| Douglas Gregor | 29dde39 | 2009-06-03 21:57:43 +0000 | [diff] [blame] | 1 | <html> |
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| 3 | <title>Precompiled Headers (PCH)</title> |
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| 18 | |
| 19 | <h1>Precompiled Headers</h1> |
| 20 | |
| 21 | <p>This document describes the design and implementation of Clang's |
| 22 | precompiled headers (PCH). If you are interested in the end-user |
| 23 | view, please see the <a |
| 24 | href="UsersManual.html#precompiledheaders">User's Manual</a>.</p> |
| 25 | |
| Douglas Gregor | 923cb23 | 2009-06-03 18:35:59 +0000 | [diff] [blame] | 26 | <p><b>Table of Contents</b></p> |
| 27 | <ul> |
| 28 | <li><a href="#usage">Using Precompiled Headers with |
| Daniel Dunbar | 69cfd86 | 2009-12-11 23:17:03 +0000 | [diff] [blame] | 29 | <tt>clang</tt></a></li> |
| Douglas Gregor | 923cb23 | 2009-06-03 18:35:59 +0000 | [diff] [blame] | 30 | <li><a href="#philosophy">Design Philosophy</a></li> |
| 31 | <li><a href="#contents">Precompiled Header Contents</a> |
| 32 | <ul> |
| 33 | <li><a href="#metadata">Metadata Block</a></li> |
| 34 | <li><a href="#sourcemgr">Source Manager Block</a></li> |
| 35 | <li><a href="#preprocessor">Preprocessor Block</a></li> |
| 36 | <li><a href="#types">Types Block</a></li> |
| 37 | <li><a href="#decls">Declarations Block</a></li> |
| 38 | <li><a href="#stmt">Statements and Expressions</a></li> |
| 39 | <li><a href="#idtable">Identifier Table Block</a></li> |
| 40 | <li><a href="#method-pool">Method Pool Block</a></li> |
| 41 | </ul> |
| 42 | </li> |
| Douglas Gregor | 4c0397f | 2009-06-03 21:55:35 +0000 | [diff] [blame] | 43 | <li><a href="#tendrils">Precompiled Header Integration |
| 44 | Points</a></li> |
| Douglas Gregor | 0084ead | 2009-06-03 21:41:31 +0000 | [diff] [blame] | 45 | </ul> |
| Douglas Gregor | 923cb23 | 2009-06-03 18:35:59 +0000 | [diff] [blame] | 46 | |
| Daniel Dunbar | 69cfd86 | 2009-12-11 23:17:03 +0000 | [diff] [blame] | 47 | <h2 id="usage">Using Precompiled Headers with <tt>clang</tt></h2> |
| Douglas Gregor | 32110df | 2009-05-20 00:16:32 +0000 | [diff] [blame] | 48 | |
| Daniel Dunbar | 69cfd86 | 2009-12-11 23:17:03 +0000 | [diff] [blame] | 49 | <p>The Clang compiler frontend, <tt>clang -cc1</tt>, supports two command line |
| 50 | options for generating and using PCH files.<p> |
| Douglas Gregor | 32110df | 2009-05-20 00:16:32 +0000 | [diff] [blame] | 51 | |
| Daniel Dunbar | 69cfd86 | 2009-12-11 23:17:03 +0000 | [diff] [blame] | 52 | <p>To generate PCH files using <tt>clang -cc1</tt>, use the option |
| Douglas Gregor | 32110df | 2009-05-20 00:16:32 +0000 | [diff] [blame] | 53 | <b><tt>-emit-pch</tt></b>: |
| 54 | |
| Daniel Dunbar | 69cfd86 | 2009-12-11 23:17:03 +0000 | [diff] [blame] | 55 | <pre> $ clang -cc1 test.h -emit-pch -o test.h.pch </pre> |
| Douglas Gregor | 32110df | 2009-05-20 00:16:32 +0000 | [diff] [blame] | 56 | |
| 57 | <p>This option is transparently used by <tt>clang</tt> when generating |
| 58 | PCH files. The resulting PCH file contains the serialized form of the |
| 59 | compiler's internal representation after it has completed parsing and |
| 60 | semantic analysis. The PCH file can then be used as a prefix header |
| 61 | with the <b><tt>-include-pch</tt></b> option:</p> |
| 62 | |
| 63 | <pre> |
| Daniel Dunbar | 69cfd86 | 2009-12-11 23:17:03 +0000 | [diff] [blame] | 64 | $ clang -cc1 -include-pch test.h.pch test.c -o test.s |
| Douglas Gregor | 32110df | 2009-05-20 00:16:32 +0000 | [diff] [blame] | 65 | </pre> |
| 66 | |
| Douglas Gregor | 923cb23 | 2009-06-03 18:35:59 +0000 | [diff] [blame] | 67 | <h2 id="philosophy">Design Philosophy</h2> |
| Douglas Gregor | 32110df | 2009-05-20 00:16:32 +0000 | [diff] [blame] | 68 | |
| 69 | <p>Precompiled headers are meant to improve overall compile times for |
| 70 | projects, so the design of precompiled headers is entirely driven by |
| 71 | performance concerns. The use case for precompiled headers is |
| 72 | relatively simple: when there is a common set of headers that is |
| 73 | included in nearly every source file in the project, we |
| 74 | <i>precompile</i> that bundle of headers into a single precompiled |
| 75 | header (PCH file). Then, when compiling the source files in the |
| 76 | project, we load the PCH file first (as a prefix header), which acts |
