docs/PTHInternals.html

   1 <html>
   2   <head>
   3   <title>Pretokenized Headers (PTH)</title>
   4   <link type="text/css" rel="stylesheet" href="../menu.css" />
   5   <link type="text/css" rel="stylesheet" href="../content.css" />
   6   <style type="text/css">
   7     td {
   8     vertical-align: top;
   9     }
  10   </style>
  11 </head>
  12 <body>
  13
  14 <!--#include virtual="../menu.html.incl"-->
  15
  16 <div id="content">
  17
  18 <h1>Pretokenized Headers (PTH)</h1>
  19
  20 <p>This document first describes the low-level
  21 interface for using PTH and then briefly elaborates on its design and
  22 implementation.  If you are interested in the end-user view, please see the
  23 <a href="UsersManual.html#precompiledheaders">User's Manual</a>.</p>
  24
  25
  26 <h2>Using Pretokenized Headers with <tt>clang</tt> (Low-level Interface)</h2>
  27
  28 <p>The Clang compiler frontend, <tt>clang -cc1</tt>, supports three command line
  29 options for generating and using PTH files.<p>
  30
  31 <p>To generate PTH files using <tt>clang -cc1</tt>, use the option
  32 <b><tt>-emit-pth</tt></b>:
  33
  34 <pre> $ clang -cc1 test.h -emit-pth -o test.h.pth </pre>
  35
  36 <p>This option is transparently used by <tt>clang</tt> when generating PTH
  37 files. Similarly, PTH files can be used as prefix headers using the
  38 <b><tt>-include-pth</tt></b> option:</p>
  39
  40 <pre>
  41   $ clang -cc1 -include-pth test.h.pth test.c -o test.s
  42 </pre>
  43
  44 <p>Alternatively, Clang's PTH files can be used as a raw &quot;token-cache&quot;
  45 (or &quot;content&quot; cache) of the source included by the original header
  46 file. This means that the contents of the PTH file are searched as substitutes
  47 for <em>any</em> source files that are used by <tt>clang -cc1</tt> to process a
  48 source file. This is done by specifying the <b><tt>-token-cache</tt></b>
  49 option:</p>
  50
  51 <pre>
  52   $ cat test.h
  53   #include &lt;stdio.h&gt;
  54   $ clang -cc1 -emit-pth test.h -o test.h.pth
  55   $ cat test.c
  56   #include "test.h"
  57   $ clang -cc1 test.c -o test -token-cache test.h.pth
  58 </pre>
  59
  60 <p>In this example the contents of <tt>stdio.h</tt> (and the files it includes)
  61 will be retrieved from <tt>test.h.pth</tt>, as the PTH file is being used in
  62 this case as a raw cache of the contents of <tt>test.h</tt>. This is a low-level
  63 interface used to both implement the high-level PTH interface as well as to
  64 provide alternative means to use PTH-style caching.</p>
  65
  66 <h2>PTH Design and Implementation</h2>
  67
  68 <p>Unlike GCC's precompiled headers, which cache the full ASTs and preprocessor
  69 state of a header file, Clang's pretokenized header files mainly cache the raw
  70 lexer <em>tokens</em> that are needed to segment the stream of characters in a
  71 source file into keywords, identifiers, and operators. Consequently, PTH serves
  72 to mainly directly speed up the lexing and preprocessing of a source file, while
  73 parsing and type-checking must be completely redone every time a PTH file is
  74 used.</p>
  75
  76 <h3>Basic Design Tradeoffs</h3>
  77
  78 <p>In the long term there are plans to provide an alternate PCH implementation
  79 for Clang that also caches the work for parsing and type checking the contents
  80 of header files. The current implementation of PCH in Clang as pretokenized
  81 header files was motivated by the following factors:<p>
  82
  83 <ul>
  84
  85 <li><p><b>Language independence</b>: PTH files work with any language that
  86 Clang's lexer can handle, including C, Objective-C, and (in the early stages)
  87 C++. This means development on language features at the parsing level or above
  88 (which is basically almost all interesting pieces) does not require PTH to be
  89 modified.</p></li>
  90
  91 <li><b>Simple design</b>: Relatively speaking, PTH has a simple design and
  92 implementation, making it easy to test. Further, because the machinery for PTH
  93 resides at the lower-levels of the Clang library stack it is fairly
  94 straightforward to profile and optimize.</li>
  95 </ul>
  96
  97 <p>Further, compared to GCC's PCH implementation (which is the dominate
  98 precompiled header file implementation that Clang can be directly compared
  99 against) the PTH design in Clang yields several attractive features:</p>
 100
 101 <ul>
 102
 103 <li><p><b>Architecture independence</b>: In contrast to GCC's PCH files (and
 104 those of several other compilers), Clang's PTH files are architecture
 105 independent, requiring only a single PTH file when building an program for
 106 multiple architectures.</p>
 107
 108 <p>For example, on Mac OS X one may wish to
 109 compile a &quot;universal binary&quot; that runs on PowerPC, 32-bit Intel
 110 (i386), and 64-bit Intel architectures. In contrast, GCC requires a PCH file for
 111 each architecture, as the definitions of types in the AST are
 112 architecture-specific. Since a Clang PTH file essentially represents a lexical
 113 cache of header files, a single PTH file can be safely used when compiling for
 114 multiple architectures. This can also reduce compile times because only a single
