1 //===----------------------------------------------------------------------===// 2 // C Language Family Front-end 3 //===----------------------------------------------------------------------===// 4 Chris Lattner 5 6 I. Introduction: 7 8 clang: noun 9 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. This 14 design point allows reuse of the tools in new and novel ways. However, building 15 the tools as libraries isn't enough: they must have clean APIs, be as 16 decoupled from each other as possible, and be easy to modify/extend. This 17 requires clean layering, decent design, and avoiding tying the libraries to a 18 specific use. Oh yeah, did I mention that we want the resultant libraries to 19 be as fast as possible? :) 20 21 This front-end is built as a component of the LLVM toolkit that can be used 22 with the LLVM backend or independently of it. In this spirit, the API has been 23 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 on 31 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 and 35 manipulating the AST (visitors, pretty printers, etc). This 36 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 library 43 invokes coarse-grained 'Actions' provided by the client to do 44 stuff (e.g. libsema builds ASTs). This depends on liblex. 45 46 libsema - Provides a set of parser actions to build a standardized AST 47 for programs. AST's are 'streamed' out a top-level declaration 48 at a time, allowing clients to use decl-at-a-time processing, 49 build up entire translation units, or even build 'whole 50 program' ASTs depending on how they use the APIs. This depends 51 on libast and libparse. 52 53 librewrite - Fast, scalable rewriting of source code. This operates on 54 the raw syntactic text of source code, allowing a client 55 to insert and delete text in very large source files using 56 the same source location information embedded in ASTs. This 57 is intended to be a low-level API that is useful for 58 higher-level clients and libraries such as code refactoring. 59 60 libanalysis - Source-level dataflow analysis useful for performing analyses 61 such as computing live variables. It also includes a 62 path-sensitive "graph-reachability" engine for writing 63 analyses that reason about different possible paths of 64 execution through source code. This is currently being 65 employed to write a set of checks for finding bugs in software. 66 67 libcodegen - Lower the AST to LLVM IR for optimization & codegen. Depends 68 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 it 74 easy to reuse individual parts or replace pieces if desired. For example, to 75 build a preprocessor, you take the Basic and Lexer libraries. If you want an 76 indexer, you take those plus the Parser library and provide some actions for 77 indexing. If you want a refactoring, static analysis, or source-to-source 78 compiler tool, it makes sense to take those plus the AST building and semantic 79 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 interesting 83 components, including a C++ front-end, ObjC support, and a whole lot of other 84 things. 85 86 Finally, it should be pointed out that the goal here is to build something that 87 is high-quality and industrial-strength: all the obnoxious features of the C 88 family must be correctly supported (trigraphs, preprocessor arcana, K&R-style 89 prototypes, GCC/MS extensions, etc). It cannot be used if it is not 'real'. 90 91 92 II. Usage of clang driver: 93 94 * Basic Command-Line Options: 95 - Help: clang --help 96 - Standard GCC options accepted: -E, -I*, -i*, -pedantic, -std=c90, etc. 97 - To make diagnostics more gcc-like: -fno-caret-diagnostics -fno-show-column 98 - Enable metric printing: -stats 99 100 * -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 some 108 semantic analysis (some errors and warnings are not produced). 109 110 * -parse-noop parses code without building an AST. This is useful 111 for timing the cost of the parser without including AST building 112 time. 113 114 * -parse-ast builds ASTs, but doesn't print them. This is most 115 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 expected 120 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 by 125 invoking Graphviz. 126 127 For more information on getting Graphviz to work with clang/LLVM, 128 see: http://llvm.org/docs/ProgrammersManual.html#ViewGraph 129 130 131 III. 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 they 135 highlight regions of interesting code (e.g. the LHS and RHS of a binop). 136 * Full diagnostic customization by client (can format diagnostics however they 137 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, preprocessing 144 and lexing. 145 * Defers exposing platform-specific stuff to as late as possible, tracks use of 146 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 and 148 does not categorize identifiers as types or variables -- this is up to the 149 parser to decide. 150 151 Potential 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 is 155 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 function 158 that texually depends on it: recompile only those functions that need it. 159 This is aka 'incremental parsing'. 160 161 162 IV. Missing Functionality / Improvements 163 164 Lexer: 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 with 168 -fextended-identifiers. 169 * -fpreprocessed mode. 170 171 Preprocessor: 172 * #assert/#unassert 173 * MSExtension: "L#param" stringizes to a wide string literal. 174 * Add support for -M* 175 176 Traditional Preprocessor: 177 * Currently, we have none. :) 178 179
