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Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	1	<html>
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Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	17
Chris Lattner	5c3074f	2009-04-20 04:37:38 +0000	[diff] [blame]	18	<h1>Pretokenized Headers (PTH)</h1>
Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	19
Chris Lattner	5c3074f	2009-04-20 04:37:38 +0000	[diff] [blame]	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>
Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	24
				25
Ted Kremenek	b7fd6b0	2009-04-09 18:17:39 +0000	[diff] [blame]	26	<h2>Using Pretokenized Headers with <tt>clang-cc</tt> (Low-level Interface)</h2>
Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	27
Chris Lattner	cdf59da	2009-04-08 05:50:25 +0000	[diff] [blame]	28	<p>The low-level Clang compiler tool, <tt>clang-cc</tt>, supports three command
				29	line options for generating and using PTH files.<p>
Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	30
Ted Kremenek	b7fd6b0	2009-04-09 18:17:39 +0000	[diff] [blame]	31	<p>To generate PTH files using <tt>clang-cc</tt>, use the option
				32	<b><tt>-emit-pth</tt></b>:
				33
				34	<pre> $ clang-cc test.h -emit-pth -o test.h.pth </pre>
Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	35
				36	<p>This option is transparently used by <tt>clang</tt> when generating PTH
Ted Kremenek	b7fd6b0	2009-04-09 18:17:39 +0000	[diff] [blame]	37	files. Similarly, PTH files can be used as prefix headers using the
				38	<b><tt>-include-pth</tt></b> option:</p>
Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	39
				40	<pre>
				41	$ clang-cc -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-cc</tt> to process a
Ted Kremenek	b7fd6b0	2009-04-09 18:17:39 +0000	[diff] [blame]	48	source file. This is done by specifying the <b><tt>-token-cache</tt></b>
				49	option:</p>
Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	50
				51	<pre>
				52	$ cat test.h
Chris Lattner	0a06999	2009-04-08 06:00:32 +0000	[diff] [blame]	53	#include <stdio.h>
Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	54	$ clang-cc -emit-pth test.h -o test.h.pth
				55	$ cat test.c
				56	#include "test.h"
				57	$ clang-cc 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>
Ted Kremenek	07f08d2	2009-04-09 18:03:21 +0000	[diff] [blame]	84
Ted Kremenek	5890c63	2009-04-09 18:22:40 +0000	[diff] [blame]	85	<li><p><b>Language independence</b>: PTH files work with any language that
Ted Kremenek	07f08d2	2009-04-09 18:03:21 +0000	[diff] [blame]	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>
Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	90
Ted Kremenek	5890c63	2009-04-09 18:22:40 +0000	[diff] [blame]	91	<li><b>Simple design</b>: Relatively speaking, PTH has a simple design and
Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	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
Ted Kremenek	5890c63	2009-04-09 18:22:40 +0000	[diff] [blame]	103	<li><p><b>Architecture independence</b>: In contrast to GCC's PCH files (and
Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	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
Ted Kremenek	5890c63	2009-04-09 18:22:40 +0000	[diff] [blame]	117	<li><p><b>Reduced memory pressure</b>: Similar to GCC,
Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	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-cc</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
Ted Kremenek	5890c63	2009-04-09 18:22:40 +0000	[diff] [blame]	126	<li><p><b>Fast generation</b>: PTH files can be generated in a small fraction
Ted Kremenek	07f08d2	2009-04-09 18:03:21 +0000	[diff] [blame]	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
Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	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
Ted Kremenek	07f08d2	2009-04-09 18:03:21 +0000	[diff] [blame]	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>
Ted Kremenek	797a247	2009-04-08 05:07:30 +0000	[diff] [blame]	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-cc</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
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