| 77 | as a stand-in for that bundle of headers.</p> |
| 78 | |
| 79 | <p>A precompiled header implementation improves performance when:</p> |
| 80 | <ul> |
| 81 | <li>Loading the PCH file is significantly faster than re-parsing the |
| 82 | bundle of headers stored within the PCH file. Thus, a precompiled |
| 83 | header design attempts to minimize the cost of reading the PCH |
| 84 | file. Ideally, this cost should not vary with the size of the |
| 85 | precompiled header file.</li> |
| 86 | |
| 87 | <li>The cost of generating the PCH file initially is not so large |
| 88 | that it counters the per-source-file performance improvement due to |
| 89 | eliminating the need to parse the bundled headers in the first |
| 90 | place. This is particularly important on multi-core systems, because |
| 91 | PCH file generation serializes the build when all compilations |
| 92 | require the PCH file to be up-to-date.</li> |
| 93 | </ul> |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 94 | |
| 95 | <p>Clang's precompiled headers are designed with a compact on-disk |
| 96 | representation, which minimizes both PCH creation time and the time |
| 97 | required to initially load the PCH file. The PCH file itself contains |
| 98 | a serialized representation of Clang's abstract syntax trees and |
| 99 | supporting data structures, stored using the same compressed bitstream |
| 100 | as <a href="http://llvm.org/docs/BitCodeFormat.html">LLVM's bitcode |
| 101 | file format</a>.</p> |
| 102 | |
| 103 | <p>Clang's precompiled headers are loaded "lazily" from disk. When a |
| 104 | PCH file is initially loaded, Clang reads only a small amount of data |
| 105 | from the PCH file to establish where certain important data structures |
| 106 | are stored. The amount of data read in this initial load is |
| 107 | independent of the size of the PCH file, such that a larger PCH file |
| 108 | does not lead to longer PCH load times. The actual header data in the |
| 109 | PCH file--macros, functions, variables, types, etc.--is loaded only |
| 110 | when it is referenced from the user's code, at which point only that |
| 111 | entity (and those entities it depends on) are deserialized from the |
| 112 | PCH file. With this approach, the cost of using a precompiled header |
| 113 | for a translation unit is proportional to the amount of code actually |
| 114 | used from the header, rather than being proportional to the size of |
| Douglas Gregor | 4c0397f | 2009-06-03 21:55:35 +0000 | [diff] [blame] | 115 | the header itself.</p> |
| 116 | |
| 117 | <p>When given the <code>-print-stats</code> option, Clang produces |
| 118 | statistics describing how much of the precompiled header was actually |
| 119 | loaded from disk. For a simple "Hello, World!" program that includes |
| 120 | the Apple <code>Cocoa.h</code> header (which is built as a precompiled |
| 121 | header), this option illustrates how little of the actual precompiled |
| 122 | header is required:</p> |
| 123 | |
| 124 | <pre> |
| 125 | *** PCH Statistics: |
| 126 | 933 stat cache hits |
| 127 | 4 stat cache misses |
| 128 | 895/39981 source location entries read (2.238563%) |
| 129 | 19/15315 types read (0.124061%) |
| 130 | 20/82685 declarations read (0.024188%) |
| 131 | 154/58070 identifiers read (0.265197%) |
| 132 | 0/7260 selectors read (0.000000%) |
| 133 | 0/30842 statements read (0.000000%) |
| 134 | 4/8400 macros read (0.047619%) |
| 135 | 1/4995 lexical declcontexts read (0.020020%) |
| 136 | 0/4413 visible declcontexts read (0.000000%) |
| 137 | 0/7230 method pool entries read (0.000000%) |
| 138 | 0 method pool misses |
| 139 | </pre> |
| 140 | |
| 141 | <p>For this small program, only a tiny fraction of the source |
| 142 | locations, types, declarations, identifiers, and macros were actually |
| 143 | deserialized from the precompiled header. These statistics can be |
| 144 | useful to determine whether the precompiled header implementation can |
| 145 | be improved by making more of the implementation lazy.</p> |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 146 | |
| Sebastian Redl | a93e3b5 | 2010-07-08 22:01:51 +0000 | [diff] [blame] | 147 | <p>Precompiled headers can be chained. When you create a PCH while |
| 148 | including an existing PCH, Clang can create the new PCH by referencing |
