we get to the point where the ``Builder`` is set up. The first line -creates a new `basic block `_ -(named "entry"), which is inserted into ``TheFunction``. The second line -then tells the builder that new instructions should be inserted into the -end of the new basic block. Basic blocks in LLVM are an important part -of functions that define the `Control Flow -Graph `_. Since we -don't have any control flow, our functions will only contain one block -at this point. We'll fix this in `Chapter 5 `_ :). - -Next we add the function arguments to the NamedValues map (after first clearing -it out) so that they're accessible to ``VariableExprAST`` nodes. - -.. code-block:: c++ - - if (Value *RetVal = Body->codegen()) { - // Finish off the function. - Builder.CreateRet(RetVal); - - // Validate the generated code, checking for consistency. - verifyFunction(*TheFunction); - - return TheFunction; - } - -Once the insertion point has been set up and the NamedValues map populated, -we call the ``codegen()`` method for the root expression of the function. If no -error happens, this emits code to compute the expression into the entry block -and returns the value that was computed. Assuming no error, we then create an -LLVM `ret instruction <../LangRef.html#ret-instruction>`_, which completes the function. -Once the function is built, we call ``verifyFunction``, which is -provided by LLVM. This function does a variety of consistency checks on -the generated code, to determine if our compiler is doing everything -right. Using this is important: it can catch a lot of bugs. Once the -function is finished and validated, we return it. - -.. code-block:: c++ - - // Error reading body, remove function. - TheFunction->eraseFromParent(); - return nullptr; - } - -The only piece left here is handling of the error case. For simplicity, -we handle this by merely deleting the function we produced with the -``eraseFromParent`` method. This allows the user to redefine a function -that they incorrectly typed in before: if we didn't delete it, it would -live in the symbol table, with a body, preventing future redefinition. - -This code does have a bug, though: If the ``FunctionAST::codegen()`` method -finds an existing IR Function, it does not validate its signature against the -definition's own prototype. This means that an earlier 'extern' declaration will -take precedence over the function definition's signature, which can cause -codegen to fail, for instance if the function arguments are named differently. -There are a number of ways to fix this bug, see what you can come up with! Here -is a testcase: - -:: - - extern foo(a); # ok, defines foo. - def foo(b) b; # Error: Unknown variable name. (decl using 'a' takes precedence). - -Driver Changes and Closing Thoughts -=================================== - -For now, code generation to LLVM doesn't really get us much, except that -we can look at the pretty IR calls. The sample code inserts calls to -codegen into the "``HandleDefinition``", "``HandleExtern``" etc -functions, and then dumps out the LLVM IR. This gives a nice way to look -at the LLVM IR for simple functions. For example: - -:: - - ready> 4+5; - Read top-level expression: - define double @0() { - entry: - ret double 9.000000e+00 - } - -Note how the parser turns the top-level expression into anonymous -functions for us. This will be handy when we add `JIT -support `_ in the next chapter. Also note that the -code is very literally transcribed, no optimizations are being performed -except simple constant folding done by IRBuilder. We will `add -optimizations `_ explicitly in the next -chapter. - -:: - - ready> def foo(a b) a*a + 2*a*b + b*b; - Read function definition: - define double @foo(double %a, double %b) { - entry: - %multmp = fmul double %a, %a - %multmp1 = fmul double 2.000000e+00, %a - %multmp2 = fmul double %multmp1, %b - %addtmp = fadd double %multmp, %multmp2 - %multmp3 = fmul double %b, %b - %addtmp4 = fadd double %addtmp, %multmp3 - ret double %addtmp4 - } - -This shows some simple arithmetic. Notice the striking similarity to the -LLVM builder calls that we use to create the instructions. - -:: - - ready> def bar(a) foo(a, 4.0) + bar(31337); - Read function definition: - define double @bar(double %a) { - entry: - %calltmp = call double @foo(double %a, double 4.000000e+00) - %calltmp1 = call double @bar(double 3.133700e+04) - %addtmp = fadd double %calltmp, %calltmp1 - ret double %addtmp - } - -This shows some function calls. Note that this function will take a long -time to execute if you call it. In the future we'll add conditional -control flow to actually make recursion useful :). - -:: - - ready> extern cos(x); - Read extern: - declare double @cos(double) - - ready> cos(1.234); - Read top-level expression: - define double @1() { - entry: - %calltmp = call double @cos(double 1.234000e+00) - ret double %calltmp - } - -This shows an extern for the libm "cos" function, and a call to it. - -.. TODO:: Abandon Pygments' horrible `llvm` lexer. It just totally gives up - on highlighting this due to the first line. - -:: - - ready> ^D - ; ModuleID = 'my cool jit' - - define double @0() { - entry: - %addtmp = fadd double 4.000000e+00, 5.000000e+00 - ret double %addtmp - } - - define double @foo(double %a, double %b) { - entry: - %multmp = fmul double %a, %a - %multmp1 = fmul double 2.000000e+00, %a - %multmp2 = fmul double %multmp1, %b - %addtmp = fadd double %multmp, %multmp2 - %multmp3 = fmul double %b, %b - %addtmp4 = fadd double %addtmp, %multmp3 - ret double %addtmp4 - } - - define double @bar(double %a) { - entry: - %calltmp = call double @foo(double %a, double 4.000000e+00) - %calltmp1 = call double @bar(double 3.133700e+04) - %addtmp = fadd double %calltmp, %calltmp1 - ret double %addtmp - } - - declare double @cos(double) - - define double @1() { - entry: - %calltmp = call double @cos(double 1.234000e+00) - ret double %calltmp - } - -When you quit the current demo (by sending an EOF via CTRL+D on Linux -or CTRL+Z and ENTER on Windows), it dumps out the IR for the entire -module generated. Here you can see the big picture with all the -functions referencing each other. - -This wraps up the third chapter of the Kaleidoscope tutorial. Up next, -we'll describe how to `add JIT codegen and optimizer -support `_ to this so we can actually start running -code! - -Full Code Listing -================= - -Here is the complete code listing for our running example, enhanced with -the LLVM code generator. Because this uses the LLVM libraries, we need -to link them in. To do this, we use the -`llvm-config `_ tool to inform -our makefile/command line about which options to use: - -.. code-block:: bash - - # Compile - clang++ -g -O3 toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core` -o toy - # Run - ./toy - -Here is the code: - -.. literalinclude:: ../../examples/Kaleidoscope/Chapter3/toy.cpp - :language: c++ - -`Next: Adding JIT and Optimizer Support `_ - +The Kaleidoscope Tutorial has `moved to another location `_ . diff --git a/docs/tutorial/LangImpl04.rst b/docs/tutorial/LangImpl04.rst index bdd21d6cd4a..1ff4dc8af44 100644 --- a/docs/tutorial/LangImpl04.rst +++ b/docs/tutorial/LangImpl04.rst @@ -1,659 +1,7 @@ -============================================== -Kaleidoscope: Adding JIT and Optimizer Support -============================================== +:orphan: -.. contents:: - :local: - -Chapter 4 Introduction -====================== - -Welcome to Chapter 4 of the "`Implementing a language with -LLVM `_" tutorial. Chapters 1-3 described the implementation -of a simple language and added support for generating LLVM IR. This -chapter describes two new techniques: adding optimizer support to your -language, and adding JIT compiler support. These additions will -demonstrate how to get nice, efficient code for the Kaleidoscope -language. - -Trivial Constant Folding -======================== - -Our demonstration for Chapter 3 is elegant and easy to extend. -Unfortunately, it does not produce wonderful code. The IRBuilder, -however, does give us obvious optimizations when compiling simple code: - -:: - - ready> def test(x) 1+2+x; - Read function definition: - define double @test(double %x) { - entry: - %addtmp = fadd double 3.000000e+00, %x - ret double %addtmp - } - -This code is not a literal transcription of the AST built by parsing the -input. That would be: - -:: - - ready> def test(x) 1+2+x; - Read function definition: - define double @test(double %x) { - entry: - %addtmp = fadd double 2.000000e+00, 1.000000e+00 - %addtmp1 = fadd double %addtmp, %x - ret double %addtmp1 - } - -Constant folding, as seen above, in particular, is a very common and -very important optimization: so much so that many language implementors -implement constant folding support in their AST representation. - -With LLVM, you don't need this support in the AST. Since all calls to -build LLVM IR go through the LLVM IR builder, the builder itself checked -to see if there was a constant folding opportunity when you call it. If -so, it just does the constant fold and return the constant instead of -creating an instruction. - -Well, that was easy :). In practice, we recommend always using -``IRBuilder`` when generating code like this. It has no "syntactic -overhead" for its use (you don't have to uglify your compiler with -constant checks everywhere) and it can dramatically reduce the amount of -LLVM IR that is generated in some cases (particular for languages with a -macro preprocessor or that use a lot of constants). - -On the other hand, the ``IRBuilder`` is limited by the fact that it does -all of its analysis inline with the code as it is built. If you take a -slightly more complex example: - -:: - - ready> def test(x) (1+2+x)*(x+(1+2)); - ready> Read function definition: - define double @test(double %x) { - entry: - %addtmp = fadd double 3.000000e+00, %x - %addtmp1 = fadd double %x, 3.000000e+00 - %multmp = fmul double %addtmp, %addtmp1 - ret double %multmp - } - -In this case, the LHS and RHS of the multiplication are the same value. -We'd really like to see this generate "``tmp = x+3; result = tmp*tmp;``" -instead of computing "``x+3``" twice. - -Unfortunately, no amount of local analysis will be able to detect and -correct this. This requires two transformations: reassociation of -expressions (to make the add's lexically identical) and Common -Subexpression Elimination (CSE) to delete the redundant add instruction. -Fortunately, LLVM provides a broad range of optimizations that you can -use, in the form of "passes". - -LLVM Optimization Passes -======================== - -.. warning:: - - Due to the transition to the new PassManager infrastructure this tutorial - is based on ``llvm::legacy::FunctionPassManager`` which can be found in - `LegacyPassManager.h `_. - For the purpose of the this tutorial the above should be used until - the pass manager transition is complete. - -LLVM provides many optimization passes, which do many different sorts of -things and have different tradeoffs. Unlike other systems, LLVM doesn't -hold to the mistaken notion that one set of optimizations is right for -all languages and for all situations. LLVM allows a compiler implementor -to make complete decisions about what optimizations to use, in which -order, and in what situation. - -As a concrete example, LLVM supports both "whole module" passes, which -look across as large of body of code as they can (often a whole file, -but if run at link time, this can be a substantial portion of the whole -program). It also supports and includes "per-function" passes which just -operate on a single function at a time, without looking at other -functions. For more information on passes and how they are run, see the -`How to Write a Pass <../WritingAnLLVMPass.html>`_ document and the -`List of LLVM Passes <../Passes.html>`_. - -For Kaleidoscope, we are currently generating functions on the fly, one -at a time, as the user types them in. We aren't shooting for the -ultimate optimization experience in this setting, but we also want to -catch the easy and quick stuff where possible. As such, we will choose -to run a few per-function optimizations as the user types the function -in. If we wanted to make a "static Kaleidoscope compiler", we would use -exactly the code we have now, except that we would defer running the -optimizer until the entire file has been parsed. - -In order to get per-function optimizations going, we need to set up a -`FunctionPassManager <../WritingAnLLVMPass.html#what-passmanager-doesr>`_ to hold -and organize the LLVM optimizations that we want to run. Once we have -that, we can add a set of optimizations to run. We'll need a new -FunctionPassManager for each module that we want to optimize, so we'll -write a function to create and initialize both the module and pass manager -for us: - -.. code-block:: c++ - - void InitializeModuleAndPassManager(void) { - // Open a new module. - TheModule = llvm::make_unique("my cool jit", TheContext); - - // Create a new pass manager attached to it. - TheFPM = llvm::make_unique(TheModule.get()); - - // Do simple "peephole" optimizations and bit-twiddling optzns. - TheFPM->add(createInstructionCombiningPass()); - // Reassociate expressions. - TheFPM->add(createReassociatePass()); - // Eliminate Common SubExpressions. - TheFPM->add(createGVNPass()); - // Simplify the control flow graph (deleting unreachable blocks, etc). - TheFPM->add(createCFGSimplificationPass()); - - TheFPM->doInitialization(); - } - -This code initializes the global module ``TheModule``, and the function pass -manager ``TheFPM``, which is attached to ``TheModule``. Once the pass manager is -set up, we use a series of "add" calls to add a bunch of LLVM passes. - -In this case, we choose to add four optimization passes. -The passes we choose here are a pretty standard set -of "cleanup" optimizations that are useful for a wide variety of code. I won't -delve into what they do but, believe me, they are a good starting place :). - -Once the PassManager is set up, we need to make use of it. We do this by -running it after our newly created function is constructed (in -``FunctionAST::codegen()``), but before it is returned to the client: - -.. code-block:: c++ - - if (Value *RetVal = Body->codegen()) { - // Finish off the function. - Builder.CreateRet(RetVal); - - // Validate the generated code, checking for consistency. - verifyFunction(*TheFunction); - - // Optimize the function. - TheFPM->run(*TheFunction); - - return TheFunction; - } - -As you can see, this is pretty straightforward. The -``FunctionPassManager`` optimizes and updates the LLVM Function\* in -place, improving (hopefully) its body. With this in place, we can try -our test above again: - -:: - - ready> def test(x) (1+2+x)*(x+(1+2)); - ready> Read function definition: - define double @test(double %x) { - entry: - %addtmp = fadd double %x, 3.000000e+00 - %multmp = fmul double %addtmp, %addtmp - ret double %multmp - } - -As expected, we now get our nicely optimized code, saving a floating -point add instruction from every execution of this function. - -LLVM provides a wide variety of optimizations that can be used in -certain circumstances. Some `documentation about the various -passes <../Passes.html>`_ is available, but it isn't very complete. -Another good source of ideas can come from looking at the passes that -``Clang`` runs to get started. The "``opt``" tool allows you to -experiment with passes from the command line, so you can see if they do -anything. - -Now that we have reasonable code coming out of our front-end, let's talk -about executing it! - -Adding a JIT Compiler +===================== +Kaleidoscope Tutorial ===================== -Code that is available in LLVM IR can have a wide variety of tools -applied to it. For example, you can run optimizations on it (as we did -above), you can dump it out in textual or binary forms, you can compile -the code to an assembly file (.s) for some target, or you can JIT -compile it. The nice thing about the LLVM IR representation is that it -is the "common currency" between many different parts of the compiler. - -In this section, we'll add JIT compiler support to our interpreter. The -basic idea that we want for Kaleidoscope is to have the user enter -function bodies as they do now, but immediately evaluate the top-level -expressions they type in. For example, if they type in "1 + 2;", we -should evaluate and print out 3. If they define a function, they should -be able to call it from the command line. - -In order to do this, we first prepare the environment to create code for -the current native target and declare and initialize the JIT. This is -done by calling some ``InitializeNativeTarget\*`` functions and -adding a global variable ``TheJIT``, and initializing it in -``main``: - -.. code-block:: c++ - - static std::unique_ptr TheJIT; - ... - int main() { - InitializeNativeTarget(); - InitializeNativeTargetAsmPrinter(); - InitializeNativeTargetAsmParser(); - - // Install standard binary operators. - // 1 is lowest precedence. - BinopPrecedence['<'] = 10; - BinopPrecedence['+'] = 20; - BinopPrecedence['-'] = 20; - BinopPrecedence['*'] = 40; // highest. - - // Prime the first token. - fprintf(stderr, "ready> "); - getNextToken(); - - TheJIT = llvm::make_unique(); - - // Run the main "interpreter loop" now. - MainLoop(); - - return 0; - } - -We also need to setup the data layout for the JIT: - -.. code-block:: c++ - - void InitializeModuleAndPassManager(void) { - // Open a new module. - TheModule = llvm::make_unique("my cool jit", TheContext); - TheModule->setDataLayout(TheJIT->getTargetMachine().createDataLayout()); - - // Create a new pass manager attached to it. - TheFPM = llvm::make_unique(TheModule.get()); - ... - -The KaleidoscopeJIT class is a simple JIT built specifically for these -tutorials, available inside the LLVM source code -at llvm-src/examples/Kaleidoscope/include/KaleidoscopeJIT.h. -In later chapters we will look at how it works and extend it with -new features, but for now we will take it as given. Its API is very simple: -``addModule`` adds an LLVM IR module to the JIT, making its functions -available for execution; ``removeModule`` removes a module, freeing any -memory associated with the code in that module; and ``findSymbol`` allows us -to look up pointers to the compiled code. - -We can take this simple API and change our code that parses top-level expressions to -look like this: - -.. code-block:: c++ - - static void HandleTopLevelExpression() { - // Evaluate a top-level expression into an anonymous function. - if (auto FnAST = ParseTopLevelExpr()) { - if (FnAST->codegen()) { - - // JIT the module containing the anonymous expression, keeping a handle so - // we can free it later. - auto H = TheJIT->addModule(std::move(TheModule)); - InitializeModuleAndPassManager(); - - // Search the JIT for the __anon_expr symbol. - auto ExprSymbol = TheJIT->findSymbol("__anon_expr"); - assert(ExprSymbol && "Function not found"); - - // Get the symbol's address and cast it to the right type (takes no - // arguments, returns a double) so we can call it as a native