slurp some content from the PTH doc into the user's doc.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@69569 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 60 insertions, 0 deletions

diff --git a/docs/UsersManual.html b/docs/UsersManual.html
index 8f8951f87e..cfed51a202 100644
--- a/docs/UsersManual.html
+++ b/docs/UsersManual.html
@@ -180,6 +180,66 @@ other high level options like -c, -g, etc.
 <h3 id="precompiledheaders">Precompiled Headers</h3>
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+<p><a href="http://en.wikipedia.org/wiki/Precompiled_header">Precompiled
+headers</a> are a general approach employed by many compilers to reduce
+compilation time. The underlying motivation of the approach is that it is
+common for the same (and often large) header files to be included by
+multiple source files. Consequently, compile times can often be greatly improved
+by caching some of the (redundant) work done by a compiler to process headers.
+Precompiled header files, which represent one of many ways to implement
+this optimization, are literally files that represent an on-disk cache that
+contains the vital information necessary to reduce some of the work
+needed to process a corresponding header file. While details of precompiled
+headers vary between compilers, precompiled headers have been shown to be a
+highly effective at speeding up program compilation on systems with very large
+system headers (e.g., Mac OS/X).</p>
+
+<p>Clang supports an implementation of precompiled headers known as
+<em>pre-tokenized headers</em> (PTH). Clang's pre-tokenized headers support most
+of same interfaces as GCC's pre-compiled headers (as well as others) but are
+completely different in their implementation.  If you are interested in how
+PTH is implemented, please see the <a href="PTHInternals.html">PTH Internals
+ document</a>.</p>
+
+<h4>Generating a PTH File</h4>
+
+<p>To generate a PTH file using Clang, one invokes Clang with
+the <b><tt>-x <i>&lt;language&gt;</i>-header</tt></b> option. This mirrors the
+interface in GCC for generating PCH files:</p>
+
+<pre>
+  $ gcc -x c-header test.h -o test.h.gch
+  $ clang -x c-header test.h -o test.h.pth
+</pre>
+
+<h4>Using a PTH File</h4>
+
+<p>A PTH file can then be used as a prefix header when a
+<b><tt>-include</tt></b> option is passed to <tt>clang</tt>:</p>
+
+<pre>
+  $ clang -include test.h test.c -o test
+</pre>
+
+<p>The <tt>clang</tt> driver will first check if a PTH file for <tt>test.h</tt>
+is available; if so, the contents of <tt>test.h</tt> (and the files it includes)
+will be processed from the PTH file. Otherwise, Clang falls back to
+directly processing the content of <tt>test.h</tt>. This mirrors the behavior of
+GCC.</p>
+
+<p><b>NOTE:</b> Clang does <em>not</em> automatically used PTH files
+for headers that are directly included within a source file. For example:</p>
+
+<pre>
+  $ clang -x c-header test.h -o test.h.pth
+  $ cat test.c
+  #include "test.h"
+  $ clang test.c -o test
+</pre>
+
+<p>In this example, <tt>clang</tt> will not automatically use the PTH file for
+<tt>test.h</tt> since <tt>test.h</tt> was included directly in the source file
+and not specified on the command line using <tt>-include</tt>.</p>
 
 
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