| 149 | the original file and only writing the new data to the new file. For |
| 150 | example, you could create a PCH out of all the headers that are very |
| 151 | commonly used throughout your project, and then create a PCH for every |
| 152 | single source file in the project that includes the code that is |
| 153 | specific to that file, so that recompiling the file itself is very fast, |
| 154 | without duplicating the data from the common headers for every file.</p> |
| 155 | |
| Douglas Gregor | 923cb23 | 2009-06-03 18:35:59 +0000 | [diff] [blame] | 156 | <h2 id="contents">Precompiled Header Contents</h2> |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 157 | |
| 158 | <img src="PCHLayout.png" align="right" alt="Precompiled header layout"> |
| 159 | |
| 160 | <p>Clang's precompiled headers are organized into several different |
| 161 | blocks, each of which contains the serialized representation of a part |
| 162 | of Clang's internal representation. Each of the blocks corresponds to |
| 163 | either a block or a record within <a |
| 164 | href="http://llvm.org/docs/BitCodeFormat.html">LLVM's bitstream |
| 165 | format</a>. The contents of each of these logical blocks are described |
| 166 | below.</p> |
| 167 | |
| Douglas Gregor | 4c0397f | 2009-06-03 21:55:35 +0000 | [diff] [blame] | 168 | <p>For a given precompiled header, the <a |
| 169 | href="http://llvm.org/cmds/llvm-bcanalyzer.html"><code>llvm-bcanalyzer</code></a> |
| 170 | utility can be used to examine the actual structure of the bitstream |
| 171 | for the precompiled header. This information can be used both to help |
| 172 | understand the structure of the precompiled header and to isolate |
| 173 | areas where precompiled headers can still be optimized, e.g., through |
| 174 | the introduction of abbreviations.</p> |
| 175 | |
| Douglas Gregor | 923cb23 | 2009-06-03 18:35:59 +0000 | [diff] [blame] | 176 | <h3 id="metadata">Metadata Block</h3> |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 177 | |
| 178 | <p>The metadata block contains several records that provide |
| 179 | information about how the precompiled header was built. This metadata |
| 180 | is primarily used to validate the use of a precompiled header. For |
| Douglas Gregor | fe3f223 | 2009-06-03 18:26:16 +0000 | [diff] [blame] | 181 | example, a precompiled header built for a 32-bit x86 target cannot be used |
| 182 | when compiling for a 64-bit x86 target. The metadata block contains |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 183 | information about:</p> |
| 184 | |
| 185 | <dl> |
| 186 | <dt>Language options</dt> |
| 187 | <dd>Describes the particular language dialect used to compile the |
| 188 | PCH file, including major options (e.g., Objective-C support) and more |
| 189 | minor options (e.g., support for "//" comments). The contents of this |
| 190 | record correspond to the <code>LangOptions</code> class.</dd> |
| Douglas Gregor | 32110df | 2009-05-20 00:16:32 +0000 | [diff] [blame] | 191 | |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 192 | <dt>Target architecture</dt> |
| 193 | <dd>The target triple that describes the architecture, platform, and |
| 194 | ABI for which the PCH file was generated, e.g., |
| 195 | <code>i386-apple-darwin9</code>.</dd> |
| 196 | |
| 197 | <dt>PCH version</dt> |
| 198 | <dd>The major and minor version numbers of the precompiled header |
| 199 | format. Changes in the minor version number should not affect backward |
| 200 | compatibility, while changes in the major version number imply that a |
| 201 | newer compiler cannot read an older precompiled header (and |
| 202 | vice-versa).</dd> |
| 203 | |
| 204 | <dt>Original file name</dt> |
| 205 | <dd>The full path of the header that was used to generate the |
| Douglas Gregor | 5accbb9 | 2009-06-03 16:06:22 +0000 | [diff] [blame] | 206 | precompiled header.</dd> |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 207 | |
| 208 | <dt>Predefines buffer</dt> |
| 209 | <dd>Although not explicitly stored as part of the metadata, the |
| 210 | predefines buffer is used in the validation of the precompiled header. |
| 211 | The predefines buffer itself contains code generated by the compiler |
| 212 | to initialize the preprocessor state according to the current target, |
| 213 | platform, and command-line options. For example, the predefines buffer |