function. - double (*FP)() = (double (*)())(intptr_t)ExprSymbol.getAddress(); - fprintf(stderr, "Evaluated to %f\n", FP()); - - // Delete the anonymous expression module from the JIT. - TheJIT->removeModule(H); - } - -If parsing and codegen succeeed, the next step is to add the module containing -the top-level expression to the JIT. We do this by calling addModule, which -triggers code generation for all the functions in the module, and returns a -handle that can be used to remove the module from the JIT later. Once the module -has been added to the JIT it can no longer be modified, so we also open a new -module to hold subsequent code by calling ``InitializeModuleAndPassManager()``. - -Once we've added the module to the JIT we need to get a pointer to the final -generated code. We do this by calling the JIT's findSymbol method, and passing -the name of the top-level expression function: ``__anon_expr``. Since we just -added this function, we assert that findSymbol returned a result. - -Next, we get the in-memory address of the ``__anon_expr`` function by calling -``getAddress()`` on the symbol. Recall that we compile top-level expressions -into a self-contained LLVM function that takes no arguments and returns the -computed double. Because the LLVM JIT compiler matches the native platform ABI, -this means that you can just cast the result pointer to a function pointer of -that type and call it directly. This means, there is no difference between JIT -compiled code and native machine code that is statically linked into your -application. - -Finally, since we don't support re-evaluation of top-level expressions, we -remove the module from the JIT when we're done to free the associated memory. -Recall, however, that the module we created a few lines earlier (via -``InitializeModuleAndPassManager``) is still open and waiting for new code to be -added. - -With just these two changes, let's see how Kaleidoscope works now! - -:: - - ready> 4+5; - Read top-level expression: - define double @0() { - entry: - ret double 9.000000e+00 - } - - Evaluated to 9.000000 - -Well this looks like it is basically working. The dump of the function -shows the "no argument function that always returns double" that we -synthesize for each top-level expression that is typed in. This -demonstrates very basic functionality, but can we do more? - -:: - - ready> def testfunc(x y) x + y*2; - Read function definition: - define double @testfunc(double %x, double %y) { - entry: - %multmp = fmul double %y, 2.000000e+00 - %addtmp = fadd double %multmp, %x - ret double %addtmp - } - - ready> testfunc(4, 10); - Read top-level expression: - define double @1() { - entry: - %calltmp = call double @testfunc(double 4.000000e+00, double 1.000000e+01) - ret double %calltmp - } - - Evaluated to 24.000000 - - ready> testfunc(5, 10); - ready> LLVM ERROR: Program used external function 'testfunc' which could not be resolved! - - -Function definitions and calls also work, but something went very wrong on that -last line. The call looks valid, so what happened? As you may have guessed from -the API a Module is a unit of allocation for the JIT, and testfunc was part -of the same module that contained anonymous expression. When we removed that -module from the JIT to free the memory for the anonymous expression, we deleted -the definition of ``testfunc`` along with it. Then, when we tried to call -testfunc a second time, the JIT could no longer find it. - -The easiest way to fix this is to put the anonymous expression in a separate -module from the rest of the function definitions. The JIT will happily resolve -function calls across module boundaries, as long as each of the functions called -has a prototype, and is added to the JIT before it is called. By putting the -anonymous expression in a different module we can delete it without affecting -the rest of the functions. - -In fact, we're going to go a step further and put every function in its own -module. Doing so allows us to exploit a useful property of the KaleidoscopeJIT -that will make our environment more REPL-like: Functions can be added to the -JIT more than once (unlike a module where every function must have a unique -definition). When you look up a symbol in KaleidoscopeJIT it will always return -the most recent definition: - -:: - - ready> def foo(x) x + 1; - Read function definition: - define double @foo(double %x) { - entry: - %addtmp = fadd double %x, 1.000000e+00 - ret double %addtmp - } - - ready> foo(2); - Evaluated to 3.000000 - - ready> def foo(x) x + 2; - define double @foo(double %x) { - entry: - %addtmp = fadd double %x, 2.000000e+00 - ret double %addtmp - } - - ready> foo(2); - Evaluated to 4.000000 - - -To allow each function to live in its own module we'll need a way to -re-generate previous function declarations into each new module we open: - -.. code-block:: c++ - - static std::unique_ptr TheJIT; - - ... - - Function *getFunction(std::string Name) { - // First, see if the function has already been added to the current module. - if (auto *F = TheModule->getFunction(Name)) - return F; - - // If not, check whether we can codegen the declaration from some existing - // prototype. - auto FI = FunctionProtos.find(Name); - if (FI != FunctionProtos.end()) - return FI->second->codegen(); - - // If no existing prototype exists, return null. - return nullptr; - } - - ... - - Value *CallExprAST::codegen() { - // Look up the name in the global module table. - Function *CalleeF = getFunction(Callee); - - ... - - Function *FunctionAST::codegen() { - // Transfer ownership of the prototype to the FunctionProtos map, but keep a - // reference to it for use below. - auto &P = *Proto; - FunctionProtos[Proto->getName()] = std::move(Proto); - Function *TheFunction = getFunction(P.getName()); - if (!TheFunction) - return nullptr; - - -To enable this, we'll start by adding a new global, ``FunctionProtos``, that -holds the most recent prototype for each function. We'll also add a convenience -method, ``getFunction()``, to replace calls to ``TheModule->getFunction()``. -Our convenience method searches ``TheModule`` for an existing function -declaration, falling back to generating a new declaration from FunctionProtos if -it doesn't find one. In ``CallExprAST::codegen()`` we just need to replace the -call to ``TheModule->getFunction()``. In ``FunctionAST::codegen()`` we need to -update the FunctionProtos map first, then call ``getFunction()``. With this -done, we can always obtain a function declaration in the current module for any -previously declared function. - -We also need to update HandleDefinition and HandleExtern: - -.. code-block:: c++ - - static void HandleDefinition() { - if (auto FnAST = ParseDefinition()) { - if (auto *FnIR = FnAST->codegen()) { - fprintf(stderr, "Read function definition:"); - FnIR->print(errs()); - fprintf(stderr, "\n"); - TheJIT->addModule(std::move(TheModule)); - InitializeModuleAndPassManager(); - } - } else { - // Skip token for error recovery. - getNextToken(); - } - } - - static void HandleExtern() { - if (auto ProtoAST = ParseExtern()) { - if (auto *FnIR = ProtoAST->codegen()) { - fprintf(stderr, "Read extern: "); - FnIR->print(errs()); - fprintf(stderr, "\n"); - FunctionProtos[ProtoAST->getName()] = std::move(ProtoAST); - } - } else { - // Skip token for error recovery. - getNextToken(); - } - } - -In HandleDefinition, we add two lines to transfer the newly defined function to -the JIT and open a new module. In HandleExtern, we just need to add one line to -add the prototype to FunctionProtos. - -With these changes made, let's try our REPL again (I removed the dump of the -anonymous functions this time, you should get the idea by now :) : - -:: - - ready> def foo(x) x + 1; - ready> foo(2); - Evaluated to 3.000000 - - ready> def foo(x) x + 2; - ready> foo(2); - Evaluated to 4.000000 - -It works! - -Even with this simple code, we get some surprisingly powerful capabilities - -check this out: - -:: - - ready> extern sin(x); - Read extern: - declare double @sin(double) - - ready> extern cos(x); - Read extern: - declare double @cos(double) - - ready> sin(1.0); - Read top-level expression: - define double @2() { - entry: - ret double 0x3FEAED548F090CEE - } - - Evaluated to 0.841471 - - ready> def foo(x) sin(x)*sin(x) + cos(x)*cos(x); - Read function definition: - define double @foo(double %x) { - entry: - %calltmp = call double @sin(double %x) - %multmp = fmul double %calltmp, %calltmp - %calltmp2 = call double @cos(double %x) - %multmp4 = fmul double %calltmp2, %calltmp2 - %addtmp = fadd double %multmp, %multmp4 - ret double %addtmp - } - - ready> foo(4.0); - Read top-level expression: - define double @3() { - entry: - %calltmp = call double @foo(double 4.000000e+00) - ret double %calltmp - } - - Evaluated to 1.000000 - -Whoa, how does the JIT know about sin and cos? The answer is surprisingly -simple: The KaleidoscopeJIT has a straightforward symbol resolution rule that -it uses to find symbols that aren't available in any given module: First -it searches all the modules that have already been added to the JIT, from the -most recent to the oldest, to find the newest definition. If no definition is -found inside the JIT, it falls back to calling "``dlsym("sin")``" on the -Kaleidoscope process itself. Since "``sin``" is defined within the JIT's -address space, it simply patches up calls in the module to call the libm -version of ``sin`` directly. But in some cases this even goes further: -as sin and cos are names of standard math functions, the constant folder -will directly evaluate the function calls to the correct result when called -with constants like in the "``sin(1.0)``" above. - -In the future we'll see how tweaking this symbol resolution rule can be used to -enable all sorts of useful features, from security (restricting the set of -symbols available to JIT'd code), to dynamic code generation based on symbol -names, and even lazy compilation. - -One immediate benefit of the symbol resolution rule is that we can now extend -the language by writing arbitrary C++ code to implement operations. For example, -if we add: - -.. code-block:: c++ - - #ifdef _WIN32 - #define DLLEXPORT __declspec(dllexport) - #else - #define DLLEXPORT - #endif - - /// putchard - putchar that takes a double and returns 0. - extern "C" DLLEXPORT double putchard(double X) { - fputc((char)X, stderr); - return 0; - } - -Note, that for Windows we need to actually export the functions because -the dynamic symbol loader will use GetProcAddress to find the symbols. - -Now we can produce simple output to the console by using things like: -"``extern putchard(x); putchard(120);``", which prints a lowercase 'x' -on the console (120 is the ASCII code for 'x'). Similar code could be -used to implement file I/O, console input, and many other capabilities -in Kaleidoscope. - -This completes the JIT and optimizer chapter of the Kaleidoscope -tutorial. At this point, we can compile a non-Turing-complete -programming language, optimize and JIT compile it in a user-driven way. -Next up we'll look into `extending the language with control flow -constructs `_, tackling some interesting LLVM IR issues -along the way. - -Full Code Listing -================= - -Here is the complete code listing for our running example, enhanced with -the LLVM JIT and optimizer. To build this example, use: - -.. code-block:: bash - - # Compile - clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core mcjit native` -O3 -o toy - # Run - ./toy - -If you are compiling this on Linux, make sure to add the "-rdynamic" -option as well. This makes sure that the external functions are resolved -properly at runtime. - -Here is the code: - -.. literalinclude:: ../../examples/Kaleidoscope/Chapter4/toy.cpp - :language: c++ - -`Next: Extending the language: control flow `_ - +The Kaleidoscope Tutorial has `moved to another location `_ . diff --git a/docs/tutorial/LangImpl05-cfg.png b/docs/tutorial/LangImpl05-cfg.png deleted file mode 100644 index cdba92ff6c5..00000000000 Binary files a/docs/tutorial/LangImpl05-cfg.png and /dev/null differ diff --git a/docs/tutorial/LangImpl05.rst b/docs/tutorial/LangImpl05.rst index dad24890e12..1ff4dc8af44 100644 --- a/docs/tutorial/LangImpl05.rst +++ b/docs/tutorial/LangImpl05.rst @@ -1,814 +1,7 @@ -================================================== -Kaleidoscope: Extending the Language: Control Flow -================================================== +:orphan: -.. contents:: - :local: - -Chapter 5 Introduction -====================== - -Welcome to Chapter 5 of the "`Implementing a language with -LLVM `_" tutorial. Parts 1-4 described the implementation of -the simple Kaleidoscope language and included support for generating -LLVM IR, followed by optimizations and a JIT compiler. Unfortunately, as -presented, Kaleidoscope is mostly useless: it has no control flow other -than call and return. This means that you can't have conditional -branches in the code, significantly limiting its power. In this episode -of "build that compiler", we'll extend Kaleidoscope to have an -if/then/else expression plus a simple 'for' loop. - -If/Then/Else -============ - -Extending Kaleidoscope to support if/then/else is quite straightforward. -It basically requires adding support for this "new" concept to the -lexer, parser, AST, and LLVM code emitter. This example is nice, because -it shows how easy it is to "grow" a language over time, incrementally -extending it as new ideas are discovered. - -Before we get going on "how" we add this extension, let's talk about -"what" we want. The basic idea is that we want to be able to write this -sort of thing: - -:: - - def fib(x) - if x < 3 then - 1 - else - fib(x-1)+fib(x-2); - -In Kaleidoscope, every construct is an expression: there are no -statements. As such, the if/then/else expression needs to return a value -like any other. Since we're using a mostly functional form, we'll have -it evaluate its conditional, then return the 'then' or 'else' value -based on how the condition was resolved. This is very similar to the C -"?:" expression. - -The semantics of the if/then/else expression is that it evaluates the -condition to a boolean equality value: 0.0 is considered to be false and -everything else is considered to be true. If the condition is true, the -first subexpression is evaluated and returned, if the condition is -false, the second subexpression is evaluated and returned. Since -Kaleidoscope allows side-effects, this behavior is important to nail -down. - -Now that we know what we "want", let's break this down into its -constituent pieces. - -Lexer Extensions for If/Then/Else ---------------------------------- - -The lexer extensions are straightforward. First we add new enum values -for the relevant tokens: - -.. code-block:: c++ - - // control - tok_if = -6, - tok_then = -7, - tok_else = -8, - -Once we have that, we recognize the new keywords in the lexer. This is -pretty simple stuff: - -.. code-block:: c++ - - ... - if (IdentifierStr == "def") - return tok_def; - if (IdentifierStr == "extern") - return tok_extern; - if (IdentifierStr == "if") - return tok_if; - if (IdentifierStr == "then") - return tok_then; - if (IdentifierStr == "else") - return tok_else; - return tok_identifier; - -AST Extensions for If/Then/Else -------------------------------- - -To represent the new expression we add a new AST node for it: - -.. code-block:: c++ - - /// IfExprAST - Expression class for if/then/else. - class IfExprAST : public ExprAST { - std::unique_ptr Cond, Then, Else; - - public: - IfExprAST(std::unique_ptr Cond, std::unique_ptr Then, - std::unique_ptr Else) - : Cond(std::move(Cond)), Then(std::move(Then)), Else(std::move(Else)) {} - - Value *codegen() override; - }; - -The AST node just has pointers to the various subexpressions. - -Parser Extensions for If/Then/Else ----------------------------------- - -Now that we have the relevant tokens coming from the lexer and we have -the AST node to build, our parsing logic is relatively straightforward. -First we define a new parsing function: - -.. code-block:: c++ - - /// ifexpr ::= 'if' expression 'then' expression 'else' expression - static std::unique_ptr ParseIfExpr() { - getNextToken(); // eat the if. - - // condition. - auto Cond = ParseExpression(); - if (!Cond) - return nullptr; - - if (CurTok != tok_then) - return LogError("expected then"); - getNextToken(); // eat the then - - auto Then = ParseExpression(); - if (!Then) - return nullptr; - - if (CurTok != tok_else) - return LogError("expected else"); - - getNextToken(); - - auto Else = ParseExpression(); - if (!Else) - return nullptr; - - return llvm::make_unique(std::move(Cond), std::move(Then), - std::move(Else)); - } - -Next we hook it up as a primary expression: - -.. code-block:: c++ - - static std::unique_ptr ParsePrimary() { - switch (CurTok) { - default: - return LogError("unknown token when expecting an expression"); - case tok_identifier: - return ParseIdentifierExpr(); - case tok_number: - return ParseNumberExpr(); - case '(': - return ParseParenExpr(); - case tok_if: - return ParseIfExpr(); - } - } - -LLVM IR for If/Then/Else ------------------------- - -Now that we have it parsing and building the AST, the final piece is -adding LLVM code generation support. This is the most interesting part -of the if/then/else example, because this is where it starts to -introduce new concepts. All of the code above has been thoroughly -described in previous chapters. - -To motivate the code we want to produce, let's take a look at a simple -example. Consider: - -:: - - extern foo(); - extern bar(); - def baz(x) if x then foo() else bar(); - -If you disable optimizations, the code you'll (soon) get from -Kaleidoscope looks like this: - -.. code-block:: llvm - - declare double @foo() - - declare double @bar() - - define double @baz(double %x) { - entry: - %ifcond = fcmp one double %x, 0.000000e+00 - br i1 %ifcond, label %then, label %else - - then: ; preds = %entry - %calltmp = call double @foo() - br label %ifcont - - else: ; preds = %entry - %calltmp1 = call double @bar() - br label %ifcont - - ifcont: ; preds = %else, %then - %iftmp = phi double [ %calltmp, %then ], [ %calltmp1, %else ] - ret double %iftmp - } - -To visualize the control flow graph, you can use a nifty feature of the -LLVM '`opt `_' tool. If you put this LLVM -IR into "t.ll" and run "``llvm-as < t.ll | opt -analyze -view-cfg``", `a -window will pop up <../ProgrammersManual.html#viewing-graphs-while-debugging-code>`_ and you'll -see this graph: - -.. figure:: LangImpl05-cfg.png - :align: center - :alt: Example CFG - - Example CFG - -Another way to get this is to call "``F->viewCFG()``" or -"``F->viewCFGOnly()``" (where F is a "``Function*``") either by -inserting actual calls into the code and recompiling or by calling these -in the debugger. LLVM has many nice features for visualizing various -graphs. - -Getting back to the generated code, it is fairly simple: the entry block -evaluates the conditional expression ("x" in our case here) and compares -the result to 0.0 with the "``fcmp one``" instruction ('one' is "Ordered -and Not