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1//===----------------------------------------------------------------------===//2// C Language Family Front-end3//===----------------------------------------------------------------------===//4 Chris Lattner5 6I. Introduction:7 8 clang: noun9 1. A loud, resonant, metallic sound.10 2. The strident call of a crane or goose.11 3. C-language family front-end toolkit.12 13 The world needs better compiler tools, tools which are built as libraries. This14 design point allows reuse of the tools in new and novel ways. However, building15 the tools as libraries isn't enough: they must have clean APIs, be as16 decoupled from each other as possible, and be easy to modify/extend. This17 requires clean layering, decent design, and avoiding tying the libraries to a18 specific use. Oh yeah, did I mention that we want the resultant libraries to19 be as fast as possible? :)20 21 This front-end is built as a component of the LLVM toolkit that can be used22 with the LLVM backend or independently of it. In this spirit, the API has been23 carefully designed as the following components:24 25 libsupport - Basic support library, reused from LLVM.26 27 libsystem - System abstraction library, reused from LLVM.28 29 libbasic - Diagnostics, SourceLocations, SourceBuffer abstraction,30 file system caching for input source files. This depends on31 libsupport and libsystem.32 33 libast - Provides classes to represent the C AST, the C type system,34 builtin functions, and various helpers for analyzing and35 manipulating the AST (visitors, pretty printers, etc). This36 library depends on libbasic.37 38 39 liblex - C/C++/ObjC lexing and preprocessing, identifier hash table,40 pragma handling, tokens, and macros. This depends on libbasic.41 42 libparse - C (for now) parsing and local semantic analysis. This library43 invokes coarse-grained 'Actions' provided by the client to do44 stuff (e.g. libsema builds ASTs). This depends on liblex.45 46 libsema - Provides a set of parser actions to build a standardized AST47 for programs. AST's are 'streamed' out a top-level declaration48 at a time, allowing clients to use decl-at-a-time processing,49 build up entire translation units, or even build 'whole50 program' ASTs depending on how they use the APIs. This depends51 on libast and libparse.52 53 librewrite - Fast, scalable rewriting of source code. This operates on54 the raw syntactic text of source code, allowing a client55 to insert and delete text in very large source files using56 the same source location information embedded in ASTs. This57 is intended to be a low-level API that is useful for58 higher-level clients and libraries such as code refactoring.59 60 libanalysis - Source-level dataflow analysis useful for performing analyses61 such as computing live variables. It also includes a62 path-sensitive "graph-reachability" engine for writing63 analyses that reason about different possible paths of64 execution through source code. This is currently being65 employed to write a set of checks for finding bugs in software.66 67 libcodegen - Lower the AST to LLVM IR for optimization & codegen. Depends68 on libast.69 70 clang - An example driver, client of the libraries at various levels.71 This depends on all these libraries, and on LLVM VMCore.72 73 This front-end has been intentionally built as a DAG of libraries, making it74 easy to reuse individual parts or replace pieces if desired. For example, to75 build a preprocessor, you take the Basic and Lexer libraries. If you want an76 indexer, you take those plus the Parser library and provide some actions for77 indexing. If you want a refactoring, static analysis, or source-to-source78 compiler tool, it makes sense to take those plus the AST building and semantic79 analyzer library. Finally, if you want to use this with the LLVM backend,80 you'd take these components plus the AST to LLVM lowering code.81 82 In the future I hope this toolkit will grow to include new and interesting83 components, including a C++ front-end, ObjC support, and a whole lot of other84 things.85 86 Finally, it should be pointed out that the goal here is to build something that87 is high-quality and industrial-strength: all the obnoxious features of the C88 family must be correctly supported (trigraphs, preprocessor arcana, K&R-style89 prototypes, GCC/MS extensions, etc). It cannot be used if it is not 'real'.90 91 92II. Usage of clang driver:93 94 * Basic Command-Line Options:95 - Help: clang --help96 - Standard GCC options accepted: -E, -I*, -i*, -pedantic, -std=c90, etc.97 - To make diagnostics more gcc-like: -fno-caret-diagnostics -fno-show-column98 - Enable metric printing: -stats99 100 * -fsyntax-only is currently the default mode.101 102 * -E mode works the same way as GCC.103 104 * -Eonly mode does all preprocessing, but does not print the output,105 useful for timing the preprocessor.106 107 * -fsyntax-only is currently partially implemented, lacking some108 semantic analysis (some errors and warnings are not produced).109 110 * -parse-noop parses code without building an AST. This is useful111 for timing the cost of the parser without including AST building112 time.113 114 * -parse-ast builds ASTs, but doesn't print them. This is most115 useful for timing AST building vs -parse-noop.116 117 * -parse-ast-print pretty prints most expression and statements nodes.118 119 * -parse-ast-check checks that diagnostic messages that are expected120 are reported and that those which are reported are expected.121 122 * -dump-cfg builds ASTs and then CFGs. CFGs are then pretty-printed.123 124 * -view-cfg builds ASTs and then CFGs. CFGs are then visualized by125 invoking Graphviz.126 127 For more information on getting Graphviz to work with clang/LLVM,128 see: https://llvm.org/docs/ProgrammersManual.html#ViewGraph129 130 131III. Current advantages over GCC:132 133 * Column numbers are fully tracked (no 256 col limit, no GCC-style pruning).134 * All diagnostics have column numbers, includes 'caret diagnostics', and they135 highlight regions of interesting code (e.g. the LHS and RHS of a binop).136 * Full diagnostic customization by client (can format diagnostics however they137 like, e.g. in an IDE or refactoring tool) through DiagnosticClient interface.138 * Built as a framework, can be reused by multiple tools.139 * All languages supported linked into same library (no cc1,cc1obj, ...).140 * mmap's code in read-only, does not dirty the pages like GCC (mem footprint).141 * LLVM License, can be linked into non-GPL projects.142 * Full diagnostic control, per diagnostic. Diagnostics are identified by ID.143 * Significantly faster than GCC at semantic analysis, parsing, preprocessing144 and lexing.145 * Defers exposing platform-specific stuff to as late as possible, tracks use of146 platform-specific features (e.g. #ifdef PPC) to allow 'portable bytecodes'.147 * The lexer doesn't rely on the "lexer hack": it has no notion of scope and148 does not categorize identifiers as types or variables -- this is up to the149 parser to decide.150 151Potential Future Features:152 153 * Fine grained diag control within the source (#pragma enable/disable warning).154 * Better token tracking within macros? (Token came from this line, which is155 a macro argument instantiated here, recursively instantiated here).156 * Fast #import with a module system.157 * Dependency tracking: change to header file doesn't recompile every function158 that texually depends on it: recompile only those functions that need it.159 This is aka 'incremental parsing'.160 161 162IV. Missing Functionality / Improvements163 164Lexer:165 * Source character mapping. GCC supports ASCII and UTF-8.166 See GCC options: -ftarget-charset and -ftarget-wide-charset.167 * Universal character support. Experimental in GCC, enabled with168 -fextended-identifiers.169 * -fpreprocessed mode.170 171Preprocessor:172 * #assert/#unassert173 * MSExtension: "L#param" stringizes to a wide string literal.174 * Add support for -M*175 176Traditional Preprocessor:177 * Currently, we have none. :)178 179