mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-11-01 08:23:21 +01:00
366 lines
17 KiB
HTML
366 lines
17 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
|
|
"http://www.w3.org/TR/html4/strict.dtd">
|
|
|
|
<html>
|
|
<head>
|
|
<title>Kaleidoscope: Conclusion and other useful LLVM tidbits</title>
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|
<meta name="author" content="Chris Lattner">
|
|
<link rel="stylesheet" href="../llvm.css" type="text/css">
|
|
</head>
|
|
|
|
<body>
|
|
|
|
<div class="doc_title">Kaleidoscope: Conclusion and other useful LLVM
|
|
tidbits</div>
|
|
|
|
<ul>
|
|
<li><a href="index.html">Up to Tutorial Index</a></li>
|
|
<li>Chapter 8
|
|
<ol>
|
|
<li><a href="#conclusion">Tutorial Conclusion</a></li>
|
|
<li><a href="#llvmirproperties">Properties of LLVM IR</a>
|
|
<ul>
|
|
<li><a href="#targetindep">Target Independence</a></li>
|
|
<li><a href="#safety">Safety Guarantees</a></li>
|
|
<li><a href="#langspecific">Language-Specific Optimizations</a></li>
|
|
</ul>
|
|
</li>
|
|
<li><a href="#tipsandtricks">Tips and Tricks</a>
|
|
<ul>
|
|
<li><a href="#offsetofsizeof">Implementing portable
|
|
offsetof/sizeof</a></li>
|
|
<li><a href="#gcstack">Garbage Collected Stack Frames</a></li>
|
|
</ul>
|
|
</li>
|
|
</ol>
|
|
</li>
|
|
</ul>
|
|
|
|
|
|
<div class="doc_author">
|
|
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section"><a name="conclusion">Tutorial Conclusion</a></div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>Welcome to the the final chapter of the "<a href="index.html">Implementing a
|
|
language with LLVM</a>" 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. :)</p>
|
|
|
|
<p>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, and an
|
|
interactive run-loop (with a JIT!) by-hand in under 700 lines of
|
|
(non-comment/non-blank) code.</p>
|
|
|
|
<p>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.
|
|
</p>
|
|
|
|
<p>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:</p>
|
|
|
|
<ul>
|
|
<li><b>global variables</b> - 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
|
|
<tt>GlobalVariable</tt> class.</li>
|
|
|
|
<li><b>typed variables</b> - 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*.</li>
|
|
|
|
<li><b>arrays, structs, vectors, etc</b> - 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 <a
|
|
href="../LangRef.html#i_getelementptr">getelementptr</a> instruction works: it
|
|
is so nifty/unconventional, it <a
|
|
href="../GetElementPtr.html">has its own FAQ</a>! If you add support
|
|
for recursive types (e.g. linked lists), make sure to read the <a
|
|
href="../ProgrammersManual.html#TypeResolve">section in the LLVM
|
|
Programmer's Manual</a> that describes how to construct them.</li>
|
|
|
|
<li><b>standard runtime</b> - 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.</li>
|
|
|
|
<li><b>memory management</b> - 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 <a href="../GarbageCollection.html">Accurate Garbage Collection</a>
|
|
including algorithms that move objects and need to scan/update the stack.</li>
|
|
|
|
<li><b>debugger support</b> - LLVM supports generation of <a
|
|
href="../SourceLevelDebugging.html">DWARF Debug info</a> which is understood by
|
|
common debuggers like GDB. Adding support for debug info is fairly
|
|
straightforward. The best way to understand it is to compile some C/C++ code
|
|
with "<tt>llvm-gcc -g -O0</tt>" and taking a look at what it produces.</li>
|
|
|
|
<li><b>exception handling support</b> - LLVM supports generation of <a
|
|
href="../ExceptionHandling.html">zero cost exceptions</a> 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.</li>
|
|
|
|
<li><b>object orientation, generics, database access, complex numbers,
|
|
geometric programming, ...</b> - Really, there is
|
|
no end of crazy features that you can add to the language.</li>
|
|
|
|
<li><b>unusual domains</b> - 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?</li>
|
|
|
|
</ul>
|
|
|
|
<p>
|
|
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 <a
|
|
href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing
|
|
list</a>: it has lots of people who are interested in languages and are often
|
|
willing to help out.