Equal"). Based on the result of this expression, the code jumps -to either the "then" or "else" blocks, which contain the expressions for -the true/false cases. - -Once the then/else blocks are finished executing, they both branch back -to the 'ifcont' block to execute the code that happens after the -if/then/else. In this case the only thing left to do is to return to the -caller of the function. The question then becomes: how does the code -know which expression to return? - -The answer to this question involves an important SSA operation: the -`Phi -operation `_. -If you're not familiar with SSA, `the wikipedia -article `_ -is a good introduction and there are various other introductions to it -available on your favorite search engine. The short version is that -"execution" of the Phi operation requires "remembering" which block -control came from. The Phi operation takes on the value corresponding to -the input control block. In this case, if control comes in from the -"then" block, it gets the value of "calltmp". If control comes from the -"else" block, it gets the value of "calltmp1". - -At this point, you are probably starting to think "Oh no! This means my -simple and elegant front-end will have to start generating SSA form in -order to use LLVM!". Fortunately, this is not the case, and we strongly -advise *not* implementing an SSA construction algorithm in your -front-end unless there is an amazingly good reason to do so. In -practice, there are two sorts of values that float around in code -written for your average imperative programming language that might need -Phi nodes: - -#. Code that involves user variables: ``x = 1; x = x + 1;`` -#. Values that are implicit in the structure of your AST, such as the - Phi node in this case. - -In `Chapter 7 `_ of this tutorial ("mutable variables"), -we'll talk about #1 in depth. For now, just believe me that you don't -need SSA construction to handle this case. For #2, you have the choice -of using the techniques that we will describe for #1, or you can insert -Phi nodes directly, if convenient. In this case, it is really -easy to generate the Phi node, so we choose to do it directly. - -Okay, enough of the motivation and overview, let's generate code! - -Code Generation for If/Then/Else --------------------------------- - -In order to generate code for this, we implement the ``codegen`` method -for ``IfExprAST``: - -.. code-block:: c++ - - Value *IfExprAST::codegen() { - Value *CondV = Cond->codegen(); - if (!CondV) - return nullptr; - - // Convert condition to a bool by comparing non-equal to 0.0. - CondV = Builder.CreateFCmpONE( - CondV, ConstantFP::get(TheContext, APFloat(0.0)), "ifcond"); - -This code is straightforward and similar to what we saw before. We emit -the expression for the condition, then compare that value to zero to get -a truth value as a 1-bit (bool) value. - -.. code-block:: c++ - - Function *TheFunction = Builder.GetInsertBlock()->getParent(); - - // Create blocks for the then and else cases. Insert the 'then' block at the - // end of the function. - BasicBlock *ThenBB = - BasicBlock::Create(TheContext, "then", TheFunction); - BasicBlock *ElseBB = BasicBlock::Create(TheContext, "else"); - BasicBlock *MergeBB = BasicBlock::Create(TheContext, "ifcont"); - - Builder.CreateCondBr(CondV, ThenBB, ElseBB); - -This code creates the basic blocks that are related to the if/then/else -statement, and correspond directly to the blocks in the example above. -The first line gets the current Function object that is being built. It -gets this by asking the builder for the current BasicBlock, and asking -that block for its "parent" (the function it is currently embedded -into). - -Once it has that, it creates three blocks. Note that it passes -"TheFunction" into the constructor for the "then" block. This causes the -constructor to automatically insert the new block into the end of the -specified function. The other two blocks are created, but aren't yet -inserted into the function. - -Once the blocks are created, we can emit the conditional branch that -chooses between them. Note that creating new blocks does not implicitly -affect the IRBuilder, so it is still inserting into the block that the -condition went into. Also note that it is creating a branch to the -"then" block and the "else" block, even though the "else" block isn't -inserted into the function yet. This is all ok: it is the standard way -that LLVM supports forward references. - -.. code-block:: c++ - - // Emit then value. - Builder.SetInsertPoint(ThenBB); - - Value *ThenV = Then->codegen(); - if (!ThenV) - return nullptr; - - Builder.CreateBr(MergeBB); - // Codegen of 'Then' can change the current block, update ThenBB for the PHI. - ThenBB = Builder.GetInsertBlock(); - -After the conditional branch is inserted, we move the builder to start -inserting into the "then" block. Strictly speaking, this call moves the -insertion point to be at the end of the specified block. However, since -the "then" block is empty, it also starts out by inserting at the -beginning of the block. :) - -Once the insertion point is set, we recursively codegen the "then" -expression from the AST. To finish off the "then" block, we create an -unconditional branch to the merge block. One interesting (and very -important) aspect of the LLVM IR is that it `requires all basic blocks -to be "terminated" <../LangRef.html#functionstructure>`_ with a `control -flow instruction <../LangRef.html#terminators>`_ such as return or -branch. This means that all control flow, *including fall throughs* must -be made explicit in the LLVM IR. If you violate this rule, the verifier -will emit an error. - -The final line here is quite subtle, but is very important. The basic -issue is that when we create the Phi node in the merge block, we need to -set up the block/value pairs that indicate how the Phi will work. -Importantly, the Phi node expects to have an entry for each predecessor -of the block in the CFG. Why then, are we getting the current block when -we just set it to ThenBB 5 lines above? The problem is that the "Then" -expression may actually itself change the block that the Builder is -emitting into if, for example, it contains a nested "if/then/else" -expression. Because calling ``codegen()`` recursively could arbitrarily change -the notion of the current block, we are required to get an up-to-date -value for code that will set up the Phi node. - -.. code-block:: c++ - - // Emit else block. - TheFunction->getBasicBlockList().push_back(ElseBB); - Builder.SetInsertPoint(ElseBB); - - Value *ElseV = Else->codegen(); - if (!ElseV) - return nullptr; - - Builder.CreateBr(MergeBB); - // codegen of 'Else' can change the current block, update ElseBB for the PHI. - ElseBB = Builder.GetInsertBlock(); - -Code generation for the 'else' block is basically identical to codegen -for the 'then' block. The only significant difference is the first line, -which adds the 'else' block to the function. Recall previously that the -'else' block was created, but not added to the function. Now that the -'then' and 'else' blocks are emitted, we can finish up with the merge -code: - -.. code-block:: c++ - - // Emit merge block. - TheFunction->getBasicBlockList().push_back(MergeBB); - Builder.SetInsertPoint(MergeBB); - PHINode *PN = - Builder.CreatePHI(Type::getDoubleTy(TheContext), 2, "iftmp"); - - PN->addIncoming(ThenV, ThenBB); - PN->addIncoming(ElseV, ElseBB); - return PN; - } - -The first two lines here are now familiar: the first adds the "merge" -block to the Function object (it was previously floating, like the else -block above). The second changes the insertion point so that newly -created code will go into the "merge" block. Once that is done, we need -to create the PHI node and set up the block/value pairs for the PHI. - -Finally, the CodeGen function returns the phi node as the value computed -by the if/then/else expression. In our example above, this returned -value will feed into the code for the top-level function, which will -create the return instruction. - -Overall, we now have the ability to execute conditional code in -Kaleidoscope. With this extension, Kaleidoscope is a fairly complete -language that can calculate a wide variety of numeric functions. Next up -we'll add another useful expression that is familiar from non-functional -languages... - -'for' Loop Expression +===================== +Kaleidoscope Tutorial ===================== -Now that we know how to add basic control flow constructs to the -language, we have the tools to add more powerful things. Let's add -something more aggressive, a 'for' expression: - -:: - - extern putchard(char); - def printstar(n) - for i = 1, i < n, 1.0 in - putchard(42); # ascii 42 = '*' - - # print 100 '*' characters - printstar(100); - -This expression defines a new variable ("i" in this case) which iterates -from a starting value, while the condition ("i < n" in this case) is -true, incrementing by an optional step value ("1.0" in this case). If -the step value is omitted, it defaults to 1.0. While the loop is true, -it executes its body expression. Because we don't have anything better -to return, we'll just define the loop as always returning 0.0. In the -future when we have mutable variables, it will get more useful. - -As before, let's talk about the changes that we need to Kaleidoscope to -support this. - -Lexer Extensions for the 'for' Loop ------------------------------------ - -The lexer extensions are the same sort of thing as for if/then/else: - -.. code-block:: c++ - - ... in enum Token ... - // control - tok_if = -6, tok_then = -7, tok_else = -8, - tok_for = -9, tok_in = -10 - - ... in gettok ... - if (IdentifierStr == "def") - return tok_def; - if (IdentifierStr == "extern") - return tok_extern; - if (IdentifierStr == "if") - return tok_if; - if (IdentifierStr == "then") - return tok_then; - if (IdentifierStr == "else") - return tok_else; - if (IdentifierStr == "for") - return tok_for; - if (IdentifierStr == "in") - return tok_in; - return tok_identifier; - -AST Extensions for the 'for' Loop ---------------------------------- - -The AST node is just as simple. It basically boils down to capturing the -variable name and the constituent expressions in the node. - -.. code-block:: c++ - - /// ForExprAST - Expression class for for/in. - class ForExprAST : public ExprAST { - std::string VarName; - std::unique_ptr Start, End, Step, Body; - - public: - ForExprAST(const std::string &VarName, std::unique_ptr Start, - std::unique_ptr End, std::unique_ptr Step, - std::unique_ptr Body) - : VarName(VarName), Start(std::move(Start)), End(std::move(End)), - Step(std::move(Step)), Body(std::move(Body)) {} - - Value *codegen() override; - }; - -Parser Extensions for the 'for' Loop ------------------------------------- - -The parser code is also fairly standard. The only interesting thing here -is handling of the optional step value. The parser code handles it by -checking to see if the second comma is present. If not, it sets the step -value to null in the AST node: - -.. code-block:: c++ - - /// forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression - static std::unique_ptr ParseForExpr() { - getNextToken(); // eat the for. - - if (CurTok != tok_identifier) - return LogError("expected identifier after for"); - - std::string IdName = IdentifierStr; - getNextToken(); // eat identifier. - - if (CurTok != '=') - return LogError("expected '=' after for"); - getNextToken(); // eat '='. - - - auto Start = ParseExpression(); - if (!Start) - return nullptr; - if (CurTok != ',') - return LogError("expected ',' after for start value"); - getNextToken(); - - auto End = ParseExpression(); - if (!End) - return nullptr; - - // The step value is optional. - std::unique_ptr Step; - if (CurTok == ',') { - getNextToken(); - Step = ParseExpression(); - if (!Step) - return nullptr; - } - - if (CurTok != tok_in) - return LogError("expected 'in' after for"); - getNextToken(); // eat 'in'. - - auto Body = ParseExpression(); - if (!Body) - return nullptr; - - return llvm::make_unique(IdName, std::move(Start), - std::move(End), std::move(Step), - std::move(Body)); - } - -And again we hook it up as a primary expression: - -.. code-block:: c++ - - static std::unique_ptr ParsePrimary() { - switch (CurTok) { - default: - return LogError("unknown token when expecting an expression"); - case tok_identifier: - return ParseIdentifierExpr(); - case tok_number: - return ParseNumberExpr(); - case '(': - return ParseParenExpr(); - case tok_if: - return ParseIfExpr(); - case tok_for: - return ParseForExpr(); - } - } - -LLVM IR for the 'for' Loop --------------------------- - -Now we get to the good part: the LLVM IR we want to generate for this -thing. With the simple example above, we get this LLVM IR (note that -this dump is generated with optimizations disabled for clarity): - -.. code-block:: llvm - - declare double @putchard(double) - - define double @printstar(double %n) { - entry: - ; initial value = 1.0 (inlined into phi) - br label %loop - - loop: ; preds = %loop, %entry - %i = phi double [ 1.000000e+00, %entry ], [ %nextvar, %loop ] - ; body - %calltmp = call double @putchard(double 4.200000e+01) - ; increment - %nextvar = fadd double %i, 1.000000e+00 - - ; termination test - %cmptmp = fcmp ult double %i, %n - %booltmp = uitofp i1 %cmptmp to double - %loopcond = fcmp one double %booltmp, 0.000000e+00 - br i1 %loopcond, label %loop, label %afterloop - - afterloop: ; preds = %loop - ; loop always returns 0.0 - ret double 0.000000e+00 - } - -This loop contains all the same constructs we saw before: a phi node, -several expressions, and some basic blocks. Let's see how this fits -together. - -Code Generation for the 'for' Loop ----------------------------------- - -The first part of codegen is very simple: we just output the start -expression for the loop value: - -.. code-block:: c++ - - Value *ForExprAST::codegen() { - // Emit the start code first, without 'variable' in scope. - Value *StartVal = Start->codegen(); - if (!StartVal) - return nullptr; - -With this out of the way, the next step is to set up the LLVM basic -block for the start of the loop body. In the case above, the whole loop -body is one block, but remember that the body code itself could consist -of multiple blocks (e.g. if it contains an if/then/else or a for/in -expression). - -.. code-block:: c++ - - // Make the new basic block for the loop header, inserting after current - // block. - Function *TheFunction = Builder.GetInsertBlock()->getParent(); - BasicBlock *PreheaderBB = Builder.GetInsertBlock(); - BasicBlock *LoopBB = - BasicBlock::Create(TheContext, "loop", TheFunction); - - // Insert an explicit fall through from the current block to the LoopBB. - Builder.CreateBr(LoopBB); - -This code is similar to what we saw for if/then/else. Because we will -need it to create the Phi node, we remember the block that falls through -into the loop. Once we have that, we create the actual block that starts -the loop and create an unconditional branch for the fall-through between -the two blocks. - -.. code-block:: c++ - - // Start insertion in LoopBB. - Builder.SetInsertPoint(LoopBB); - - // Start the PHI node with an entry for Start. - PHINode *Variable = Builder.CreatePHI(Type::getDoubleTy(TheContext), - 2, VarName.c_str()); - Variable->addIncoming(StartVal, PreheaderBB); - -Now that the "preheader" for the loop is set up, we switch to emitting -code for the loop body. To begin with, we move the insertion point and -create the PHI node for the loop induction variable. Since we already -know the incoming value for the starting value, we add it to the Phi -node. Note that the Phi will eventually get a second value for the -backedge, but we can't set it up yet (because it doesn't exist!). - -.. code-block:: c++ - - // Within the loop, the variable is defined equal to the PHI node. If it - // shadows an existing variable, we have to restore it, so save it now. - Value *OldVal = NamedValues[VarName]; - NamedValues[VarName] = Variable; - - // Emit the body of the loop. This, like any other expr, can change the - // current BB. Note that we ignore the value computed by the body, but don't - // allow an error. - if (!Body->codegen()) - return nullptr; - -Now the code starts to get more interesting. Our 'for' loop introduces a -new variable to the symbol table. This means that our symbol table can -now contain either function arguments or loop variables. To handle this, -before we codegen the body of the loop, we add the loop variable as the -current value for its name. Note that it is possible that there is a -variable of the same name in the outer scope. It would be easy to make -this an error (emit an error and return null if there is already an -entry for VarName) but we choose to allow shadowing of variables. In -order to handle this correctly, we remember the Value that we are -potentially shadowing in ``OldVal`` (which will be null if there is no -shadowed variable). - -Once the loop variable is set into the symbol table, the code -recursively codegen's the body. This allows the body to use the loop -variable: any references to it will naturally find it in the symbol -table. - -.. code-block:: c++ - - // Emit the step value. - Value *StepVal = nullptr; - if (Step) { - StepVal = Step->codegen(); - if (!StepVal) - return nullptr; - } else { - // If not specified, use 1.0. - StepVal = ConstantFP::get(TheContext, APFloat(1.0)); - } - - Value *NextVar = Builder.CreateFAdd(Variable, StepVal, "nextvar"); - -Now that the body is emitted, we compute the next value of the iteration -variable by adding the step value, or 1.0 if it isn't present. -'``NextVar``' will be the value of the loop variable on the next -iteration of the loop. - -.. code-block:: c++ - - // Compute the end condition. - Value *EndCond = End->codegen(); - if (!EndCond) - return nullptr; - - // Convert condition to a bool by comparing non-equal to 0.0. - EndCond = Builder.CreateFCmpONE( - EndCond, ConstantFP::get(TheContext, APFloat(0.0)), "loopcond"); - -Finally, we evaluate the exit value of the loop, to determine whether -the loop should exit. This mirrors the condition evaluation for the -if/then/else statement. - -.. code-block:: c++ - - // Create the "after loop" block and insert it. - BasicBlock *LoopEndBB = Builder.GetInsertBlock(); - BasicBlock *AfterBB = - BasicBlock::Create(TheContext, "afterloop", TheFunction); - - // Insert the conditional branch into the end of LoopEndBB. - Builder.CreateCondBr(EndCond, LoopBB, AfterBB); - - // Any new code will be inserted in AfterBB. - Builder.SetInsertPoint(AfterBB); - -With the code for the body of the loop complete, we just need to finish -up the control flow for it. This code remembers the end block (for the -phi node), then creates the block for the loop exit ("afterloop"). Based -on the value of the exit condition, it creates a conditional branch that -chooses between executing the loop again and exiting the loop. Any -future code is emitted in the "afterloop" block, so it sets the -insertion position to it. - -.. code-block:: c++ - - // Add a new entry to the PHI node for