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1//===----------------------------------------------------------------------===//
2// C Language Family Front-end
3//===----------------------------------------------------------------------===//
4                                                             Chris Lattner
5
6I. 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
92II. 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
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 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
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 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
162IV. Missing Functionality / Improvements
163
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 with
168   -fextended-identifiers.
169 * -fpreprocessed mode.
170
171Preprocessor:
172 * #assert/#unassert
173 * MSExtension: "L#param" stringizes to a wide string literal.
174 * Add support for -M*
175
176Traditional Preprocessor:
177 * Currently, we have none. :)
178
179