| 214 | will contain "<code>#define __STDC__ 1</code>" when we are compiling C |
| 215 | without Microsoft extensions. The predefines buffer itself is stored |
| 216 | within the <a href="#sourcemgr">source manager block</a>, but its |
| Douglas Gregor | 5accbb9 | 2009-06-03 16:06:22 +0000 | [diff] [blame] | 217 | contents are verified along with the rest of the metadata.</dd> |
| 218 | |
| 219 | </dl> |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 220 | |
| Sebastian Redl | a93e3b5 | 2010-07-08 22:01:51 +0000 | [diff] [blame] | 221 | <p>A chained PCH file (that is, one that references another PCH) has |
| 222 | a slightly different metadata block, which contains the following |
| 223 | information:</p> |
| 224 | |
| 225 | <dl> |
| 226 | <dt>Referenced file</dt> |
| 227 | <dd>The name of the referenced PCH file. It is looked up like a file |
| 228 | specified using -include-pch.</dd> |
| 229 | |
| 230 | <dt>PCH version</dt> |
| 231 | <dd>This is the same as in normal PCH files.</dd> |
| 232 | |
| 233 | <dt>Original file name</dt> |
| 234 | <dd>The full path of the header that was used to generate this |
| 235 | precompiled header.</dd> |
| 236 | |
| 237 | </dl> |
| 238 | |
| 239 | <p>The language options, target architecture and predefines buffer data |
| 240 | is taken from the end of the chain, since they have to match anyway.</p> |
| 241 | |
| Douglas Gregor | 923cb23 | 2009-06-03 18:35:59 +0000 | [diff] [blame] | 242 | <h3 id="sourcemgr">Source Manager Block</h3> |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 243 | |
| 244 | <p>The source manager block contains the serialized representation of |
| 245 | Clang's <a |
| 246 | href="InternalsManual.html#SourceLocation">SourceManager</a> class, |
| 247 | which handles the mapping from source locations (as represented in |
| 248 | Clang's abstract syntax tree) into actual column/line positions within |
| 249 | a source file or macro instantiation. The precompiled header's |
| 250 | representation of the source manager also includes information about |
| 251 | all of the headers that were (transitively) included when building the |
| 252 | precompiled header.</p> |
| 253 | |
| 254 | <p>The bulk of the source manager block is dedicated to information |
| 255 | about the various files, buffers, and macro instantiations into which |
| 256 | a source location can refer. Each of these is referenced by a numeric |
| 257 | "file ID", which is a unique number (allocated starting at 1) stored |
| 258 | in the source location. Clang serializes the information for each kind |
| 259 | of file ID, along with an index that maps file IDs to the position |
| 260 | within the PCH file where the information about that file ID is |
| 261 | stored. The data associated with a file ID is loaded only when |
| 262 | required by the front end, e.g., to emit a diagnostic that includes a |
| 263 | macro instantiation history inside the header itself.</p> |
| 264 | |
| 265 | <p>The source manager block also contains information about all of the |
| 266 | headers that were included when building the precompiled header. This |
| 267 | includes information about the controlling macro for the header (e.g., |
| 268 | when the preprocessor identified that the contents of the header |
| 269 | dependent on a macro like <code>LLVM_CLANG_SOURCEMANAGER_H</code>) |
| 270 | along with a cached version of the results of the <code>stat()</code> |
| 271 | system calls performed when building the precompiled header. The |
| 272 | latter is particularly useful in reducing system time when searching |
| 273 | for include files.</p> |
| 274 | |
| Douglas Gregor | 923cb23 | 2009-06-03 18:35:59 +0000 | [diff] [blame] | 275 | <h3 id="preprocessor">Preprocessor Block</h3> |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 276 | |
| 277 | <p>The preprocessor block contains the serialized representation of |
| 278 | the preprocessor. Specifically, it contains all of the macros that |
| 279 | have been defined by the end of the header used to build the |
| 280 | precompiled header, along with the token sequences that comprise each |
| 281 | macro. The macro definitions are only read from the PCH file when the |
| 282 | name of the macro first occurs in the program. This lazy loading of |