the backedge. - Variable->addIncoming(NextVar, LoopEndBB); - - // Restore the unshadowed variable. - if (OldVal) - NamedValues[VarName] = OldVal; - else - NamedValues.erase(VarName); - - // for expr always returns 0.0. - return Constant::getNullValue(Type::getDoubleTy(TheContext)); - } - -The final code handles various cleanups: now that we have the "NextVar" -value, we can add the incoming value to the loop PHI node. After that, -we remove the loop variable from the symbol table, so that it isn't in -scope after the for loop. Finally, code generation of the for loop -always returns 0.0, so that is what we return from -``ForExprAST::codegen()``. - -With this, we conclude the "adding control flow to Kaleidoscope" chapter -of the tutorial. In this chapter we added two control flow constructs, -and used them to motivate a couple of aspects of the LLVM IR that are -important for front-end implementors to know. In the next chapter of our -saga, we will get a bit crazier and add `user-defined -operators `_ to our poor innocent language. - -Full Code Listing -================= - -Here is the complete code listing for our running example, enhanced with -the if/then/else and for expressions. To build this example, use: - -.. code-block:: bash - - # Compile - clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core mcjit native` -O3 -o toy - # Run - ./toy - -Here is the code: - -.. literalinclude:: ../../examples/Kaleidoscope/Chapter5/toy.cpp - :language: c++ - -`Next: Extending the language: user-defined operators `_ - +The Kaleidoscope Tutorial has `moved to another location `_ . diff --git a/docs/tutorial/LangImpl06.rst b/docs/tutorial/LangImpl06.rst index 2a9f4c6b609..1ff4dc8af44 100644 --- a/docs/tutorial/LangImpl06.rst +++ b/docs/tutorial/LangImpl06.rst @@ -1,768 +1,7 @@ -============================================================ -Kaleidoscope: Extending the Language: User-defined Operators -============================================================ +:orphan: -.. contents:: - :local: - -Chapter 6 Introduction -====================== - -Welcome to Chapter 6 of the "`Implementing a language with -LLVM `_" tutorial. At this point in our tutorial, we now -have a fully functional language that is fairly minimal, but also -useful. There is still one big problem with it, however. Our language -doesn't have many useful operators (like division, logical negation, or -even any comparisons besides less-than). - -This chapter of the tutorial takes a wild digression into adding -user-defined operators to the simple and beautiful Kaleidoscope -language. This digression now gives us a simple and ugly language in -some ways, but also a powerful one at the same time. One of the great -things about creating your own language is that you get to decide what -is good or bad. In this tutorial we'll assume that it is okay to use -this as a way to show some interesting parsing techniques. - -At the end of this tutorial, we'll run through an example Kaleidoscope -application that `renders the Mandelbrot set <#kicking-the-tires>`_. This gives an -example of what you can build with Kaleidoscope and its feature set. - -User-defined Operators: the Idea -================================ - -The "operator overloading" that we will add to Kaleidoscope is more -general than in languages like C++. In C++, you are only allowed to -redefine existing operators: you can't programmatically change the -grammar, introduce new operators, change precedence levels, etc. In this -chapter, we will add this capability to Kaleidoscope, which will let the -user round out the set of operators that are supported. - -The point of going into user-defined operators in a tutorial like this -is to show the power and flexibility of using a hand-written parser. -Thus far, the parser we have been implementing uses recursive descent -for most parts of the grammar and operator precedence parsing for the -expressions. See `Chapter 2 `_ for details. By -using operator precedence parsing, it is very easy to allow -the programmer to introduce new operators into the grammar: the grammar -is dynamically extensible as the JIT runs. - -The two specific features we'll add are programmable unary operators -(right now, Kaleidoscope has no unary operators at all) as well as -binary operators. An example of this is: - -:: - - # Logical unary not. - def unary!(v) - if v then - 0 - else - 1; - - # Define > with the same precedence as <. - def binary> 10 (LHS RHS) - RHS < LHS; - - # Binary "logical or", (note that it does not "short circuit") - def binary| 5 (LHS RHS) - if LHS then - 1 - else if RHS then - 1 - else - 0; - - # Define = with slightly lower precedence than relationals. - def binary= 9 (LHS RHS) - !(LHS < RHS | LHS > RHS); - -Many languages aspire to being able to implement their standard runtime -library in the language itself. In Kaleidoscope, we can implement -significant parts of the language in the library! - -We will break down implementation of these features into two parts: -implementing support for user-defined binary operators and adding unary -operators. - -User-defined Binary Operators -============================= - -Adding support for user-defined binary operators is pretty simple with -our current framework. We'll first add support for the unary/binary -keywords: - -.. code-block:: c++ - - enum Token { - ... - // operators - tok_binary = -11, - tok_unary = -12 - }; - ... - static int gettok() { - ... - if (IdentifierStr == "for") - return tok_for; - if (IdentifierStr == "in") - return tok_in; - if (IdentifierStr == "binary") - return tok_binary; - if (IdentifierStr == "unary") - return tok_unary; - return tok_identifier; - -This just adds lexer support for the unary and binary keywords, like we -did in `previous chapters `_. One nice thing -about our current AST, is that we represent binary operators with full -generalisation by using their ASCII code as the opcode. For our extended -operators, we'll use this same representation, so we don't need any new -AST or parser support. - -On the other hand, we have to be able to represent the definitions of -these new operators, in the "def binary\| 5" part of the function -definition. In our grammar so far, the "name" for the function -definition is parsed as the "prototype" production and into the -``PrototypeAST`` AST node. To represent our new user-defined operators -as prototypes, we have to extend the ``PrototypeAST`` AST node like -this: - -.. code-block:: c++ - - /// PrototypeAST - This class represents the "prototype" for a function, - /// which captures its argument names as well as if it is an operator. - class PrototypeAST { - std::string Name; - std::vector Args; - bool IsOperator; - unsigned Precedence; // Precedence if a binary op. - - public: - PrototypeAST(const std::string &name, std::vector Args, - bool IsOperator = false, unsigned Prec = 0) - : Name(name), Args(std::move(Args)), IsOperator(IsOperator), - Precedence(Prec) {} - - Function *codegen(); - const std::string &getName() const { return Name; } - - bool isUnaryOp() const { return IsOperator && Args.size() == 1; } - bool isBinaryOp() const { return IsOperator && Args.size() == 2; } - - char getOperatorName() const { - assert(isUnaryOp() || isBinaryOp()); - return Name[Name.size() - 1]; - } - - unsigned getBinaryPrecedence() const { return Precedence; } - }; - -Basically, in addition to knowing a name for the prototype, we now keep -track of whether it was an operator, and if it was, what precedence -level the operator is at. The precedence is only used for binary -operators (as you'll see below, it just doesn't apply for unary -operators). Now that we have a way to represent the prototype for a -user-defined operator, we need to parse it: - -.. code-block:: c++ - - /// prototype - /// ::= id '(' id* ')' - /// ::= binary LETTER number? (id, id) - static std::unique_ptr ParsePrototype() { - std::string FnName; - - unsigned Kind = 0; // 0 = identifier, 1 = unary, 2 = binary. - unsigned BinaryPrecedence = 30; - - switch (CurTok) { - default: - return LogErrorP("Expected function name in prototype"); - case tok_identifier: - FnName = IdentifierStr; - Kind = 0; - getNextToken(); - break; - case tok_binary: - getNextToken(); - if (!isascii(CurTok)) - return LogErrorP("Expected binary operator"); - FnName = "binary"; - FnName += (char)CurTok; - Kind = 2; - getNextToken(); - - // Read the precedence if present. - if (CurTok == tok_number) { - if (NumVal < 1 || NumVal > 100) - return LogErrorP("Invalid precedence: must be 1..100"); - BinaryPrecedence = (unsigned)NumVal; - getNextToken(); - } - break; - } - - if (CurTok != '(') - return LogErrorP("Expected '(' in prototype"); - - std::vector ArgNames; - while (getNextToken() == tok_identifier) - ArgNames.push_back(IdentifierStr); - if (CurTok != ')') - return LogErrorP("Expected ')' in prototype"); - - // success. - getNextToken(); // eat ')'. - - // Verify right number of names for operator. - if (Kind && ArgNames.size() != Kind) - return LogErrorP("Invalid number of operands for operator"); - - return llvm::make_unique(FnName, std::move(ArgNames), Kind != 0, - BinaryPrecedence); - } - -This is all fairly straightforward parsing code, and we have already -seen a lot of similar code in the past. One interesting part about the -code above is the couple lines that set up ``FnName`` for binary -operators. This builds names like "binary@" for a newly defined "@" -operator. It then takes advantage of the fact that symbol names in the -LLVM symbol table are allowed to have any character in them, including -embedded nul characters. - -The next interesting thing to add, is codegen support for these binary -operators. Given our current structure, this is a simple addition of a -default case for our existing binary operator node: - -.. code-block:: c++ - - Value *BinaryExprAST::codegen() { - Value *L = LHS->codegen(); - Value *R = RHS->codegen(); - if (!L || !R) - return nullptr; - - switch (Op) { - case '+': - return Builder.CreateFAdd(L, R, "addtmp"); - case '-': - return Builder.CreateFSub(L, R, "subtmp"); - case '*': - return Builder.CreateFMul(L, R, "multmp"); - case '<': - L = Builder.CreateFCmpULT(L, R, "cmptmp"); - // Convert bool 0/1 to double 0.0 or 1.0 - return Builder.CreateUIToFP(L, Type::getDoubleTy(TheContext), - "booltmp"); - default: - break; - } - - // If it wasn't a builtin binary operator, it must be a user defined one. Emit - // a call to it. - Function *F = getFunction(std::string("binary") + Op); - assert(F && "binary operator not found!"); - - Value *Ops[2] = { L, R }; - return Builder.CreateCall(F, Ops, "binop"); - } - -As you can see above, the new code is actually really simple. It just -does a lookup for the appropriate operator in the symbol table and -generates a function call to it. Since user-defined operators are just -built as normal functions (because the "prototype" boils down to a -function with the right name) everything falls into place. - -The final piece of code we are missing, is a bit of top-level magic: - -.. code-block:: c++ - - Function *FunctionAST::codegen() { - // Transfer ownership of the prototype to the FunctionProtos map, but keep a - // reference to it for use below. - auto &P = *Proto; - FunctionProtos[Proto->getName()] = std::move(Proto); - Function *TheFunction = getFunction(P.getName()); - if (!TheFunction) - return nullptr; - - // If this is an operator, install it. - if (P.isBinaryOp()) - BinopPrecedence[P.getOperatorName()] = P.getBinaryPrecedence(); - - // Create a new basic block to start insertion into. - BasicBlock *BB = BasicBlock::Create(TheContext, "entry", TheFunction); - ... - -Basically, before codegening a function, if it is a user-defined -operator, we register it in the precedence table. This allows the binary -operator parsing logic we already have in place to handle it. Since we -are working on a fully-general operator precedence parser, this is all -we need to do to "extend the grammar". - -Now we have useful user-defined binary operators. This builds a lot on -the previous framework we built for other operators. Adding unary -operators is a bit more challenging, because we don't have any framework -for it yet - let's see what it takes. - -User-defined Unary Operators -============================ - -Since we don't currently support unary operators in the Kaleidoscope -language, we'll need to add everything to support them. Above, we added -simple support for the 'unary' keyword to the lexer. In addition to -that, we need an AST node: - -.. code-block:: c++ - - /// UnaryExprAST - Expression class for a unary operator. - class UnaryExprAST : public ExprAST { - char Opcode; - std::unique_ptr Operand; - - public: - UnaryExprAST(char Opcode, std::unique_ptr Operand) - : Opcode(Opcode), Operand(std::move(Operand)) {} - - Value *codegen() override; - }; - -This AST node is very simple and obvious by now. It directly mirrors the -binary operator AST node, except that it only has one child. With this, -we need to add the parsing logic. Parsing a unary operator is pretty -simple: we'll add a new function to do it: - -.. code-block:: c++ - - /// unary - /// ::= primary - /// ::= '!' unary - static std::unique_ptr ParseUnary() { - // If the current token is not an operator, it must be a primary expr. - if (!isascii(CurTok) || CurTok == '(' || CurTok == ',') - return ParsePrimary(); - - // If this is a unary operator, read it. - int Opc = CurTok; - getNextToken(); - if (auto Operand = ParseUnary()) - return llvm::make_unique(Opc, std::move(Operand)); - return nullptr; - } - -The grammar we add is pretty straightforward here. If we see a unary -operator when parsing a primary operator, we eat the operator as a -prefix and parse the remaining piece as another unary operator. This -allows us to handle multiple unary operators (e.g. "!!x"). Note that -unary operators can't have ambiguous parses like binary operators can, -so there is no need for precedence information. - -The problem with this function, is that we need to call ParseUnary from -somewhere. To do this, we change previous callers of ParsePrimary to -call ParseUnary instead: - -.. code-block:: c++ - - /// binoprhs - /// ::= ('+' unary)* - static std::unique_ptr ParseBinOpRHS(int ExprPrec, - std::unique_ptr LHS) { - ... - // Parse the unary expression after the binary operator. - auto RHS = ParseUnary(); - if (!RHS) - return nullptr; - ... - } - /// expression - /// ::= unary binoprhs - /// - static std::unique_ptr ParseExpression() { - auto LHS = ParseUnary(); - if (!LHS) - return nullptr; - - return ParseBinOpRHS(0, std::move(LHS)); - } - -With these two simple changes, we are now able to parse unary operators -and build the AST for them. Next up, we need to add parser support for -prototypes, to parse the unary operator prototype. We extend the binary -operator code above with: - -.. code-block:: c++ - - /// prototype - /// ::= id '(' id* ')' - /// ::= binary LETTER number? (id, id) - /// ::= unary LETTER (id) - static std::unique_ptr ParsePrototype() { - std::string FnName; - - unsigned Kind = 0; // 0 = identifier, 1 = unary, 2 = binary. - unsigned BinaryPrecedence = 30; - - switch (CurTok) { - default: - return LogErrorP("Expected function name in prototype"); - case tok_identifier: - FnName = IdentifierStr; - Kind = 0; - getNextToken(); - break; - case tok_unary: - getNextToken(); - if (!isascii(CurTok)) - return LogErrorP("Expected unary operator"); - FnName = "unary"; - FnName += (char)CurTok; - Kind = 1; - getNextToken(); - break; - case tok_binary: - ... - -As with binary operators, we name unary operators with a name that -includes the operator character. This assists us at code generation -time. Speaking of, the final piece we need to add is codegen support for -unary operators. It looks like this: - -.. code-block:: c++ - - Value *UnaryExprAST::codegen() { - Value *OperandV = Operand->codegen(); - if (!OperandV) - return nullptr; - - Function *F = getFunction(std::string("unary") + Opcode); - if (!F) - return LogErrorV("Unknown unary operator"); - - return Builder.CreateCall(F, OperandV, "unop"); - } - -This code is similar to, but simpler than, the code for binary -operators. It is simpler primarily because it doesn't need to handle any -predefined operators. - -Kicking the Tires -================= - -It is somewhat hard to believe, but with a few simple extensions we've -covered in the last chapters, we have grown a real-ish language. With -this, we can do a lot of interesting things, including I/O, math, and a -bunch of other things. For example, we can now add a nice sequencing -operator (printd is defined to print out the specified value and a -newline): - -:: - - ready> extern printd(x); - Read extern: - declare double @printd(double) - - ready> def binary : 1 (x y) 0; # Low-precedence operator that ignores operands. - ... - ready> printd(123) : printd(456) : printd(789); - 123.000000 - 456.000000 - 789.000000 - Evaluated to 0.000000 - -We can also define a bunch of other "primitive" operations, such as: - -:: - - # Logical unary not. - def unary!(v) - if v then - 0 - else - 1; - - # Unary negate. - def unary-(v) - 0-v; - - # Define > with the same precedence as <. - def binary> 10 (LHS RHS) - RHS < LHS; - - # Binary logical or, which does not short circuit. - def binary| 5 (LHS RHS) - if LHS then - 1 - else if RHS then - 1 - else - 0; - - # Binary logical and, which does not short circuit. - def binary& 6 (LHS RHS) - if !LHS then - 0 - else - !!RHS; - - # Define = with slightly lower precedence than relationals. - def binary = 9 (LHS RHS) - !(LHS < RHS | LHS > RHS); - - # Define ':' for sequencing: as a low-precedence operator that ignores operands - # and just returns the RHS. - def binary : 1 (x y) y; - -Given the previous if/then/else support, we can also define interesting -functions for I/O. For example, the following prints out a character -whose "density" reflects the value passed in: the lower the value, the -denser the character: - -:: - - ready> extern putchard(char); - ... - ready> def printdensity(d) - if d > 8 then - putchard(32) # ' ' - else if d > 4 then - putchard(46) # '.' - else if d > 2 then - putchard(43) # '+' - else - putchard(42); # '*' - ... - ready> printdensity(1): printdensity(2): printdensity(3): - printdensity(4): printdensity(5): printdensity(9): - putchard(10); - **++. - Evaluated to 0.000000 - -Based on these simple primitive operations, we can start to define more -interesting things. For example, here's a little function that determines -the number of iterations it takes for a certain function in the complex -plane to diverge: - -:: - - # Determine whether the specific location diverges. - # Solve for z = z^2 + c in the complex plane. - def mandelconverger(real imag iters creal cimag) - if iters > 255 | (real*real + imag*imag > 4) then - iters - else - mandelconverger(real*real - imag*imag + creal, - 2*real*imag + cimag, - iters+1, creal, cimag); - - # Return the number of iterations required for the iteration to escape - def mandelconverge(real imag) - mandelconverger(real, imag, 0, real, imag); - -This "``z = z2 + c``" function is a beautiful little creature that is -the basis for computation of the `Mandelbrot -Set `_. Our -``mandelconverge`` function returns the number of iterations that it -takes for a complex orbit to escape, saturating to 255. This is not a -very useful function by itself, but if you plot its value over a -two-dimensional plane, you can see the Mandelbrot set. Given that we are -limited to using putchard here, our amazing graphical output is limited, -but we can whip together something using the density plotter above: - -:: - - # Compute and plot the mandelbrot set with the specified 2 dimensional range - # info. - def mandelhelp(xmin xmax xstep ymin ymax ystep) - for y = ymin, y < ymax, ystep in ( - (for x = xmin, x < xmax, xstep in - printdensity(mandelconverge(x,y))) - : putchard(10) - ) - - # mandel - This is a convenient helper function for plotting the mandelbrot set - # from the specified position with the specified Magnification. - def mandel(realstart imagstart realmag imagmag) - mandelhelp(realstart, realstart+realmag*78, realmag, - imagstart, imagstart+imagmag*40, imagmag); - -Given this, we can try plotting out the mandelbrot set! Lets try it out: - -:: - - ready> mandel(-2.3, -1.3, 0.05, 0.07); - *******************************+++++++++++************************************* - *************************+++++++++++++++++++++++******************************* - **********************+++++++++++++++++++++++++++++**************************** - *******************+++++++++++++++++++++.. ...