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 "token-cache" 45 (or "content" 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 <stdio.h> 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 "universal binary" 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> 178