|
|
</p>
|
|
|
|
<p>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.</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section"><a name="llvmirproperties">Properties of the LLVM
|
|
IR</a></div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>We have a couple common questions about code in the LLVM IR form - lets just
|
|
get these out of the way right now, shall we?</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsubsection"><a name="targetindep">Target
|
|
Independence</a></div>
|
|
<!-- ======================================================================= -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>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).</p>
|
|
|
|
<p>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.</p>
|
|
|
|
<p>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?).</p>
|
|
|
|
<p>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:</p>
|
|
|
|
<div class="doc_code">
|
|
<pre>
|
|
#ifdef __i386__
|
|
int X = 1;
|
|
#else
|
|
int X = 42;
|
|
#endif
|
|
</pre>
|
|
</div>
|
|
|
|
<p>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.</p>
|
|
|
|
<p>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.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsubsection"><a name="safety">Safety Guarantees</a></div>
|
|
<!-- ======================================================================= -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>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.</p>
|
|
|
|
<p>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 <a
|
|
href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">llvmdev mailing
|
|
list</a> if you are interested in more details.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsubsection"><a name="langspecific">Language-Specific
|
|
Optimizations</a></div>
|
|
<!-- ======================================================================= -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>One thing about LLVM that turns off many people is that it does not solve all
|
|
the world's problems in one system (sorry 'world hunger', someone else will have
|
|
to solve you some other day). One specific complaint is that people perceive
|
|
LLVM as being incapable of performing high-level language-specific optimization:
|
|
LLVM "loses too much information".</p>
|
|
|
|
<p>Unfortunately, this is really not the place to give you a full and unified
|
|
version of "Chris Lattner's theory of compiler design". Instead, I'll make a
|
|
few observations:</p>
|
|
|
|
<p>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.</p>
|
|
|
|
<p>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.</p>
|
|
|
|
<p>Third, it is <em>possible and easy</em> 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.</p>
|
|
|
|
<p>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 llvmdev 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.
|
|
</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section"><a name="tipsandtricks">Tips and Tricks</a></div>
|
|
<!-- *********************************************************************** -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>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.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsubsection"><a name="offsetofsizeof">Implementing portable
|
|
offsetof/sizeof</a></div>
|
|
<!-- ======================================================================= -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>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.</p>
|
|
|
|
<p>Unfortunately, this can vary widely across targets: for example the width of
|
|
a pointer is trivially target-specific. However, there is a <a
|
|
href="http://nondot.org/sabre/LLVMNotes/SizeOf-OffsetOf-VariableSizedStructs.txt">clever
|
|
way to use the getelementptr instruction</a> that allows you to compute this
|
|
in a portable way.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<div class="doc_subsubsection"><a name="gcstack">Garbage Collected
|
|
Stack Frames</a></div>
|
|
<!-- ======================================================================= -->
|
|
|
|
<div class="doc_text">
|
|
|
|
<p>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 <a
|
|
href="http://nondot.org/sabre/LLVMNotes/ExplicitlyManagedStackFrames.txt">LLVM
|
|
does support them,</a> if you want. It requires your front-end to convert the
|
|
code into <a
|
|
href="http://en.wikipedia.org/wiki/Continuation-passing_style">Continuation
|
|
Passing Style</a> and the use of tail calls (which LLVM also supports).</p>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<hr>
|
|
<address>
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
|
|
<a href="http://validator.w3.org/check/referer"><img
|
|
src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
|
|
|
|
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
|
|
<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date$
|
|
</address>
|
|
</body>
|
|
</html>
|