 115 PTH file needs to be generated during a build instead of several.</p></li>
 116
 117 <li><p><b>Reduced memory pressure</b>: Similar to GCC,
 118 Clang reads PTH files via the use of memory mapping (i.e., <tt>mmap</tt>).
 119 Clang, however, memory maps PTH files as read-only, meaning that multiple
 120 invocations of <tt>clang -cc1</tt> can share the same pages in memory from a
 121 memory-mapped PTH file. In comparison, GCC also memory maps its PCH files but
 122 also modifies those pages in memory, incurring the copy-on-write costs. The
 123 read-only nature of PTH can greatly reduce memory pressure for builds involving
 124 multiple cores, thus improving overall scalability.</p></li>
 125
 126 <li><p><b>Fast generation</b>: PTH files can be generated in a small fraction
 127 of the time needed to generate GCC's PCH files. Since PTH/PCH generation is a
 128 serial operation that typically blocks progress during a build, faster
 129 generation time leads to improved processor utilization with parallel builds on
 130 multicore machines.</p></li>
 131
 132 </ul>
 133
 134 <p>Despite these strengths, PTH's simple design suffers some algorithmic
 135 handicaps compared to other PCH strategies such as those used by GCC. While PTH
 136 can greatly speed up the processing time of a header file, the amount of work
 137 required to process a header file is still roughly linear in the size of the
 138 header file. In contrast, the amount of work done by GCC to process a
 139 precompiled header is (theoretically) constant (the ASTs for the header are
 140 literally memory mapped into the compiler). This means that only the pieces of
 141 the header file that are referenced by the source file including the header are
 142 the only ones the compiler needs to process during actual compilation. While
 143 GCC's particular implementation of PCH mitigates some of these algorithmic
 144 strengths via the use of copy-on-write pages, the approach itself can
 145 fundamentally dominate at an algorithmic level, especially when one considers
 146 header files of arbitrary size.</p>
 147
 148 <p>There are plans to potentially implement an complementary PCH implementation
 149 for Clang based on the lazy deserialization of ASTs. This approach would
 150 theoretically have the same constant-time algorithmic advantages just mentioned
 151 but would also retain some of the strengths of PTH such as reduced memory
 152 pressure (ideal for multi-core builds).</p>
 153
 154 <h3>Internal PTH Optimizations</h3>
 155
 156 <p>While the main optimization employed by PTH is to reduce lexing time of
 157 header files by caching pre-lexed tokens, PTH also employs several other
 158 optimizations to speed up the processing of header files:</p>
 159
 160 <ul>
 161
 162 <li><p><em><tt>stat</tt> caching</em>: PTH files cache information obtained via
 163 calls to <tt>stat</tt> that <tt>clang -cc1</tt> uses to resolve which files are
 164 included by <tt>#include</tt> directives. This greatly reduces the overhead
 165 involved in context-switching to the kernel to resolve included files.</p></li>
 166
 167 <li><p><em>Fasting skipping of <tt>#ifdef</tt>...<tt>#endif</tt> chains</em>:
 168 PTH files record the basic structure of nested preprocessor blocks. When the
 169 condition of the preprocessor block is false, all of its tokens are immediately
 170 skipped instead of requiring them to be handled by Clang's
 171 preprocessor.</p></li>
 172
 173 </ul>
 174
 175 </div>
 176 </body>
 177 </html>