mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-11-24 03:33:20 +01:00
179 lines
8.2 KiB
Plaintext
179 lines
8.2 KiB
Plaintext
//===----------------------------------------------------------------------===//
|
|
// C Language Family Front-end
|
|
//===----------------------------------------------------------------------===//
|
|
Chris Lattner
|
|
|
|
I. Introduction:
|
|
|
|
clang: noun
|
|
1. A loud, resonant, metallic sound.
|
|
2. The strident call of a crane or goose.
|
|
3. C-language family front-end toolkit.
|
|
|
|
The world needs better compiler tools, tools which are built as libraries. This
|
|
design point allows reuse of the tools in new and novel ways. However, building
|
|
the tools as libraries isn't enough: they must have clean APIs, be as
|
|
decoupled from each other as possible, and be easy to modify/extend. This
|
|
requires clean layering, decent design, and avoiding tying the libraries to a
|
|
specific use. Oh yeah, did I mention that we want the resultant libraries to
|
|
be as fast as possible? :)
|
|
|
|
This front-end is built as a component of the LLVM toolkit that can be used
|
|
with the LLVM backend or independently of it. In this spirit, the API has been
|
|
carefully designed as the following components:
|
|
|
|
libsupport - Basic support library, reused from LLVM.
|
|
|
|
libsystem - System abstraction library, reused from LLVM.
|
|
|
|
libbasic - Diagnostics, SourceLocations, SourceBuffer abstraction,
|
|
file system caching for input source files. This depends on
|
|
libsupport and libsystem.
|
|
|
|
libast - Provides classes to represent the C AST, the C type system,
|
|
builtin functions, and various helpers for analyzing and
|
|
manipulating the AST (visitors, pretty printers, etc). This
|
|
library depends on libbasic.
|
|
|
|
|
|
liblex - C/C++/ObjC lexing and preprocessing, identifier hash table,
|
|
pragma handling, tokens, and macros. This depends on libbasic.
|
|
|
|
libparse - C (for now) parsing and local semantic analysis. This library
|
|
invokes coarse-grained 'Actions' provided by the client to do
|
|
stuff (e.g. libsema builds ASTs). This depends on liblex.
|
|
|
|
libsema - Provides a set of parser actions to build a standardized AST
|
|
for programs. AST's are 'streamed' out a top-level declaration
|
|
at a time, allowing clients to use decl-at-a-time processing,
|
|
build up entire translation units, or even build 'whole
|
|
program' ASTs depending on how they use the APIs. This depends
|
|
on libast and libparse.
|
|
|
|
librewrite - Fast, scalable rewriting of source code. This operates on
|
|
the raw syntactic text of source code, allowing a client
|
|
to insert and delete text in very large source files using
|
|
the same source location information embedded in ASTs. This
|
|
is intended to be a low-level API that is useful for
|
|
higher-level clients and libraries such as code refactoring.
|
|
|
|
libanalysis - Source-level dataflow analysis useful for performing analyses
|
|
such as computing live variables. It also includes a
|
|
path-sensitive "graph-reachability" engine for writing
|
|
analyses that reason about different possible paths of
|
|
execution through source code. This is currently being
|
|
employed to write a set of checks for finding bugs in software.
|
|
|
|
libcodegen - Lower the AST to LLVM IR for optimization & codegen. Depends
|
|
on libast.
|
|
|
|
clang - An example driver, client of the libraries at various levels.
|
|
This depends on all these libraries, and on LLVM VMCore.
|
|
|
|
This front-end has been intentionally built as a DAG of libraries, making it
|
|
easy to reuse individual parts or replace pieces if desired. For example, to
|
|
build a preprocessor, you take the Basic and Lexer libraries. If you want an
|
|
indexer, you take those plus the Parser library and provide some actions for
|
|
indexing. If you want a refactoring, static analysis, or source-to-source
|
|
compiler tool, it makes sense to take those plus the AST building and semantic
|
|
analyzer library. Finally, if you want to use this with the LLVM backend,
|
|
you'd take these components plus the AST to LLVM lowering code.
|
|
|
|
In the future I hope this toolkit will grow to include new and interesting
|
|
components, including a C++ front-end, ObjC support, and a whole lot of other
|
|
things.