++++++++************************* - *****************++++++++++++++++++++++.... ...+++++++++*********************** - ***************+++++++++++++++++++++++..... ...+++++++++********************* - **************+++++++++++++++++++++++.... ....+++++++++******************** - *************++++++++++++++++++++++...... .....++++++++******************* - ************+++++++++++++++++++++....... .......+++++++****************** - ***********+++++++++++++++++++.... ... .+++++++***************** - **********+++++++++++++++++....... .+++++++**************** - *********++++++++++++++........... ...+++++++*************** - ********++++++++++++............ ...++++++++************** - ********++++++++++... .......... .++++++++************** - *******+++++++++..... .+++++++++************* - *******++++++++...... ..+++++++++************* - *******++++++....... ..+++++++++************* - *******+++++...... ..+++++++++************* - *******.... .... ...+++++++++************* - *******.... . ...+++++++++************* - *******+++++...... ...+++++++++************* - *******++++++....... ..+++++++++************* - *******++++++++...... .+++++++++************* - *******+++++++++..... ..+++++++++************* - ********++++++++++... .......... .++++++++************** - ********++++++++++++............ ...++++++++************** - *********++++++++++++++.......... ...+++++++*************** - **********++++++++++++++++........ .+++++++**************** - **********++++++++++++++++++++.... ... ..+++++++**************** - ***********++++++++++++++++++++++....... .......++++++++***************** - ************+++++++++++++++++++++++...... ......++++++++****************** - **************+++++++++++++++++++++++.... ....++++++++******************** - ***************+++++++++++++++++++++++..... ...+++++++++********************* - *****************++++++++++++++++++++++.... ...++++++++*********************** - *******************+++++++++++++++++++++......++++++++************************* - *********************++++++++++++++++++++++.++++++++*************************** - *************************+++++++++++++++++++++++******************************* - ******************************+++++++++++++************************************ - ******************************************************************************* - ******************************************************************************* - ******************************************************************************* - Evaluated to 0.000000 - ready> mandel(-2, -1, 0.02, 0.04); - **************************+++++++++++++++++++++++++++++++++++++++++++++++++++++ - ***********************++++++++++++++++++++++++++++++++++++++++++++++++++++++++ - *********************+++++++++++++++++++++++++++++++++++++++++++++++++++++++++. - *******************+++++++++++++++++++++++++++++++++++++++++++++++++++++++++... - *****************+++++++++++++++++++++++++++++++++++++++++++++++++++++++++..... - ***************++++++++++++++++++++++++++++++++++++++++++++++++++++++++........ - **************++++++++++++++++++++++++++++++++++++++++++++++++++++++........... - ************+++++++++++++++++++++++++++++++++++++++++++++++++++++.............. - ***********++++++++++++++++++++++++++++++++++++++++++++++++++........ . - **********++++++++++++++++++++++++++++++++++++++++++++++............. - ********+++++++++++++++++++++++++++++++++++++++++++.................. - *******+++++++++++++++++++++++++++++++++++++++....................... - ******+++++++++++++++++++++++++++++++++++........................... - *****++++++++++++++++++++++++++++++++............................ - *****++++++++++++++++++++++++++++............................... - ****++++++++++++++++++++++++++...... ......................... - ***++++++++++++++++++++++++......... ...... ........... - ***++++++++++++++++++++++............ - **+++++++++++++++++++++.............. - **+++++++++++++++++++................ - *++++++++++++++++++................. - *++++++++++++++++............ ... - *++++++++++++++.............. - *+++....++++................ - *.......... ........... - * - *.......... ........... - *+++....++++................ - *++++++++++++++.............. - *++++++++++++++++............ ... - *++++++++++++++++++................. - **+++++++++++++++++++................ - **+++++++++++++++++++++.............. - ***++++++++++++++++++++++............ - ***++++++++++++++++++++++++......... ...... ........... - ****++++++++++++++++++++++++++...... ......................... - *****++++++++++++++++++++++++++++............................... - *****++++++++++++++++++++++++++++++++............................ - ******+++++++++++++++++++++++++++++++++++........................... - *******+++++++++++++++++++++++++++++++++++++++....................... - ********+++++++++++++++++++++++++++++++++++++++++++.................. - Evaluated to 0.000000 - ready> mandel(-0.9, -1.4, 0.02, 0.03); - ******************************************************************************* - ******************************************************************************* - ******************************************************************************* - **********+++++++++++++++++++++************************************************ - *+++++++++++++++++++++++++++++++++++++++*************************************** - +++++++++++++++++++++++++++++++++++++++++++++********************************** - ++++++++++++++++++++++++++++++++++++++++++++++++++***************************** - ++++++++++++++++++++++++++++++++++++++++++++++++++++++************************* - +++++++++++++++++++++++++++++++++++++++++++++++++++++++++********************** - +++++++++++++++++++++++++++++++++.........++++++++++++++++++******************* - +++++++++++++++++++++++++++++++.... ......+++++++++++++++++++**************** - +++++++++++++++++++++++++++++....... ........+++++++++++++++++++************** - ++++++++++++++++++++++++++++........ ........++++++++++++++++++++************ - +++++++++++++++++++++++++++......... .. ...+++++++++++++++++++++********** - ++++++++++++++++++++++++++........... ....++++++++++++++++++++++******** - ++++++++++++++++++++++++............. .......++++++++++++++++++++++****** - +++++++++++++++++++++++............. ........+++++++++++++++++++++++**** - ++++++++++++++++++++++........... ..........++++++++++++++++++++++*** - ++++++++++++++++++++........... .........++++++++++++++++++++++* - ++++++++++++++++++............ ...........++++++++++++++++++++ - ++++++++++++++++............... .............++++++++++++++++++ - ++++++++++++++................. ...............++++++++++++++++ - ++++++++++++.................. .................++++++++++++++ - +++++++++.................. .................+++++++++++++ - ++++++........ . ......... ..++++++++++++ - ++............ ...... ....++++++++++ - .............. ...++++++++++ - .............. ....+++++++++ - .............. .....++++++++ - ............. ......++++++++ - ........... .......++++++++ - ......... ........+++++++ - ......... ........+++++++ - ......... ....+++++++ - ........ ...+++++++ - ....... ...+++++++ - ....+++++++ - .....+++++++ - ....+++++++ - ....+++++++ - ....+++++++ - Evaluated to 0.000000 - ready> ^D - -At this point, you may be starting to realize that Kaleidoscope is a -real and powerful language. It may not be self-similar :), but it can be -used to plot things that are! - -With this, we conclude the "adding user-defined operators" chapter of -the tutorial. We have successfully augmented our language, adding the -ability to extend the language in the library, and we have shown how -this can be used to build a simple but interesting end-user application -in Kaleidoscope. At this point, Kaleidoscope can build a variety of -applications that are functional and can call functions with -side-effects, but it can't actually define and mutate a variable itself. - -Strikingly, variable mutation is an important feature of some languages, -and it is not at all obvious how to `add support for mutable -variables `_ without having to add an "SSA construction" -phase to your front-end. In the next chapter, we will describe how you -can add variable mutation without building SSA in your front-end. - -Full Code Listing -================= - -Here is the complete code listing for our running example, enhanced with -the support for user-defined operators. To build this example, use: - -.. code-block:: bash - - # Compile - clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core mcjit native` -O3 -o toy - # Run - ./toy - -On some platforms, you will need to specify -rdynamic or --Wl,--export-dynamic when linking. This ensures that symbols defined in -the main executable are exported to the dynamic linker and so are -available for symbol resolution at run time. This is not needed if you -compile your support code into a shared library, although doing that -will cause problems on Windows. - -Here is the code: - -.. literalinclude:: ../../examples/Kaleidoscope/Chapter6/toy.cpp - :language: c++ - -`Next: Extending the language: mutable variables / SSA -construction `_ +===================== +Kaleidoscope Tutorial +===================== +The Kaleidoscope Tutorial has `moved to another location `_ . diff --git a/docs/tutorial/LangImpl07.rst b/docs/tutorial/LangImpl07.rst index 582645f449b..1ff4dc8af44 100644 --- a/docs/tutorial/LangImpl07.rst +++ b/docs/tutorial/LangImpl07.rst @@ -1,883 +1,7 @@ -======================================================= -Kaleidoscope: Extending the Language: Mutable Variables -======================================================= +:orphan: -.. contents:: - :local: - -Chapter 7 Introduction -====================== - -Welcome to Chapter 7 of the "`Implementing a language with -LLVM `_" tutorial. In chapters 1 through 6, we've built a -very respectable, albeit simple, `functional programming -language `_. In our -journey, we learned some parsing techniques, how to build and represent -an AST, how to build LLVM IR, and how to optimize the resultant code as -well as JIT compile it. - -While Kaleidoscope is interesting as a functional language, the fact -that it is functional makes it "too easy" to generate LLVM IR for it. In -particular, a functional language makes it very easy to build LLVM IR -directly in `SSA -form `_. -Since LLVM requires that the input code be in SSA form, this is a very -nice property and it is often unclear to newcomers how to generate code -for an imperative language with mutable variables. - -The short (and happy) summary of this chapter is that there is no need -for your front-end to build SSA form: LLVM provides highly tuned and -well tested support for this, though the way it works is a bit -unexpected for some. - -Why is this a hard problem? -=========================== - -To understand why mutable variables cause complexities in SSA -construction, consider this extremely simple C example: - -.. code-block:: c - - int G, H; - int test(_Bool Condition) { - int X; - if (Condition) - X = G; - else - X = H; - return X; - } - -In this case, we have the variable "X", whose value depends on the path -executed in the program. Because there are two different possible values -for X before the return instruction, a PHI node is inserted to merge the -two values. The LLVM IR that we want for this example looks like this: - -.. code-block:: llvm - - @G = weak global i32 0 ; type of @G is i32* - @H = weak global i32 0 ; type of @H is i32* - - define i32 @test(i1 %Condition) { - entry: - br i1 %Condition, label %cond_true, label %cond_false - - cond_true: - %X.0 = load i32* @G - br label %cond_next - - cond_false: - %X.1 = load i32* @H - br label %cond_next - - cond_next: - %X.2 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ] - ret i32 %X.2 - } - -In this example, the loads from the G and H global variables are -explicit in the LLVM IR, and they live in the then/else branches of the -if statement (cond\_true/cond\_false). In order to merge the incoming -values, the X.2 phi node in the cond\_next block selects the right value -to use based on where control flow is coming from: if control flow comes -from the cond\_false block, X.2 gets the value of X.1. Alternatively, if -control flow comes from cond\_true, it gets the value of X.0. The intent -of this chapter is not to explain the details of SSA form. For more -information, see one of the many `online -references `_. - -The question for this article is "who places the phi nodes when lowering -assignments to mutable variables?". The issue here is that LLVM -*requires* that its IR be in SSA form: there is no "non-ssa" mode for -it. However, SSA construction requires non-trivial algorithms and data -structures, so it is inconvenient and wasteful for every front-end to -have to reproduce this logic. - -Memory in LLVM -============== - -The 'trick' here is that while LLVM does require all register values to -be in SSA form, it does not require (or permit) memory objects to be in -SSA form. In the example above, note that the loads from G and H are -direct accesses to G and H: they are not renamed or versioned. This -differs from some other compiler systems, which do try to version memory -objects. In LLVM, instead of encoding dataflow analysis of memory into -the LLVM IR, it is handled with `Analysis -Passes <../WritingAnLLVMPass.html>`_ which are computed on demand. - -With this in mind, the high-level idea is that we want to make a stack -variable (which lives in memory, because it is on the stack) for each -mutable object in a function. To take advantage of this trick, we need -to talk about how LLVM represents stack variables. - -In LLVM, all memory accesses are explicit with load/store instructions, -and it is carefully designed not to have (or need) an "address-of" -operator. Notice how the type of the @G/@H global variables is actually -"i32\*" even though the variable is defined as "i32". What this means is -that @G defines *space* for an i32 in the global data area, but its -*name* actually refers to the address for that space. Stack variables -work the same way, except that instead of being declared with global -variable definitions, they are declared with the `LLVM alloca -instruction <../LangRef.html#alloca-instruction>`_: - -.. code-block:: llvm - - define i32 @example() { - entry: - %X = alloca i32 ; type of %X is i32*. - ... - %tmp = load i32* %X ; load the stack value %X from the stack. - %tmp2 = add i32 %tmp, 1 ; increment it - store i32 %tmp2, i32* %X ; store it back - ... - -This code shows an example of how you can declare and manipulate a stack -variable in the LLVM IR. Stack memory allocated with the alloca -instruction is fully general: you can pass the address of the stack slot -to functions, you can store it in other variables, etc. In our example -above, we could rewrite the example to use the alloca technique to avoid -using a PHI node: - -.. code-block:: llvm - - @G = weak global i32 0 ; type of @G is i32* - @H = weak global i32 0 ; type of @H is i32* - - define i32 @test(i1 %Condition) { - entry: - %X = alloca i32 ; type of %X is i32*. - br i1 %Condition, label %cond_true, label %cond_false - - cond_true: - %X.0 = load i32* @G - store i32 %X.0, i32* %X ; Update X - br label %cond_next - - cond_false: - %X.1 = load i32* @H - store i32 %X.1, i32* %X ; Update X - br label %cond_next - - cond_next: - %X.2 = load i32* %X ; Read X - ret i32 %X.2 - } - -With this, we have discovered a way to handle arbitrary mutable -variables without the need to create Phi nodes at all: - -#. Each mutable variable becomes a stack allocation. -#. Each read of the variable becomes a load from the stack. -#. Each update of the variable becomes a store to the stack. -#. Taking the address of a variable just uses the stack address - directly. - -While this solution has solved our immediate problem, it introduced -another one: we have now apparently introduced a lot of stack traffic -for very simple and common operations, a major performance problem. -Fortunately for us, the LLVM optimizer has a highly-tuned optimization -pass named "mem2reg" that handles this case, promoting allocas like this -into SSA registers, inserting Phi nodes as appropriate. If you run this -example through the pass, for example, you'll get: - -.. code-block:: bash - - $ llvm-as < example.ll | opt -mem2reg | llvm-dis - @G = weak global i32 0 - @H = weak global i32 0 - - define i32 @test(i1 %Condition) { - entry: - br i1 %Condition, label %cond_true, label %cond_false - - cond_true: - %X.0 = load i32* @G - br label %cond_next - - cond_false: - %X.1 = load i32* @H - br label %cond_next - - cond_next: - %X.01 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ] - ret i32 %X.01 - } - -The mem2reg pass implements the standard "iterated dominance frontier" -algorithm for constructing SSA form and has a number of optimizations -that speed up (very common) degenerate cases. The mem2reg optimization -pass is the answer to dealing with mutable variables, and we highly -recommend that you depend on it. Note that mem2reg only works on -variables in certain circumstances: - -#. mem2reg is alloca-driven: it looks for allocas and if it can handle - them, it promotes them. It does not apply to global variables or heap - allocations. -#. mem2reg only looks for alloca instructions in the entry block of the - function. Being in the entry block guarantees that the alloca is only - executed once, which makes analysis simpler. -#. mem2reg only promotes allocas whose uses are direct loads and stores. - If the address of the stack object is passed to a function, or if any - funny pointer arithmetic is involved, the alloca will not be - promoted. -#. mem2reg only