diff options
Diffstat (limited to 'docs/PTHInternals.html')
| -rw-r--r-- | docs/PTHInternals.html | 179 |
1 files changed, 0 insertions, 179 deletions
diff --git a/docs/PTHInternals.html b/docs/PTHInternals.html deleted file mode 100644 index b15f68160676..000000000000 --- a/docs/PTHInternals.html +++ /dev/null @@ -1,179 +0,0 @@ -<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" - "http://www.w3.org/TR/html4/strict.dtd"> -<html> - <head> - <title>Pretokenized Headers (PTH)</title> - <link type="text/css" rel="stylesheet" href="../menu.css"> - <link type="text/css" rel="stylesheet" href="../content.css"> - <style type="text/css"> - td { - vertical-align: top; - } - </style> -</head> -<body> - -<!--#include virtual="../menu.html.incl"--> - -<div id="content"> - -<h1>Pretokenized Headers (PTH)</h1> - -<p>This document first describes the low-level -interface for using PTH and then briefly elaborates on its design and -implementation. If you are interested in the end-user view, please see the -<a href="UsersManual.html#precompiledheaders">User's Manual</a>.</p> - - -<h2>Using Pretokenized Headers with <tt>clang</tt> (Low-level Interface)</h2> - -<p>The Clang compiler frontend, <tt>clang -cc1</tt>, supports three command line -options for generating and using PTH files.<p> - -<p>To generate PTH files using <tt>clang -cc1</tt>, use the option -<b><tt>-emit-pth</tt></b>: - -<pre> $ clang -cc1 test.h -emit-pth -o test.h.pth </pre> - -<p>This option is transparently used by <tt>clang</tt> when generating PTH -files. Similarly, PTH files can be used as prefix headers using the -<b><tt>-include-pth</tt></b> option:</p> - -<pre> - $ clang -cc1 -include-pth test.h.pth test.c -o test.s -</pre> - -<p>Alternatively, Clang's PTH files can be used as a raw "token-cache" -(or "content" cache) of the source included by the original header -file. This means that the contents of the PTH file are searched as substitutes -for <em>any</em> source files that are used by <tt>clang -cc1</tt> to process a -source file. This is done by specifying the <b><tt>-token-cache</tt></b> -option:</p> - -<pre> - $ cat test.h - #include <stdio.h> - $ clang -cc1 -emit-pth test.h -o test.h.pth - $ cat test.c - #include "test.h" - $ clang -cc1 test.c -o test -token-cache test.h.pth -</pre> - -<p>In this example the contents of <tt>stdio.h</tt> (and the files it includes) -will be retrieved from <tt>test.h.pth</tt>, as the PTH file is being used in -this case as a raw cache of the contents of <tt>test.h</tt>. This is a low-level -interface used to both implement the high-level PTH interface as well as to -provide alternative means to use PTH-style caching.</p> - -<h2>PTH Design and Implementation</h2> - -<p>Unlike GCC's precompiled headers, which cache the full ASTs and preprocessor -state of a header file, Clang's pretokenized header files mainly cache the raw -lexer <em>tokens</em> that are needed to segment the stream of characters in a -source file into keywords, identifiers, and operators. Consequently, PTH serves -to mainly directly speed up the lexing and preprocessing of a source file, while -parsing and type-checking must be completely redone every time a PTH file is -used.</p> - -<h3>Basic Design Tradeoffs</h3> - -<p>In the long term there are plans to provide an alternate PCH implementation -for Clang that also caches the work for parsing and type checking the contents -of header files. The current implementation of PCH in Clang as pretokenized -header files was motivated by the following factors:<p> - -<ul> - -<li><p><b>Language independence</b>: PTH files work with any language that -Clang's lexer can handle, including C, Objective-C, and (in the early stages) -C++. This means development on language features at the parsing level or above -(which is basically almost all interesting pieces) does not require PTH to be -modified.</p></li> - -<li><b>Simple design</b>: Relatively speaking, PTH has a simple design and -implementation, making it easy to test. Further, because the machinery for PTH -resides at the lower-levels of the Clang library stack it is fairly -straightforward to profile and optimize.