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  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
  26   <p><b>Table of Contents</b></p>
  27   <ul>
  28     <li><a href="#usage">Using Precompiled Headers with
  29     <tt>clang</tt></a></li>
  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>
  43     <li><a href="#tendrils">Precompiled Header Integration
  44     Points</a></li>
  45 </ul>
  46
  47 <h2 id="usage">Using Precompiled Headers with <tt>clang</tt></h2>
  48
  49 <p>The Clang compiler frontend, <tt>clang -cc1</tt>, supports two command line
  50 options for generating and using PCH files.<p>
  51
  52 <p>To generate PCH files using <tt>clang -cc1</tt>, use the option
  53 <b><tt>-emit-pch</tt></b>:
  54
  55 <pre> $ clang -cc1 test.h -emit-pch -o test.h.pch </pre>
  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>
  64   $ clang -cc1 -include-pch test.h.pch test.c -o test.s
  65 </pre>
  66
  67 <h2 id="philosophy">Design Philosophy</h2>
  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>
  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
 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>
 146
 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
 156 <h2 id="contents">Precompiled Header Contents</h2>
 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
 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
 176 <h3 id="metadata">Metadata Block</h3>
 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
 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
 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>
 191
 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
 206 precompiled header.</dd>
 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
 217 contents are verified along with the rest of the metadata.</dd>
 218
 219 </dl>
 220
 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
 242 <h3 id="sourcemgr">Source Manager Block</h3>
 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
 275 <h3 id="preprocessor">Preprocessor Block</h3>
 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
 283 macro definitions is triggered by lookups into the <a
 284  href="#idtable">identifier table</a>.</p>
 285
 286 <h3 id="types">Types Block</h3>
 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
 312 <h3 id="decls">Declarations Block</h3>
 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
 329 declarations in the translation unit. These declarations (such as
 330 functions or struct types) may also contain other declarations inside
 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
 346 declaration context will be deserialized. There are two representations
 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-&gt;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
 375 <h3 id="stmt">Statements and Expressions</h3>
 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
 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
 394 subexpressions of an expression are stored, in reverse order, prior to the expression
 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">
 401   <tr><td><code>IntegerLiteral(5)</code></td></tr>
 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>
 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
 410 record it encounters, builds the appropriate abstract syntax tree node,
 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>
 417
 418 <h3 id="idtable">Identifier Table Block</h3>
 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
 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>
 445
 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
 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
 451 <h3 id="method-pool">Method Pool Block</h3>
 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
 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
 527 </div>
 528
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