|
|
|
|
Finally, it should be pointed out that the goal here is to build something that
|
|
is high-quality and industrial-strength: all the obnoxious features of the C
|
|
family must be correctly supported (trigraphs, preprocessor arcana, K&R-style
|
|
prototypes, GCC/MS extensions, etc). It cannot be used if it is not 'real'.
|
|
|
|
|
|
II. Usage of clang driver:
|
|
|
|
* Basic Command-Line Options:
|
|
- Help: clang --help
|
|
- Standard GCC options accepted: -E, -I*, -i*, -pedantic, -std=c90, etc.
|
|
- To make diagnostics more gcc-like: -fno-caret-diagnostics -fno-show-column
|
|
- Enable metric printing: -stats
|
|
|
|
* -fsyntax-only is currently the default mode.
|
|
|
|
* -E mode works the same way as GCC.
|
|
|
|
* -Eonly mode does all preprocessing, but does not print the output,
|
|
useful for timing the preprocessor.
|
|
|
|
* -fsyntax-only is currently partially implemented, lacking some
|
|
semantic analysis (some errors and warnings are not produced).
|
|
|
|
* -parse-noop parses code without building an AST. This is useful
|
|
for timing the cost of the parser without including AST building
|
|
time.
|
|
|
|
* -parse-ast builds ASTs, but doesn't print them. This is most
|
|
useful for timing AST building vs -parse-noop.
|
|
|
|
* -parse-ast-print pretty prints most expression and statements nodes.
|
|
|
|
* -parse-ast-check checks that diagnostic messages that are expected
|
|
are reported and that those which are reported are expected.
|
|
|
|
* -dump-cfg builds ASTs and then CFGs. CFGs are then pretty-printed.
|
|
|
|
* -view-cfg builds ASTs and then CFGs. CFGs are then visualized by
|
|
invoking Graphviz.
|
|
|
|
For more information on getting Graphviz to work with clang/LLVM,
|
|
see: http://llvm.org/docs/ProgrammersManual.html#ViewGraph
|
|
|
|
|
|
III. Current advantages over GCC:
|
|
|
|
* Column numbers are fully tracked (no 256 col limit, no GCC-style pruning).
|
|
* All diagnostics have column numbers, includes 'caret diagnostics', and they
|
|
highlight regions of interesting code (e.g. the LHS and RHS of a binop).
|
|
* Full diagnostic customization by client (can format diagnostics however they
|
|
like, e.g. in an IDE or refactoring tool) through DiagnosticClient interface.
|
|
* Built as a framework, can be reused by multiple tools.
|
|
* All languages supported linked into same library (no cc1,cc1obj, ...).
|
|
* mmap's code in read-only, does not dirty the pages like GCC (mem footprint).
|
|
* LLVM License, can be linked into non-GPL projects.
|
|
* Full diagnostic control, per diagnostic. Diagnostics are identified by ID.
|
|
* Significantly faster than GCC at semantic analysis, parsing, preprocessing
|
|
and lexing.
|
|
* Defers exposing platform-specific stuff to as late as possible, tracks use of
|
|
platform-specific features (e.g. #ifdef PPC) to allow 'portable bytecodes'.
|
|
* The lexer doesn't rely on the "lexer hack": it has no notion of scope and
|
|
does not categorize identifiers as types or variables -- this is up to the
|
|
parser to decide.
|
|
|
|
Potential Future Features:
|
|
|
|
* Fine grained diag control within the source (#pragma enable/disable warning).
|
|
* Better token tracking within macros? (Token came from this line, which is
|
|
a macro argument instantiated here, recursively instantiated here).
|
|
* Fast #import with a module system.
|
|
* Dependency tracking: change to header file doesn't recompile every function
|
|
that texually depends on it: recompile only those functions that need it.
|
|
This is aka 'incremental parsing'.
|
|
|
|
|
|
IV. Missing Functionality / Improvements
|
|
|
|
Lexer:
|
|
* Source character mapping. GCC supports ASCII and UTF-8.
|
|
See GCC options: -ftarget-charset and -ftarget-wide-charset.
|
|
* Universal character support. Experimental in GCC, enabled with
|
|
-fextended-identifiers.
|
|
* -fpreprocessed mode.
|
|
|
|
Preprocessor:
|
|
* #assert/#unassert
|
|
* MSExtension: "L#param" stringizes to a wide string literal.
|
|
* Add support for -M*
|
|
|
|
Traditional Preprocessor:
|
|
* Currently, we have none. :)
|
|
|