</li> -</ul> - -<p>Further, compared to GCC's PCH implementation (which is the dominate -precompiled header file implementation that Clang can be directly compared -against) the PTH design in Clang yields several attractive features:</p> - -<ul> - -<li><p><b>Architecture independence</b>: In contrast to GCC's PCH files (and -those of several other compilers), Clang's PTH files are architecture -independent, requiring only a single PTH file when building an program for -multiple architectures.</p> - -<p>For example, on Mac OS X one may wish to -compile a "universal binary" that runs on PowerPC, 32-bit Intel -(i386), and 64-bit Intel architectures. In contrast, GCC requires a PCH file for -each architecture, as the definitions of types in the AST are -architecture-specific. Since a Clang PTH file essentially represents a lexical -cache of header files, a single PTH file can be safely used when compiling for -multiple architectures. This can also reduce compile times because only a single -PTH file needs to be generated during a build instead of several.</p></li> - -<li><p><b>Reduced memory pressure</b>: Similar to GCC, -Clang reads PTH files via the use of memory mapping (i.e., <tt>mmap</tt>). -Clang, however, memory maps PTH files as read-only, meaning that multiple -invocations of <tt>clang -cc1</tt> can share the same pages in memory from a -memory-mapped PTH file. In comparison, GCC also memory maps its PCH files but -also modifies those pages in memory, incurring the copy-on-write costs. The -read-only nature of PTH can greatly reduce memory pressure for builds involving -multiple cores, thus improving overall scalability.</p></li> - -<li><p><b>Fast generation</b>: PTH files can be generated in a small fraction -of the time needed to generate GCC's PCH files. Since PTH/PCH generation is a -serial operation that typically blocks progress during a build, faster -generation time leads to improved processor utilization with parallel builds on -multicore machines.</p></li> - -</ul> - -<p>Despite these strengths, PTH's simple design suffers some algorithmic -handicaps compared to other PCH strategies such as those used by GCC. While PTH -can greatly speed up the processing time of a header file, the amount of work -required to process a header file is still roughly linear in the size of the -header file. In contrast, the amount of work done by GCC to process a -precompiled header is (theoretically) constant (the ASTs for the header are -literally memory mapped into the compiler). This means that only the pieces of -the header file that are referenced by the source file including the header are -the only ones the compiler needs to process during actual compilation. While -GCC's particular implementation of PCH mitigates some of these algorithmic -strengths via the use of copy-on-write pages, the approach itself can -fundamentally dominate at an algorithmic level, especially when one considers -header files of arbitrary size.</p> - -<p>There are plans to potentially implement an complementary PCH implementation -for Clang based on the lazy deserialization of ASTs. This approach would -theoretically have the same constant-time algorithmic advantages just mentioned -but would also retain some of the strengths of PTH such as reduced memory -pressure (ideal for multi-core builds).</p> - -<h3>Internal PTH Optimizations</h3> - -<p>While the main optimization employed by PTH is to reduce lexing time of -header files by caching pre-lexed tokens, PTH also employs several other -optimizations to speed up the processing of header files:</p> - -<ul> - -<li><p><em><tt>stat</tt> caching</em>: PTH files cache information obtained via -calls to <tt>stat</tt> that <tt>clang -cc1</tt> uses to resolve which files are -included by <tt>#include</tt> directives. This greatly reduces the overhead -involved in context-switching to the kernel to resolve included files.</p></li> - -<li><p><em>Fasting skipping of <tt>#ifdef</tt>...<tt>#endif</tt> chains</em>: -PTH files record the basic structure of nested preprocessor blocks. When the -condition of the preprocessor block is false, all of its tokens are immediately -skipped instead of requiring them to be handled by Clang's -preprocessor.</p></li> - -</ul> - -</div> -</body> -</html> |