works on allocas of `first - class <../LangRef.html#first-class-types>`_ values (such as pointers, - scalars and vectors), and only if the array size of the allocation is - 1 (or missing in the .ll file). mem2reg is not capable of promoting - structs or arrays to registers. Note that the "sroa" pass is - more powerful and can promote structs, "unions", and arrays in many - cases. - -All of these properties are easy to satisfy for most imperative -languages, and we'll illustrate it below with Kaleidoscope. The final -question you may be asking is: should I bother with this nonsense for my -front-end? Wouldn't it be better if I just did SSA construction -directly, avoiding use of the mem2reg optimization pass? In short, we -strongly recommend that you use this technique for building SSA form, -unless there is an extremely good reason not to. Using this technique -is: - -- Proven and well tested: clang uses this technique - for local mutable variables. As such, the most common clients of LLVM - are using this to handle a bulk of their variables. You can be sure - that bugs are found fast and fixed early. -- Extremely Fast: mem2reg has a number of special cases that make it - fast in common cases as well as fully general. For example, it has - fast-paths for variables that are only used in a single block, - variables that only have one assignment point, good heuristics to - avoid insertion of unneeded phi nodes, etc. -- Needed for debug info generation: `Debug information in - LLVM <../SourceLevelDebugging.html>`_ relies on having the address of - the variable exposed so that debug info can be attached to it. This - technique dovetails very naturally with this style of debug info. - -If nothing else, this makes it much easier to get your front-end up and -running, and is very simple to implement. Let's extend Kaleidoscope with -mutable variables now! - -Mutable Variables in Kaleidoscope -================================= - -Now that we know the sort of problem we want to tackle, let's see what -this looks like in the context of our little Kaleidoscope language. -We're going to add two features: - -#. The ability to mutate variables with the '=' operator. -#. The ability to define new variables. - -While the first item is really what this is about, we only have -variables for incoming arguments as well as for induction variables, and -redefining those only goes so far :). Also, the ability to define new -variables is a useful thing regardless of whether you will be mutating -them. Here's a motivating example that shows how we could use these: - -:: - - # Define ':' for sequencing: as a low-precedence operator that ignores operands - # and just returns the RHS. - def binary : 1 (x y) y; - - # Recursive fib, we could do this before. - def fib(x) - if (x < 3) then - 1 - else - fib(x-1)+fib(x-2); - - # Iterative fib. - def fibi(x) - var a = 1, b = 1, c in - (for i = 3, i < x in - c = a + b : - a = b : - b = c) : - b; - - # Call it. - fibi(10); - -In order to mutate variables, we have to change our existing variables -to use the "alloca trick". Once we have that, we'll add our new -operator, then extend Kaleidoscope to support new variable definitions. - -Adjusting Existing Variables for Mutation -========================================= - -The symbol table in Kaleidoscope is managed at code generation time by -the '``NamedValues``' map. This map currently keeps track of the LLVM -"Value\*" that holds the double value for the named variable. In order -to support mutation, we need to change this slightly, so that -``NamedValues`` holds the *memory location* of the variable in question. -Note that this change is a refactoring: it changes the structure of the -code, but does not (by itself) change the behavior of the compiler. All -of these changes are isolated in the Kaleidoscope code generator. - -At this point in Kaleidoscope's development, it only supports variables -for two things: incoming arguments to functions and the induction -variable of 'for' loops. For consistency, we'll allow mutation of these -variables in addition to other user-defined variables. This means that -these will both need memory locations. - -To start our transformation of Kaleidoscope, we'll change the -NamedValues map so that it maps to AllocaInst\* instead of Value\*. Once -we do this, the C++ compiler will tell us what parts of the code we need -to update: - -.. code-block:: c++ - - static std::map NamedValues; - -Also, since we will need to create these allocas, we'll use a helper -function that ensures that the allocas are created in the entry block of -the function: - -.. code-block:: c++ - - /// CreateEntryBlockAlloca - Create an alloca instruction in the entry block of - /// the function. This is used for mutable variables etc. - static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction, - const std::string &VarName) { - IRBuilder<> TmpB(&TheFunction->getEntryBlock(), - TheFunction->getEntryBlock().begin()); - return TmpB.CreateAlloca(Type::getDoubleTy(TheContext), 0, - VarName.c_str()); - } - -This funny looking code creates an IRBuilder object that is pointing at -the first instruction (.begin()) of the entry block. It then creates an -alloca with the expected name and returns it. Because all values in -Kaleidoscope are doubles, there is no need to pass in a type to use. - -With this in place, the first functionality change we want to make belongs to -variable references. In our new scheme, variables live on the stack, so -code generating a reference to them actually needs to produce a load -from the stack slot: - -.. code-block:: c++ - - Value *VariableExprAST::codegen() { - // Look this variable up in the function. - Value *V = NamedValues[Name]; - if (!V) - return LogErrorV("Unknown variable name"); - - // Load the value. - return Builder.CreateLoad(V, Name.c_str()); - } - -As you can see, this is pretty straightforward. Now we need to update -the things that define the variables to set up the alloca. We'll start -with ``ForExprAST::codegen()`` (see the `full code listing <#id1>`_ for -the unabridged code): - -.. code-block:: c++ - - Function *TheFunction = Builder.GetInsertBlock()->getParent(); - - // Create an alloca for the variable in the entry block. - AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName); - - // Emit the start code first, without 'variable' in scope. - Value *StartVal = Start->codegen(); - if (!StartVal) - return nullptr; - - // Store the value into the alloca. - Builder.CreateStore(StartVal, Alloca); - ... - - // Compute the end condition. - Value *EndCond = End->codegen(); - if (!EndCond) - return nullptr; - - // Reload, increment, and restore the alloca. This handles the case where - // the body of the loop mutates the variable. - Value *CurVar = Builder.CreateLoad(Alloca); - Value *NextVar = Builder.CreateFAdd(CurVar, StepVal, "nextvar"); - Builder.CreateStore(NextVar, Alloca); - ... - -This code is virtually identical to the code `before we allowed mutable -variables `_. The big difference is that we -no longer have to construct a PHI node, and we use load/store to access -the variable as needed. - -To support mutable argument variables, we need to also make allocas for -them. The code for this is also pretty simple: - -.. code-block:: c++ - - Function *FunctionAST::codegen() { - ... - Builder.SetInsertPoint(BB); - - // Record the function arguments in the NamedValues map. - NamedValues.clear(); - for (auto &Arg : TheFunction->args()) { - // Create an alloca for this variable. - AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, Arg.getName()); - - // Store the initial value into the alloca. - Builder.CreateStore(&Arg, Alloca); - - // Add arguments to variable symbol table. - NamedValues[Arg.getName()] = Alloca; - } - - if (Value *RetVal = Body->codegen()) { - ... - -For each argument, we make an alloca, store the input value to the -function into the alloca, and register the alloca as the memory location -for the argument. This method gets invoked by ``FunctionAST::codegen()`` -right after it sets up the entry block for the function. - -The final missing piece is adding the mem2reg pass, which allows us to -get good codegen once again: - -.. code-block:: c++ - - // Promote allocas to registers. - TheFPM->add(createPromoteMemoryToRegisterPass()); - // Do simple "peephole" optimizations and bit-twiddling optzns. - TheFPM->add(createInstructionCombiningPass()); - // Reassociate expressions. - TheFPM->add(createReassociatePass()); - ... - -It is interesting to see what the code looks like before and after the -mem2reg optimization runs. For example, this is the before/after code -for our recursive fib function. Before the optimization: - -.. code-block:: llvm - - define double @fib(double %x) { - entry: - %x1 = alloca double - store double %x, double* %x1 - %x2 = load double, double* %x1 - %cmptmp = fcmp ult double %x2, 3.000000e+00 - %booltmp = uitofp i1 %cmptmp to double - %ifcond = fcmp one double %booltmp, 0.000000e+00 - br i1 %ifcond, label %then, label %else - - then: ; preds = %entry - br label %ifcont - - else: ; preds = %entry - %x3 = load double, double* %x1 - %subtmp = fsub double %x3, 1.000000e+00 - %calltmp = call double @fib(double %subtmp) - %x4 = load double, double* %x1 - %subtmp5 = fsub double %x4, 2.000000e+00 - %calltmp6 = call double @fib(double %subtmp5) - %addtmp = fadd double %calltmp, %calltmp6 - br label %ifcont - - ifcont: ; preds = %else, %then - %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ] - ret double %iftmp - } - -Here there is only one variable (x, the input argument) but you can -still see the extremely simple-minded code generation strategy we are -using. In the entry block, an alloca is created, and the initial input -value is stored into it. Each reference to the variable does a reload -from the stack. Also, note that we didn't modify the if/then/else -expression, so it still inserts a PHI node. While we could make an -alloca for it, it is actually easier to create a PHI node for it, so we -still just make the PHI. - -Here is the code after the mem2reg pass runs: - -.. code-block:: llvm - - define double @fib(double %x) { - entry: - %cmptmp = fcmp ult double %x, 3.000000e+00 - %booltmp = uitofp i1 %cmptmp to double - %ifcond = fcmp one double %booltmp, 0.000000e+00 - br i1 %ifcond, label %then, label %else - - then: - br label %ifcont - - else: - %subtmp = fsub double %x, 1.000000e+00 - %calltmp = call double @fib(double %subtmp) - %subtmp5 = fsub double %x, 2.000000e+00 - %calltmp6 = call double @fib(double %subtmp5) - %addtmp = fadd double %calltmp, %calltmp6 - br label %ifcont - - ifcont: ; preds = %else, %then - %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ] - ret double %iftmp - } - -This is a trivial case for mem2reg, since there are no redefinitions of -the variable. The point of showing this is to calm your tension about -inserting such blatent inefficiencies :). - -After the rest of the optimizers run, we get: - -.. code-block:: llvm - - define double @fib(double %x) { - entry: - %cmptmp = fcmp ult double %x, 3.000000e+00 - %booltmp = uitofp i1 %cmptmp to double - %ifcond = fcmp ueq double %booltmp, 0.000000e+00 - br i1 %ifcond, label %else, label %ifcont - - else: - %subtmp = fsub double %x, 1.000000e+00 - %calltmp = call double @fib(double %subtmp) - %subtmp5 = fsub double %x, 2.000000e+00 - %calltmp6 = call double @fib(double %subtmp5) - %addtmp = fadd double %calltmp, %calltmp6 - ret double %addtmp - - ifcont: - ret double 1.000000e+00 - } - -Here we see that the simplifycfg pass decided to clone the return -instruction into the end of the 'else' block. This allowed it to -eliminate some branches and the PHI node. - -Now that all symbol table references are updated to use stack variables, -we'll add the assignment operator. - -New Assignment Operator -======================= - -With our current framework, adding a new assignment operator is really -simple. We will parse it just like any other binary operator, but handle -it internally (instead of allowing the user to define it). The first -step is to set a precedence: - -.. code-block:: c++ - - int main() { - // Install standard binary operators. - // 1 is lowest precedence. - BinopPrecedence['='] = 2; - BinopPrecedence['<'] = 10; - BinopPrecedence['+'] = 20; - BinopPrecedence['-'] = 20; - -Now that the parser knows the precedence of the binary operator, it -takes care of all the parsing and AST generation. We just need to -implement codegen for the assignment operator. This looks like: - -.. code-block:: c++ - - Value *BinaryExprAST::codegen() { - // Special case '=' because we don't want to emit the LHS as an expression. - if (Op == '=') { - // Assignment requires the LHS to be an identifier. - VariableExprAST *LHSE = dynamic_cast(LHS.get()); - if (!LHSE) - return LogErrorV("destination of '=' must be a variable"); - -Unlike the rest of the binary operators, our assignment operator doesn't -follow the "emit LHS, emit RHS, do computation" model. As such, it is -handled as a special case before the other binary operators are handled. -The other strange thing is that it requires the LHS to be a variable. It -is invalid to have "(x+1) = expr" - only things like "x = expr" are -allowed. - -.. code-block:: c++ - - // Codegen the RHS. - Value *Val = RHS->codegen(); - if (!Val) - return nullptr; - - // Look up the name. - Value *Variable = NamedValues[LHSE->getName()]; - if (!Variable) - return LogErrorV("Unknown variable name"); - - Builder.CreateStore(Val, Variable); - return Val; - } - ... - -Once we have the variable, codegen'ing the assignment is -straightforward: we emit the RHS of the assignment, create a store, and -return the computed value. Returning a value allows for chained -assignments like "X = (Y = Z)". - -Now that we have an assignment operator, we can mutate loop variables -and arguments. For example, we can now run code like this: - -:: - - # Function to print a double. - extern printd(x); - - # Define ':' for sequencing: as a low-precedence operator that ignores operands - # and just returns the RHS. - def binary : 1 (x y) y; - - def test(x) - printd(x) : - x = 4 : - printd(x); - - test(123); - -When run, this example prints "123" and then "4", showing that we did -actually mutate the value! Okay, we have now officially implemented our -goal: getting this to work requires SSA construction in the general -case. However, to be really useful, we want the ability to define our -own local variables, let's add this next! - -User-defined Local Variables -============================ - -Adding var/in is just like any other extension we made to -Kaleidoscope: we extend the lexer, the parser, the AST and the code -generator. The first step for adding our new 'var/in' construct is to -extend the lexer. As before, this is pretty trivial, the code looks like -this: - -.. code-block:: c++ - - enum Token { - ... - // var definition - tok_var = -13 - ... - } - ... - static int gettok() { - ... - if (IdentifierStr == "in") - return tok_in; - if (IdentifierStr == "binary") - return tok_binary; - if (IdentifierStr == "unary") - return tok_unary; - if (IdentifierStr == "var") - return tok_var; - return tok_identifier; - ... - -The next step is to define the AST node that we will construct. For -var/in, it looks like this: - -.. code-block:: c++ - - /// VarExprAST - Expression class for var/in - class VarExprAST : public ExprAST { - std::vector>> VarNames; - std::unique_ptr Body; - - public: - VarExprAST(std::vector>> VarNames, - std::unique_ptr Body) - : VarNames(std::move(VarNames)), Body(std::move(Body)) {} - - Value *codegen() override; - }; - -var/in allows a list of names to be defined all at once, and each name -can optionally have an initializer value. As such, we capture this -information in the VarNames vector. Also, var/in has a body, this body -is allowed to access the variables defined by the var/in. - -With this in place, we can define the parser pieces. The first thing we -do is add it as a primary expression: - -.. code-block:: c++ - - /// primary - /// ::= identifierexpr - /// ::= numberexpr - /// ::= parenexpr - /// ::= ifexpr - /// ::= forexpr - /// ::= varexpr - static std::unique_ptr ParsePrimary() { - switch (CurTok) { - default: - return LogError("unknown token when expecting an expression"); - case tok_identifier: - return ParseIdentifierExpr(); - case tok_number: - return ParseNumberExpr(); - case '(': - return ParseParenExpr(); - case tok_if: - return ParseIfExpr(); - case tok_for: - return ParseForExpr(); - case tok_var: - return ParseVarExpr(); - } - } - -Next we define ParseVarExpr: - -.. code-block:: c++ - - /// varexpr ::= 'var' identifier ('=' expression)? - // (',' identifier ('=' expression)?)