| Chris Lattner | 57eccbe | 2009-06-13 18:11:10 +0000 | [diff] [blame] | 283 | macro definitions is triggered by lookups into the <a |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 284 | href="#idtable">identifier table</a>.</p> |
| 285 | |
| Douglas Gregor | 923cb23 | 2009-06-03 18:35:59 +0000 | [diff] [blame] | 286 | <h3 id="types">Types Block</h3> |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 287 | |
| 288 | <p>The types block contains the serialized representation of all of |
| 289 | the types referenced in the translation unit. Each Clang type node |
| 290 | (<code>PointerType</code>, <code>FunctionProtoType</code>, etc.) has a |
| 291 | corresponding record type in the PCH file. When types are deserialized |
| 292 | from the precompiled header, the data within the record is used to |
| 293 | reconstruct the appropriate type node using the AST context.</p> |
| 294 | |
| 295 | <p>Each type has a unique type ID, which is an integer that uniquely |
| 296 | identifies that type. Type ID 0 represents the NULL type, type IDs |
| 297 | less than <code>NUM_PREDEF_TYPE_IDS</code> represent predefined types |
| 298 | (<code>void</code>, <code>float</code>, etc.), while other |
| 299 | "user-defined" type IDs are assigned consecutively from |
| 300 | <code>NUM_PREDEF_TYPE_IDS</code> upward as the types are encountered. |
| 301 | The PCH file has an associated mapping from the user-defined types |
| 302 | block to the location within the types block where the serialized |
| 303 | representation of that type resides, enabling lazy deserialization of |
| 304 | types. When a type is referenced from within the PCH file, that |
| 305 | reference is encoded using the type ID shifted left by 3 bits. The |
| 306 | lower three bits are used to represent the <code>const</code>, |
| 307 | <code>volatile</code>, and <code>restrict</code> qualifiers, as in |
| 308 | Clang's <a |
| 309 | href="http://clang.llvm.org/docs/InternalsManual.html#Type">QualType</a> |
| 310 | class.</p> |
| 311 | |
| Douglas Gregor | 923cb23 | 2009-06-03 18:35:59 +0000 | [diff] [blame] | 312 | <h3 id="decls">Declarations Block</h3> |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 313 | |
| 314 | <p>The declarations block contains the serialized representation of |
| 315 | all of the declarations referenced in the translation unit. Each Clang |
| 316 | declaration node (<code>VarDecl</code>, <code>FunctionDecl</code>, |
| 317 | etc.) has a corresponding record type in the PCH file. When |
| 318 | declarations are deserialized from the precompiled header, the data |
| 319 | within the record is used to build and populate a new instance of the |
| 320 | corresponding <code>Decl</code> node. As with types, each declaration |
| 321 | node has a numeric ID that is used to refer to that declaration within |
| 322 | the PCH file. In addition, a lookup table provides a mapping from that |
| 323 | numeric ID to the offset within the precompiled header where that |
| 324 | declaration is described.</p> |
| 325 | |
| 326 | <p>Declarations in Clang's abstract syntax trees are stored |
| 327 | hierarchically. At the top of the hierarchy is the translation unit |
| 328 | (<code>TranslationUnitDecl</code>), which contains all of the |
| Chris Lattner | 57eccbe | 2009-06-13 18:11:10 +0000 | [diff] [blame] | 329 | declarations in the translation unit. These declarations (such as |
| 330 | functions or struct types) may also contain other declarations inside |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 331 | them, and so on. Within Clang, each declaration is stored within a <a |
| 332 | href="http://clang.llvm.org/docs/InternalsManual.html#DeclContext">declaration |
| 333 | context</a>, as represented by the <code>DeclContext</code> class. |
| 334 | Declaration contexts provide the mechanism to perform name lookup |
| 335 | within a given declaration (e.g., find the member named <code>x</code> |
| 336 | in a structure) and iterate over the declarations stored within a |
| 337 | context (e.g., iterate over all of the fields of a structure for |
| 338 | structure layout).</p> |
| 339 | |
| 340 | <p>In Clang's precompiled header format, deserializing a declaration |
| 341 | that is a <code>DeclContext</code> is a separate operation from |
| 342 | deserializing all of the declarations stored within that declaration |
| 343 | context. Therefore, Clang will deserialize the translation unit |
