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