* 'in' expression - static std::unique_ptr ParseVarExpr() { - getNextToken(); // eat the var. - - std::vector>> VarNames; - - // At least one variable name is required. - if (CurTok != tok_identifier) - return LogError("expected identifier after var"); - -The first part of this code parses the list of identifier/expr pairs -into the local ``VarNames`` vector. - -.. code-block:: c++ - - while (1) { - std::string Name = IdentifierStr; - getNextToken(); // eat identifier. - - // Read the optional initializer. - std::unique_ptr Init; - if (CurTok == '=') { - getNextToken(); // eat the '='. - - Init = ParseExpression(); - if (!Init) return nullptr; - } - - VarNames.push_back(std::make_pair(Name, std::move(Init))); - - // End of var list, exit loop. - if (CurTok != ',') break; - getNextToken(); // eat the ','. - - if (CurTok != tok_identifier) - return LogError("expected identifier list after var"); - } - -Once all the variables are parsed, we then parse the body and create the -AST node: - -.. code-block:: c++ - - // At this point, we have to have 'in'. - if (CurTok != tok_in) - return LogError("expected 'in' keyword after 'var'"); - getNextToken(); // eat 'in'. - - auto Body = ParseExpression(); - if (!Body) - return nullptr; - - return llvm::make_unique(std::move(VarNames), - std::move(Body)); - } - -Now that we can parse and represent the code, we need to support -emission of LLVM IR for it. This code starts out with: - -.. code-block:: c++ - - Value *VarExprAST::codegen() { - std::vector OldBindings; - - Function *TheFunction = Builder.GetInsertBlock()->getParent(); - - // Register all variables and emit their initializer. - for (unsigned i = 0, e = VarNames.size(); i != e; ++i) { - const std::string &VarName = VarNames[i].first; - ExprAST *Init = VarNames[i].second.get(); - -Basically it loops over all the variables, installing them one at a -time. For each variable we put into the symbol table, we remember the -previous value that we replace in OldBindings. - -.. code-block:: c++ - - // Emit the initializer before adding the variable to scope, this prevents - // the initializer from referencing the variable itself, and permits stuff - // like this: - // var a = 1 in - // var a = a in ... # refers to outer 'a'. - Value *InitVal; - if (Init) { - InitVal = Init->codegen(); - if (!InitVal) - return nullptr; - } else { // If not specified, use 0.0. - InitVal = ConstantFP::get(TheContext, APFloat(0.0)); - } - - AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName); - Builder.CreateStore(InitVal, Alloca); - - // Remember the old variable binding so that we can restore the binding when - // we unrecurse. - OldBindings.push_back(NamedValues[VarName]); - - // Remember this binding. - NamedValues[VarName] = Alloca; - } - -There are more comments here than code. The basic idea is that we emit -the initializer, create the alloca, then update the symbol table to -point to it. Once all the variables are installed in the symbol table, -we evaluate the body of the var/in expression: - -.. code-block:: c++ - - // Codegen the body, now that all vars are in scope. - Value *BodyVal = Body->codegen(); - if (!BodyVal) - return nullptr; - -Finally, before returning, we restore the previous variable bindings: - -.. code-block:: c++ - - // Pop all our variables from scope. - for (unsigned i = 0, e = VarNames.size(); i != e; ++i) - NamedValues[VarNames[i].first] = OldBindings[i]; - - // Return the body computation. - return BodyVal; - } - -The end result of all of this is that we get properly scoped variable -definitions, and we even (trivially) allow mutation of them :). - -With this, we completed what we set out to do. Our nice iterative fib -example from the intro compiles and runs just fine. The mem2reg pass -optimizes all of our stack variables into SSA registers, inserting PHI -nodes where needed, and our front-end remains simple: no "iterated -dominance frontier" computation anywhere in sight. - -Full Code Listing -================= - -Here is the complete code listing for our running example, enhanced with -mutable variables and var/in support. To build this example, use: - -.. code-block:: bash - - # Compile - clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core mcjit native` -O3 -o toy - # Run - ./toy - -Here is the code: - -.. literalinclude:: ../../examples/Kaleidoscope/Chapter7/toy.cpp - :language: c++ - -`Next: Compiling to Object Code `_ +===================== +Kaleidoscope Tutorial +===================== +The Kaleidoscope Tutorial has `moved to another location `_ . diff --git a/docs/tutorial/LangImpl08.rst b/docs/tutorial/LangImpl08.rst index 4f28a2adc64..1ff4dc8af44 100644 --- a/docs/tutorial/LangImpl08.rst +++ b/docs/tutorial/LangImpl08.rst @@ -1,218 +1,7 @@ -======================================== - Kaleidoscope: Compiling to Object Code -======================================== +:orphan: -.. contents:: - :local: +===================== +Kaleidoscope Tutorial +===================== -Chapter 8 Introduction -====================== - -Welcome to Chapter 8 of the "`Implementing a language with LLVM -`_" tutorial. This chapter describes how to compile our -language down to object files. - -Choosing a target -================= - -LLVM has native support for cross-compilation. You can compile to the -architecture of your current machine, or just as easily compile for -other architectures. In this tutorial, we'll target the current -machine. - -To specify the architecture that you want to target, we use a string -called a "target triple". This takes the form -``---`` (see the `cross compilation docs -`_). - -As an example, we can see what clang thinks is our current target -triple: - -:: - - $ clang --version | grep Target - Target: x86_64-unknown-linux-gnu - -Running this command may show something different on your machine as -you might be using a different architecture or operating system to me. - -Fortunately, we don't need to hard-code a target triple to target the -current machine. LLVM provides ``sys::getDefaultTargetTriple``, which -returns the target triple of the current machine. - -.. code-block:: c++ - - auto TargetTriple = sys::getDefaultTargetTriple(); - -LLVM doesn't require us to link in all the target -functionality. For example, if we're just using the JIT, we don't need -the assembly printers. Similarly, if we're only targeting certain -architectures, we can only link in the functionality for those -architectures. - -For this example, we'll initialize all the targets for emitting object -code. - -.. code-block:: c++ - - InitializeAllTargetInfos(); - InitializeAllTargets(); - InitializeAllTargetMCs(); - InitializeAllAsmParsers(); - InitializeAllAsmPrinters(); - -We can now use our target triple to get a ``Target``: - -.. code-block:: c++ - - std::string Error; - auto Target = TargetRegistry::lookupTarget(TargetTriple, Error); - - // Print an error and exit if we couldn't find the requested target. - // This generally occurs if we've forgotten to initialise the - // TargetRegistry or we have a bogus target triple. - if (!Target) { - errs() << Error; - return 1; - } - -Target Machine -============== - -We will also need a ``TargetMachine``. This class provides a complete -machine description of the machine we're targeting. If we want to -target a specific feature (such as SSE) or a specific CPU (such as -Intel's Sandylake), we do so now. - -To see which features and CPUs that LLVM knows about, we can use -``llc``. For example, let's look at x86: - -:: - - $ llvm-as < /dev/null | llc -march=x86 -mattr=help - Available CPUs for this target: - - amdfam10 - Select the amdfam10 processor. - athlon - Select the athlon processor. - athlon-4 - Select the athlon-4 processor. - ... - - Available features for this target: - - 16bit-mode - 16-bit mode (i8086). - 32bit-mode - 32-bit mode (80386). - 3dnow - Enable 3DNow! instructions. - 3dnowa - Enable 3DNow! Athlon instructions. - ... - -For our example, we'll use the generic CPU without any additional -features, options or relocation model. - -.. code-block:: c++ - - auto CPU = "generic"; - auto Features = ""; - - TargetOptions opt; - auto RM = Optional(); - auto TargetMachine = Target->createTargetMachine(TargetTriple, CPU, Features, opt, RM); - - -Configuring the Module -====================== - -We're now ready to configure our module, to specify the target and -data layout. This isn't strictly necessary, but the `frontend -performance guide <../Frontend/PerformanceTips.html>`_ recommends -this. Optimizations benefit from knowing about the target and data -layout. - -.. code-block:: c++ - - TheModule->setDataLayout(TargetMachine->createDataLayout()); - TheModule->setTargetTriple(TargetTriple); - -Emit Object Code -================ - -We're ready to emit object code! Let's define where we want to write -our file to: - -.. code-block:: c++ - - auto Filename = "output.o"; - std::error_code EC; - raw_fd_ostream dest(Filename, EC, sys::fs::F_None); - - if (EC) { - errs() << "Could not open file: " << EC.message(); - return 1; - } - -Finally, we define a pass that emits object code, then we run that -pass: - -.. code-block:: c++ - - legacy::PassManager pass; - auto FileType = TargetMachine::CGFT_ObjectFile; - - if (TargetMachine->addPassesToEmitFile(pass, dest, nullptr, FileType)) { - errs() << "TargetMachine can't emit a file of this type"; - return 1; - } - - pass.run(*TheModule); - dest.flush(); - -Putting It All Together -======================= - -Does it work? Let's give it a try. We need to compile our code, but -note that the arguments to ``llvm-config`` are different to the previous chapters. - -:: - - $ clang++ -g -O3 toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs all` -o toy - -Let's run it, and define a simple ``average`` function. Press Ctrl-D -when you're done. - -:: - - $ ./toy - ready> def average(x y) (x + y) * 0.5; - ^D - Wrote output.o - -We have an object file! To test it, let's write a simple program and -link it with our output. Here's the source code: - -.. code-block:: c++ - - #include - - extern "C" { - double average(double, double); - } - - int main() { - std::cout << "average of 3.0 and 4.0: " << average(3.0, 4.0) << std::endl; - } - -We link our program to output.o and check the result is what we -expected: - -:: - - $ clang++ main.cpp output.o -o main - $ ./main - average of 3.0 and 4.0: 3.5 - -Full Code Listing -================= - -.. literalinclude:: ../../examples/Kaleidoscope/Chapter8/toy.cpp - :language: c++ - -`Next: Adding Debug Information `_ +The Kaleidoscope Tutorial has `moved to another location `_ . diff --git a/docs/tutorial/LangImpl09.rst b/docs/tutorial/LangImpl09.rst index d81f9fa0001..1ff4dc8af44 100644 --- a/docs/tutorial/LangImpl09.rst +++ b/docs/tutorial/LangImpl09.rst @@ -1,465 +1,7 @@ -====================================== -Kaleidoscope: Adding Debug Information -====================================== +:orphan: -.. contents:: - :local: - -Chapter 9 Introduction -====================== - -Welcome to Chapter 9 of the "`Implementing a language with -LLVM `_" tutorial. In chapters 1 through 8, we've built a -decent little programming language with functions and variables. -What happens if something goes wrong though, how do you debug your -program? - -Source level debugging uses formatted data that helps a debugger -translate from binary and the state of the machine back to the -source that the programmer wrote. In LLVM we generally use a format -called `DWARF `_. DWARF is a compact encoding -that represents types, source locations, and variable locations. - -The short summary of this chapter is that we'll go through the -various things you have to add to a programming language to -support debug info, and how you translate that into DWARF. - -Caveat: For now we can't debug via the JIT, so we'll need to compile -our program down to something small and standalone. As part of this -we'll make a few modifications to the running of the language and -how programs are compiled. This means that we'll have a source file -with a simple program written in Kaleidoscope rather than the -interactive JIT. It does involve a limitation that we can only -have one "top level" command at a time to reduce the number of -changes necessary. - -Here's the sample program we'll be compiling: - -.. code-block:: python - - def fib(x) - if x < 3 then - 1 - else - fib(x-1)+fib(x-2); - - fib(10) - - -Why is this a hard problem? -=========================== - -Debug information is a hard problem for a few different reasons - mostly -centered around optimized code. First, optimization makes keeping source -locations more difficult. In LLVM IR we keep the original source location -for each IR level instruction on the instruction. Optimization passes -should keep the source locations for newly created instructions, but merged -instructions only get to keep a single location - this can cause jumping -around when stepping through optimized programs. Secondly, optimization -can move variables in ways that are either optimized out, shared in memory -with other variables, or difficult to track. For the purposes of this -tutorial we're going to avoid optimization (as you'll see with one of the -next sets of patches). - -Ahead-of-Time Compilation Mode -============================== - -To highlight only the aspects of adding debug information to a source -language without needing to worry about the complexities of JIT debugging -we're going to make a few changes to Kaleidoscope to support compiling -the IR emitted by the front end into a simple standalone program that -you can execute, debug, and see results. - -First we make our anonymous function that contains our top level -statement be our "main": - -.. code-block:: udiff - - - auto Proto = llvm::make_unique("", std::vector()); - + auto Proto = llvm::make_unique("main", std::vector()); - -just with the simple change of giving it a name. - -Then we're going to remove the command line code wherever it exists: - -.. code-block:: udiff - - @@ -1129,7 +1129,6 @@ static void HandleTopLevelExpression() { - /// top ::= definition | external | expression | ';' - static void MainLoop() { - while (1) { - - fprintf(stderr, "ready> "); - switch (CurTok) { - case tok_eof: - return; - @@ -1184,7 +1183,6 @@ int main() { - BinopPrecedence['*'] = 40; // highest. - - // Prime the first token. - - fprintf(stderr, "ready> "); - getNextToken(); - -Lastly we're going to disable all of the optimization passes and the JIT so -that the only thing that happens after we're done parsing and generating -code is that the LLVM IR goes to standard error: - -.. code-block:: udiff - - @@ -1108,17 +1108,8 @@ static void HandleExtern() { - static void HandleTopLevelExpression() { - // Evaluate a top-level expression into an anonymous function. - if (auto FnAST = ParseTopLevelExpr()) { - - if (auto *FnIR = FnAST->codegen()) { - - // We're just doing this to make sure it executes. - - TheExecutionEngine->finalizeObject(); - - // JIT the function, returning a function pointer. - - void *FPtr = TheExecutionEngine->getPointerToFunction(FnIR); - - - - // Cast it to the right type (takes no arguments, returns a double) so we - - // can call it as a native function. - - double (*FP)() = (double (*)())(intptr_t)FPtr; - - // Ignore the return value for this. - - (void)FP; - + if (!F->codegen()) { - + fprintf(stderr, "Error generating code for top level expr"); - } - } else { - // Skip token for error recovery. - @@ -1439,11 +1459,11 @@ int main() { - // target lays out data structures. - TheModule->setDataLayout(TheExecutionEngine->getDataLayout()); - OurFPM.add(new DataLayoutPass()); - +#if 0 - OurFPM.add(createBasicAliasAnalysisPass()); - // Promote allocas to registers. - OurFPM.add(createPromoteMemoryToRegisterPass()); - @@ -1218,7 +1210,7 @@ int main() { - OurFPM.add(createGVNPass()); - // Simplify the control flow graph (deleting unreachable blocks, etc). - OurFPM.add(createCFGSimplificationPass()); - - - + #endif - OurFPM.doInitialization(); - - // Set the global so the code gen can use this. - -This relatively small set of changes get us to the point that we can compile -our piece of Kaleidoscope language down to an executable program via this -command line: - -.. code-block:: bash - - Kaleidoscope-Ch9 < fib.ks | & clang -x ir - - -which gives an a.out/a.exe in the current working directory. - -Compile Unit -============ - -The top level container for a section of code in DWARF is a compile unit. -This contains the type and function data for an individual translation unit -(read: one file of source code). So the first thing we need to do is -construct one for our fib.ks file. - -DWARF Emission Setup -==================== - -Similar to the ``IRBuilder`` class we have a -`DIBuilder `_ class -that helps in constructing debug metadata for an LLVM IR file. It -corresponds 1:1 similarly to ``IRBuilder`` and LLVM IR, but with nicer names. -Using it does require that you be more familiar with DWARF terminology than -you needed to be with ``IRBuilder`` and ``Instruction`` names, but if you -read through the general documentation on the -`Metadata Format `_ it -should be a little more clear. We'll be using this class to construct all -of our IR level descriptions. Construction for it takes a module so we -need to construct it shortly after we construct our module. We've left it -as a global static variable to make it a bit easier to use. - -Next we're going to create a small container to cache some of our frequent -data. The first will be our compile unit, but we'll also write a bit of -code for our one type since we won't have to worry about multiple typed -expressions: - -.. code-block:: c++ - - static DIBuilder *DBuilder; - - struct DebugInfo { - DICompileUnit *TheCU; - DIType *DblTy; - - DIType *getDoubleTy(); - } KSDbgInfo; - - DIType *DebugInfo::getDoubleTy() { - if (DblTy) - return DblTy; - - DblTy = DBuilder->createBasicType("double", 64, dwarf::DW_ATE_float); - return DblTy; - } - -And then later on in ``main`` when we're constructing our module: - -.. code-block:: c++ - - DBuilder = new DIBuilder(*TheModule); - - KSDbgInfo.TheCU = DBuilder->createCompileUnit( - dwarf::DW_LANG_C, DBuilder->createFile("fib.ks", "."), - "Kaleidoscope Compiler", 0, "", 0); - -There are a couple of things to note here. First, while we're producing a -compile unit for a language called Kaleidoscope we used the language -constant for C. This is because a debugger wouldn't necessarily understand -the calling conventions or default ABI for a language it doesn't recognize -and we follow the C ABI in our LLVM code generation so it's the closest -thing to accurate. This ensures we can actually call functions from the -debugger and have them execute. Secondly, you'll see the "fib.ks" in the -call to ``createCompileUnit``. This is a default hard coded value since -we're using shell redirection to put our source into the Kaleidoscope -compiler. In a usual front end you'd have an input file name and it would -go there. - -One last thing as part of emitting debug information via DIBuilder is that -we need to "finalize" the debug information. The reasons are part of the -underlying API for DIBuilder, but make sure you do this near the end of -main: - -.. code-block:: c++ - - DBuilder->finalize(); - -before you dump out the module. - -Functions -========= - -Now that we have our ``Compile Unit`` and our source locations, we can add -function definitions to the debug info. So in ``PrototypeAST::codegen()`` we -add a few lines of code to describe a context for our subprogram, in this -case the "File", and the actual definition of the function itself. - -So the context: - -.. code-block:: c++ - - DIFile *Unit = DBuilder->createFile(KSDbgInfo.TheCU.getFilename(), - KSDbgInfo.TheCU.getDirectory()); - -giving us an DIFile and asking the ``Compile Unit`` we created above for the -directory and filename where we are currently. Then, for now, we use some -source locations of 0 (since our AST doesn't currently have source location -information) and construct our function definition: - -.. code-block:: c++ - - DIScope *FContext = Unit; - unsigned LineNo = 0; - unsigned ScopeLine = 0; - DISubprogram *SP = DBuilder->createFunction( - FContext, P.getName(), StringRef(), Unit, LineNo, - CreateFunctionType(TheFunction->arg_size(), Unit), - false /* internal linkage */, true /* definition */, ScopeLine, - DINode::FlagPrototyped, false); - TheFunction->setSubprogram(SP); - -and we now have an DISubprogram that contains a reference to all of our -metadata for the function. - -Source Locations -================ - -The most important thing for debug information is accurate source location - -this makes it possible to map your source code back. We have a problem though, -Kaleidoscope really doesn't have any source location information in the lexer -or parser so we'll need to add it. - -.. code-block:: c++ - - struct SourceLocation { - int Line; - int Col; - }; - static SourceLocation CurLoc; - static SourceLocation LexLoc = {1, 0}; - - static int advance() { - int LastChar = getchar(); - - if (LastChar == '\n' || LastChar == '\r') { - LexLoc.Line++; - LexLoc.Col = 0; - } else - LexLoc.Col++; - return LastChar; - } - -In this set of code we've added some functionality on how to keep track of the -line and column of the "source file". As we lex every token we set our current -current "lexical location" to the assorted line and column for the beginning -of the token. We do this by overriding all of the previous calls to -``getchar()`` with our new ``advance()`` that keeps track of the