| 344 | declaration without deserializing the declarations within that |
| 345 | translation unit. When required, the declarations stored within a |
| Chris Lattner | 57eccbe | 2009-06-13 18:11:10 +0000 | [diff] [blame] | 346 | declaration context will be deserialized. There are two representations |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 347 | of the declarations within a declaration context, which correspond to |
| 348 | the name-lookup and iteration behavior described above:</p> |
| 349 | |
| 350 | <ul> |
| 351 | <li>When the front end performs name lookup to find a name |
| 352 | <code>x</code> within a given declaration context (for example, |
| 353 | during semantic analysis of the expression <code>p->x</code>, |
| 354 | where <code>p</code>'s type is defined in the precompiled header), |
| 355 | Clang deserializes a hash table mapping from the names within that |
| 356 | declaration context to the declaration IDs that represent each |
| 357 | visible declaration with that name. The entire hash table is |
| 358 | deserialized at this point (into the <code>llvm::DenseMap</code> |
| 359 | stored within each <code>DeclContext</code> object), but the actual |
| 360 | declarations are not yet deserialized. In a second step, those |
| 361 | declarations with the name <code>x</code> will be deserialized and |
| 362 | will be used as the result of name lookup.</li> |
| 363 | |
| 364 | <li>When the front end performs iteration over all of the |
| 365 | declarations within a declaration context, all of those declarations |
| 366 | are immediately de-serialized. For large declaration contexts (e.g., |
| 367 | the translation unit), this operation is expensive; however, large |
| 368 | declaration contexts are not traversed in normal compilation, since |
| 369 | such a traversal is unnecessary. However, it is common for the code |
| 370 | generator and semantic analysis to traverse declaration contexts for |
| 371 | structs, classes, unions, and enumerations, although those contexts |
| 372 | contain relatively few declarations in the common case.</li> |
| 373 | </ul> |
| 374 | |
| Douglas Gregor | 923cb23 | 2009-06-03 18:35:59 +0000 | [diff] [blame] | 375 | <h3 id="stmt">Statements and Expressions</h3> |
| Douglas Gregor | 5accbb9 | 2009-06-03 16:06:22 +0000 | [diff] [blame] | 376 | |
| 377 | <p>Statements and expressions are stored in the precompiled header in |
| 378 | both the <a href="#types">types</a> and the <a |
| 379 | href="#decls">declarations</a> blocks, because every statement or |
| 380 | expression will be associated with either a type or declaration. The |
| 381 | actual statement and expression records are stored immediately |
| 382 | following the declaration or type that owns the statement or |
| 383 | expression. For example, the statement representing the body of a |
| 384 | function will be stored directly following the declaration of the |
| 385 | function.</p> |
| 386 | |
| 387 | <p>As with types and declarations, each statement and expression kind |
| 388 | in Clang's abstract syntax tree (<code>ForStmt</code>, |
| 389 | <code>CallExpr</code>, etc.) has a corresponding record type in the |
| 390 | precompiled header, which contains the serialized representation of |
| Douglas Gregor | fe3f223 | 2009-06-03 18:26:16 +0000 | [diff] [blame] | 391 | that statement or expression. Each substatement or subexpression |
| 392 | within an expression is stored as a separate record (which keeps most |
| 393 | records to a fixed size). Within the precompiled header, the |
| Argyrios Kyrtzidis | 86d3ca5 | 2010-09-13 17:48:02 +0000 | [diff] [blame] | 394 | subexpressions of an expression are stored, in reverse order, prior to the expression |
| Douglas Gregor | fe3f223 | 2009-06-03 18:26:16 +0000 | [diff] [blame] | 395 | that owns those expression, using a form of <a |
| 396 | href="http://en.wikipedia.org/wiki/Reverse_Polish_notation">Reverse |
| 397 | Polish Notation</a>. For example, an expression <code>3 - 4 + 5</code> |
| 398 | would be represented as follows:</p> |
| 399 | |
| 400 | <table border="1"> |
| Douglas Gregor | fe3f223 | 2009-06-03 18:26:16 +0000 | [diff] [blame] | 401 | <tr><td><code>IntegerLiteral(5)</code></td></tr> |
| Argyrios Kyrtzidis | 86d3ca5 | 2010-09-13 17:48:02 +0000 | [diff] [blame] | 402 | <tr><td><code>IntegerLiteral(4)</code></td></tr> |
| 403 | <tr><td><code>IntegerLiteral(3)</code></td></tr> |
| 404 | <tr><td><code>BinaryOperator(-)</code></td></tr> |
| Douglas Gregor | fe3f223 | 2009-06-03 18:26:16 +0000 | [diff] [blame] | 405 | <tr><td><code>BinaryOperator(+)</code></td></tr> |
| 406 | <tr><td>STOP</td></tr> |
| 407 | </table> |
| 408 | |
| 409 | <p>When reading this representation, Clang evaluates each expression |
| Argyrios Kyrtzidis | 86d3ca5 | 2010-09-13 17:48:02 +0000 | [diff] [blame] | 410 | record it encounters, builds the appropriate abstract syntax tree node, |
| Douglas Gregor | fe3f223 | 2009-06-03 18:26:16 +0000 | [diff] [blame] | 411 | and then pushes that expression on to a stack. When a record contains <i>N</i> |
| 412 | subexpressions--<code>BinaryOperator</code> has two of them--those |
| 413 | expressions are popped from the top of the stack. The special STOP |
| 414 | code indicates that we have reached the end of a serialized expression |
| 415 | or statement; other expression or statement records may follow, but |
| 416 | they are part of a different expression.</p> |
| Douglas Gregor | 5accbb9 | 2009-06-03 16:06:22 +0000 | [diff] [blame] | 417 | |
| Douglas Gregor | 923cb23 | 2009-06-03 18:35:59 +0000 | [diff] [blame] | 418 | <h3 id="idtable">Identifier Table Block</h3> |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 419 | |
| 420 | <p>The identifier table block contains an on-disk hash table that maps |
| 421 | each identifier mentioned within the precompiled header to the |
| 422 | serialized representation of the identifier's information (e.g, the |
| 423 | <code>IdentifierInfo</code> structure). The serialized representation |
| 424 | contains:</p> |
| 425 | |
| 426 | <ul> |
| 427 | <li>The actual identifier string.</li> |
| 428 | <li>Flags that describe whether this identifier is the name of a |
| 429 | built-in, a poisoned identifier, an extension token, or a |
| 430 | macro.</li> |
| 431 | <li>If the identifier names a macro, the offset of the macro |
| 432 | definition within the <a href="#preprocessor">preprocessor |
| 433 | block</a>.</li> |
| 434 | <li>If the identifier names one or more declarations visible from |
| 435 | translation unit scope, the <a href="#decls">declaration IDs</a> of these |
| 436 | declarations.</li> |
| 437 | </ul> |
| 438 | |
| 439 | <p>When a precompiled header is loaded, the precompiled header |
| 440 | mechanism introduces itself into the identifier table as an external |
| 441 | lookup source. Thus, when the user program refers to an identifier |
| 442 | that has not yet been seen, Clang will perform a lookup into the |
| Chris Lattner | 57eccbe | 2009-06-13 18:11:10 +0000 | [diff] [blame] | 443 | identifier table. If an identifier is found, its contents (macro |
| 444 | definitions, flags, top-level declarations, etc.) will be deserialized, at which point the corresponding <code>IdentifierInfo</code> structure will have the same contents it would have after parsing the headers in the precompiled header.</p> |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 445 | |
| Douglas Gregor | 5accbb9 | 2009-06-03 16:06:22 +0000 | [diff] [blame] | 446 | <p>Within the PCH file, the identifiers used to name declarations are represented with an integral value. A separate table provides a mapping from this integral value (the identifier ID) to the location within the on-disk |
| Douglas Gregor | 2cc390e | 2009-06-02 22:08:07 +0000 | [diff] [blame] | 447 | hash table where that identifier is stored. This mapping is used when |
| 448 | deserializing the name of a declaration, the identifier of a token, or |
| 449 | any other construct in the PCH file that refers to a name.</p> |
| 450 | |
| Douglas Gregor | 923cb23 | 2009-06-03 18:35:59 +0000 | [diff] [blame] | 451 | <h3 id="method-pool">Method Pool Block</h3> |
| Douglas Gregor | 5accbb9 | 2009-06-03 16:06:22 +0000 | [diff] [blame] | 452 | |
| 453 | <p>The method pool block is represented as an on-disk hash table that |
| 454 | serves two purposes: it provides a mapping from the names of |
| 455 | Objective-C selectors to the set of Objective-C instance and class |
| 456 | methods that have that particular selector (which is required for |
| 457 | semantic analysis in Objective-C) and also stores all of the selectors |
| 458 | used by entities within the precompiled header. The design of the |