information -and then we have added to all of our AST classes a source location: - -.. code-block:: c++ - - class ExprAST { - SourceLocation Loc; - - public: - ExprAST(SourceLocation Loc = CurLoc) : Loc(Loc) {} - virtual ~ExprAST() {} - virtual Value* codegen() = 0; - int getLine() const { return Loc.Line; } - int getCol() const { return Loc.Col; } - virtual raw_ostream &dump(raw_ostream &out, int ind) { - return out << ':' << getLine() << ':' << getCol() << '\n'; - } - -that we pass down through when we create a new expression: - -.. code-block:: c++ - - LHS = llvm::make_unique(BinLoc, BinOp, std::move(LHS), - std::move(RHS)); - -giving us locations for each of our expressions and variables. - -To make sure that every instruction gets proper source location information, -we have to tell ``Builder`` whenever we're at a new source location. -We use a small helper function for this: - -.. code-block:: c++ - - void DebugInfo::emitLocation(ExprAST *AST) { - DIScope *Scope; - if (LexicalBlocks.empty()) - Scope = TheCU; - else - Scope = LexicalBlocks.back(); - Builder.SetCurrentDebugLocation( - DebugLoc::get(AST->getLine(), AST->getCol(), Scope)); - } - -This both tells the main ``IRBuilder`` where we are, but also what scope -we're in. The scope can either be on compile-unit level or be the nearest -enclosing lexical block like the current function. -To represent this we create a stack of scopes: - -.. code-block:: c++ - - std::vector LexicalBlocks; - -and push the scope (function) to the top of the stack when we start -generating the code for each function: - -.. code-block:: c++ - - KSDbgInfo.LexicalBlocks.push_back(SP); - -Also, we may not forget to pop the scope back off of the scope stack at the -end of the code generation for the function: - -.. code-block:: c++ - - // Pop off the lexical block for the function since we added it - // unconditionally. - KSDbgInfo.LexicalBlocks.pop_back(); - -Then we make sure to emit the location every time we start to generate code -for a new AST object: - -.. code-block:: c++ - - KSDbgInfo.emitLocation(this); - -Variables -========= - -Now that we have functions, we need to be able to print out the variables -we have in scope. Let's get our function arguments set up so we can get -decent backtraces and see how our functions are being called. It isn't -a lot of code, and we generally handle it when we're creating the -argument allocas in ``FunctionAST::codegen``. - -.. code-block:: c++ - - // Record the function arguments in the NamedValues map. - NamedValues.clear(); - unsigned ArgIdx = 0; - for (auto &Arg : TheFunction->args()) { - // Create an alloca for this variable. - AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, Arg.getName()); - - // Create a debug descriptor for the variable. - DILocalVariable *D = DBuilder->createParameterVariable( - SP, Arg.getName(), ++ArgIdx, Unit, LineNo, KSDbgInfo.getDoubleTy(), - true); - - DBuilder->insertDeclare(Alloca, D, DBuilder->createExpression(), - DebugLoc::get(LineNo, 0, SP), - Builder.GetInsertBlock()); - - // Store the initial value into the alloca. - Builder.CreateStore(&Arg, Alloca); - - // Add arguments to variable symbol table. - NamedValues[Arg.getName()] = Alloca; - } - - -Here we're first creating the variable, giving it the scope (``SP``), -the name, source location, type, and since it's an argument, the argument -index. Next, we create an ``lvm.dbg.declare`` call to indicate at the IR -level that we've got a variable in an alloca (and it gives a starting -location for the variable), and setting a source location for the -beginning of the scope on the declare. - -One interesting thing to note at this point is that various debuggers have -assumptions based on how code and debug information was generated for them -in the past. In this case we need to do a little bit of a hack to avoid -generating line information for the function prologue so that the debugger -knows to skip over those instructions when setting a breakpoint. So in -``FunctionAST::CodeGen`` we add some more lines: - -.. code-block:: c++ - - // Unset the location for the prologue emission (leading instructions with no - // location in a function are considered part of the prologue and the debugger - // will run past them when breaking on a function) - KSDbgInfo.emitLocation(nullptr); - -and then emit a new location when we actually start generating code for the -body of the function: - -.. code-block:: c++ - - KSDbgInfo.emitLocation(Body.get()); - -With this we have enough debug information to set breakpoints in functions, -print out argument variables, and call functions. Not too bad for just a -few simple lines of code! - -Full Code Listing -================= - -Here is the complete code listing for our running example, enhanced with -debug information. To build this example, use: - -.. code-block:: bash - - # Compile - clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core mcjit native` -O3 -o toy - # Run - ./toy - -Here is the code: - -.. literalinclude:: ../../examples/Kaleidoscope/Chapter9/toy.cpp - :language: c++ - -`Next: Conclusion and other useful LLVM tidbits `_ +===================== +Kaleidoscope Tutorial +===================== +The Kaleidoscope Tutorial has `moved to another location `_ . diff --git a/docs/tutorial/LangImpl10.rst b/docs/tutorial/LangImpl10.rst index b1d19c2cdd8..1ff4dc8af44 100644 --- a/docs/tutorial/LangImpl10.rst +++ b/docs/tutorial/LangImpl10.rst @@ -1,254 +1,7 @@ -====================================================== -Kaleidoscope: Conclusion and other useful LLVM tidbits -====================================================== +:orphan: -.. contents:: - :local: - -Tutorial Conclusion -=================== - -Welcome to the final chapter of the "`Implementing a language with -LLVM `_" tutorial. In the course of this tutorial, we have -grown our little Kaleidoscope language from being a useless toy, to -being a semi-interesting (but probably still useless) toy. :) - -It is interesting to see how far we've come, and how little code it has -taken. We built the entire lexer, parser, AST, code generator, an -interactive run-loop (with a JIT!), and emitted debug information in -standalone executables - all in under 1000 lines of (non-comment/non-blank) -code. - -Our little language supports a couple of interesting features: it -supports user defined binary and unary operators, it uses JIT -compilation for immediate evaluation, and it supports a few control flow -constructs with SSA construction. - -Part of the idea of this tutorial was to show you how easy and fun it -can be to define, build, and play with languages. Building a compiler -need not be a scary or mystical process! Now that you've seen some of -the basics, I strongly encourage you to take the code and hack on it. -For example, try adding: - -- **global variables** - While global variables have questional value - in modern software engineering, they are often useful when putting - together quick little hacks like the Kaleidoscope compiler itself. - Fortunately, our current setup makes it very easy to add global - variables: just have value lookup check to see if an unresolved - variable is in the global variable symbol table before rejecting it. - To create a new global variable, make an instance of the LLVM - ``GlobalVariable`` class. -- **typed variables** - Kaleidoscope currently only supports variables - of type double. This gives the language a very nice elegance, because - only supporting one type means that you never have to specify types. - Different languages have different ways of handling this. The easiest - way is to require the user to specify types for every variable - definition, and record the type of the variable in the symbol table - along with its Value\*. -- **arrays, structs, vectors, etc** - Once you add types, you can start - extending the type system in all sorts of interesting ways. Simple - arrays are very easy and are quite useful for many different - applications. Adding them is mostly an exercise in learning how the - LLVM `getelementptr <../LangRef.html#getelementptr-instruction>`_ instruction - works: it is so nifty/unconventional, it `has its own - FAQ <../GetElementPtr.html>`_! -- **standard runtime** - Our current language allows the user to access - arbitrary external functions, and we use it for things like "printd" - and "putchard". As you extend the language to add higher-level - constructs, often these constructs make the most sense if they are - lowered to calls into a language-supplied runtime. For example, if - you add hash tables to the language, it would probably make sense to - add the routines to a runtime, instead of inlining them all the way. -- **memory management** - Currently we can only access the stack in - Kaleidoscope. It would also be useful to be able to allocate heap - memory, either with calls to the standard libc malloc/free interface - or with a garbage collector. If you would like to use garbage - collection, note that LLVM fully supports `Accurate Garbage - Collection <../GarbageCollection.html>`_ including algorithms that - move objects and need to scan/update the stack. -- **exception handling support** - LLVM supports generation of `zero - cost exceptions <../ExceptionHandling.html>`_ which interoperate with - code compiled in other languages. You could also generate code by - implicitly making every function return an error value and checking - it. You could also make explicit use of setjmp/longjmp. There are - many different ways to go here. -- **object orientation, generics, database access, complex numbers, - geometric programming, ...** - Really, there is no end of crazy - features that you can add to the language. -- **unusual domains** - We've been talking about applying LLVM to a - domain that many people are interested in: building a compiler for a - specific language. However, there are many other domains that can use - compiler technology that are not typically considered. For example, - LLVM has been used to implement OpenGL graphics acceleration, - translate C++ code to ActionScript, and many other cute and clever - things. Maybe you will be the first to JIT compile a regular - expression interpreter into native code with LLVM? - -Have fun - try doing something crazy and unusual. Building a language -like everyone else always has, is much less fun than trying something a -little crazy or off the wall and seeing how it turns out. If you get -stuck or want to talk about it, feel free to email the `llvm-dev mailing -list `_: it has lots -of people who are interested in languages and are often willing to help -out. - -Before we end this tutorial, I want to talk about some "tips and tricks" -for generating LLVM IR. These are some of the more subtle things that -may not be obvious, but are very useful if you want to take advantage of -LLVM's capabilities. - -Properties of the LLVM IR -========================= - -We have a couple of common questions about code in the LLVM IR form - -let's just get these out of the way right now, shall we? - -Target Independence -------------------- - -Kaleidoscope is an example of a "portable language": any program written -in Kaleidoscope will work the same way on any target that it runs on. -Many other languages have this property, e.g. lisp, java, haskell, -javascript, python, etc (note that while these languages are portable, -not all their libraries are). - -One nice aspect of LLVM is that it is often capable of preserving target -independence in the IR: you can take the LLVM IR for a -Kaleidoscope-compiled program and run it on any target that LLVM -supports, even emitting C code and compiling that on targets that LLVM -doesn't support natively. You can trivially tell that the Kaleidoscope -compiler generates target-independent code because it never queries for -any target-specific information when generating code. - -The fact that LLVM provides a compact, target-independent, -representation for code gets a lot of people excited. Unfortunately, -these people are usually thinking about C or a language from the C -family when they are asking questions about language portability. I say -"unfortunately", because there is really no way to make (fully general) -C code portable, other than shipping the source code around (and of -course, C source code is not actually portable in general either - ever -port a really old application from 32- to 64-bits?). - -The problem with C (again, in its full generality) is that it is heavily -laden with target specific assumptions. As one simple example, the -preprocessor often destructively removes target-independence from the -code when it processes the input text: - -.. code-block:: c - - #ifdef __i386__ - int X = 1; - #else - int X = 42; - #endif - -While it is possible to engineer more and more complex solutions to -problems like this, it cannot be solved in full generality in a way that -is better than shipping the actual source code. - -That said, there are interesting subsets of C that can be made portable. -If you are willing to fix primitive types to a fixed size (say int = -32-bits, and long = 64-bits), don't care about ABI compatibility with -existing binaries, and are willing to give up some other minor features, -you can have portable code. This can make sense for specialized domains -such as an in-kernel language. - -Safety Guarantees ------------------ - -Many of the languages above are also "safe" languages: it is impossible -for a program written in Java to corrupt its address space and crash the -process (assuming the JVM has no bugs). Safety is an interesting -property that requires a combination of language design, runtime -support, and often operating system support. - -It is certainly possible to implement a safe language in LLVM, but LLVM -IR does not itself guarantee safety. The LLVM IR allows unsafe pointer -casts, use after free bugs, buffer over-runs, and a variety of other -problems. Safety needs to be implemented as a layer on top of LLVM and, -conveniently, several groups have investigated this. Ask on the `llvm-dev -mailing list `_ if -you are interested in more details. - -Language-Specific Optimizations -------------------------------- - -One thing about LLVM that turns off many people is that it does not -solve all the world's problems in one system. One specific -complaint is that people perceive LLVM as being incapable of performing -high-level language-specific optimization: LLVM "loses too much -information". Here are a few observations about this: - -First, you're right that LLVM does lose information. For example, as of -this writing, there is no way to distinguish in the LLVM IR whether an -SSA-value came from a C "int" or a C "long" on an ILP32 machine (other -than debug info). Both get compiled down to an 'i32' value and the -information about what it came from is lost. The more general issue -here, is that the LLVM type system uses "structural equivalence" instead -of "name equivalence". Another place this surprises people is if you -have two types in a high-level language that have the same structure -(e.g. two different structs that have a single int field): these types -will compile down into a single LLVM type and it will be impossible to -tell what it came from. - -Second, while LLVM does lose information, LLVM is not a fixed target: we -continue to enhance and improve it in many different ways. In addition -to adding new features (LLVM did not always support exceptions or debug -info), we also extend the IR to capture important information for -optimization (e.g. whether an argument is sign or zero extended, -information about pointers aliasing, etc). Many of the enhancements are -user-driven: people want LLVM to include some specific feature, so they -go ahead and extend it. - -Third, it is *possible and easy* to add language-specific optimizations, -and you have a number of choices in how to do it. As one trivial -example, it is easy to add language-specific optimization passes that -"know" things about code compiled for a language. In the case of the C -family, there is an optimization pass that "knows" about the standard C -library functions. If you call "exit(0)" in main(), it knows that it is -safe to optimize that into "return 0;" because C specifies what the -'exit' function does. - -In addition to simple library knowledge, it is possible to embed a -variety of other language-specific information into the LLVM IR. If you -have a specific need and run into a wall, please bring the topic up on -the llvm-dev list. At the very worst, you can always treat LLVM as if it -were a "dumb code generator" and implement the high-level optimizations -you desire in your front-end, on the language-specific AST. - -Tips and Tricks -=============== - -There is a variety of useful tips and tricks that you come to know after -working on/with LLVM that aren't obvious at first glance. Instead of -letting everyone rediscover them, this section talks about some of these -issues. - -Implementing portable offsetof/sizeof -------------------------------------- - -One interesting thing that comes up, if you are trying to keep the code -generated by your compiler "target independent", is that you often need -to know the size of some LLVM type or the offset of some field in an -llvm structure. For example, you might need to pass the size of a type -into a function that allocates memory. - -Unfortunately, this can vary widely across targets: for example the -width of a pointer is trivially target-specific. However, there is a -`clever way to use the getelementptr -instruction `_ -that allows you to compute this in a portable way. - -Garbage Collected Stack Frames ------------------------------- - -Some languages want to explicitly manage their stack frames, often so -that they are garbage collected or to allow easy implementation of -closures. There are often better ways to implement these features than -explicit stack frames, but `LLVM does support -them, `_ -if you want. It requires your front-end to convert the code into -`Continuation Passing -Style `_ and -the use of tail calls (which LLVM also supports). +===================== +Kaleidoscope Tutorial +===================== +The Kaleidoscope Tutorial has `moved to another location `_ . diff --git a/docs/tutorial/MyFirstLanguageFrontend/index.rst b/docs/tutorial/MyFirstLanguageFrontend/index.rst index 2b589d19180..dcf9d5814fb 100644 --- a/docs/tutorial/MyFirstLanguageFrontend/index.rst +++ b/docs/tutorial/MyFirstLanguageFrontend/index.rst @@ -1,5 +1,3 @@ -:orphan: - ============================================= My First Language Frontend with LLVM Tutorial ============================================= diff --git a/docs/tutorial/index.rst b/docs/tutorial/index.rst index 844623ef215..03a2e5d53b4 100644 --- a/docs/tutorial/index.rst +++ b/docs/tutorial/index.rst @@ -10,7 +10,7 @@ Kaleidoscope: Implementing a Language with LLVM :glob: :numbered: -#. `My First Language Frontend with LLVM Tutorial `_ +#. `My First Language Frontend with LLVM Tutorial `_ This is the "Kaleidoscope" Language tutorial, showing how to implement a simple language using LLVM components in C++.