| 459 | method pool is similar to that of the <a href="#idtable">identifier |
| 460 | table</a>: the first time a particular selector is formed during the |
| 461 | compilation of the program, Clang will search in the on-disk hash |
| 462 | table of selectors; if found, Clang will read the Objective-C methods |
| 463 | associated with that selector into the appropriate front-end data |
| 464 | structure (<code>Sema::InstanceMethodPool</code> and |
| 465 | <code>Sema::FactoryMethodPool</code> for instance and class methods, |
| 466 | respectively).</p> |
| 467 | |
| 468 | <p>As with identifiers, selectors are represented by numeric values |
| 469 | within the PCH file. A separate index maps these numeric selector |
| 470 | values to the offset of the selector within the on-disk hash table, |
| 471 | and will be used when de-serializing an Objective-C method declaration |
| 472 | (or other Objective-C construct) that refers to the selector.</p> |
| 473 | |
| Douglas Gregor | 0084ead | 2009-06-03 21:41:31 +0000 | [diff] [blame] | 474 | <h2 id="tendrils">Precompiled Header Integration Points</h2> |
| 475 | |
| 476 | <p>The "lazy" deserialization behavior of precompiled headers requires |
| 477 | their integration into several completely different submodules of |
| 478 | Clang. For example, lazily deserializing the declarations during name |
| 479 | lookup requires that the name-lookup routines be able to query the |
| 480 | precompiled header to find entities within the PCH file.</p> |
| 481 | |
| 482 | <p>For each Clang data structure that requires direct interaction with |
| 483 | the precompiled header logic, there is an abstract class that provides |
| 484 | the interface between the two modules. The <code>PCHReader</code> |
| 485 | class, which handles the loading of a precompiled header, inherits |
| 486 | from all of these abstract classes to provide lazy deserialization of |
| 487 | Clang's data structures. <code>PCHReader</code> implements the |
| 488 | following abstract classes:</p> |
| 489 | |
| 490 | <dl> |
| 491 | <dt><code>StatSysCallCache</code></dt> |
| 492 | <dd>This abstract interface is associated with the |
| 493 | <code>FileManager</code> class, and is used whenever the file |
| 494 | manager is going to perform a <code>stat()</code> system call.</dd> |
| 495 | |
| 496 | <dt><code>ExternalSLocEntrySource</code></dt> |
| 497 | <dd>This abstract interface is associated with the |
| 498 | <code>SourceManager</code> class, and is used whenever the |
| 499 | <a href="#sourcemgr">source manager</a> needs to load the details |
| 500 | of a file, buffer, or macro instantiation.</dd> |
| 501 | |
| 502 | <dt><code>IdentifierInfoLookup</code></dt> |
| 503 | <dd>This abstract interface is associated with the |
| 504 | <code>IdentifierTable</code> class, and is used whenever the |
| 505 | program source refers to an identifier that has not yet been seen. |
| 506 | In this case, the precompiled header implementation searches for |
| 507 | this identifier within its <a href="#idtable">identifier table</a> |
| 508 | to load any top-level declarations or macros associated with that |
| 509 | identifier.</dd> |
| 510 | |
| 511 | <dt><code>ExternalASTSource</code></dt> |
| 512 | <dd>This abstract interface is associated with the |
| 513 | <code>ASTContext</code> class, and is used whenever the abstract |
| 514 | syntax tree nodes need to loaded from the precompiled header. It |
| 515 | provides the ability to de-serialize declarations and types |
| 516 | identified by their numeric values, read the bodies of functions |
| 517 | when required, and read the declarations stored within a |
| 518 | declaration context (either for iteration or for name lookup).</dd> |
| 519 | |
| 520 | <dt><code>ExternalSemaSource</code></dt> |
| 521 | <dd>This abstract interface is associated with the <code>Sema</code> |
| 522 | class, and is used whenever semantic analysis needs to read |
| 523 | information from the <a href="#methodpool">global method |
| 524 | pool</a>.</dd> |
| 525 | </dl> |
| 526 | |
| Douglas Gregor | 32110df | 2009-05-20 00:16:32 +0000 | [diff] [blame] | 527 | </div> |
| 528 | |
| Douglas Gregor | 4c0397f | 2009-06-03 21:55:35 +0000 | [diff] [blame] | 529 | </body> |
| Douglas Gregor | 32110df | 2009-05-20 00:16:32 +0000 | [diff] [blame] | 530 | </html> |