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docs/windows_precompiled_headers.md

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-# Windows precompiled headers
-
-Using precompiled headers on Windows can speed builds up by around 25%.
-
-Precompiled headers are used by default when GYP generates project files for
-Visual Studio 2010.
-
-When using Visual Studio 2008, use of precompiled headers is off by default (see
-discussion below). To turn on precompiled headers in your client when using
-MSVS 2008, make sure your `~\.gyp\include.gypi` file looks something like this,
-then run `gclient runhooks` to update the solution files generated by GYP:
-
-    {
-      'variables': {
-        'chromium_win_pch': 1,
-      }
-    }
-
-Since [r174228](http://src.chromium.org/viewvc/chrome?view=rev&revision=174228),
-default is using precompiled header for non `Official` build.
-
-## Discussion
-
-MSVS 2008 has some limitations in how well it handles precompiled headers.
-We've run into two issues:
-
-1.  Using precompiled headers can push our official builders over the edge of
-    the world, into the dangerous Kingdom of Oom (out of memory).
-1.  When compilation flags are changed, instead of doing the right thing and
-    rebuilding the precompiled headers and their dependents, MSVS prints a
-    warning instead, saying the precompiled header file was built with
-    different flags than the current file.
-
-Because of the above, we disabled use of precompiled headers by default, and
-required the `chromium_win_pch` flag discussed above to be set.
-
-We may be able to turn use of precompiled headers back on for Debug builds by
-default, by adding a workaround to MSVS's limitations to GYP, i.e. if it detects
-a change in compile flags it could blow away MSVS's output directory.
-
-## Troubleshooting
-
-Both of these apply to Visual Studio 2008 only.
-
-### Precompiled header is missing
-
-**Problem**: You didn't rebuild recently, and you want to build an individual
-source file (Ctrl+F7). MSVS complains that the precompiled header is missing.
-
-**Solution**: You could do a full build of the target your source file is in. If
-you'd like to avoid that, find the precompiled header generator file, located
-within a filter somewhere like `../../build/precompile.cc` in your project,
-individually build that file, then individually build the source file you
-intended to build. The `precompile.cc` file is the generator for the precompiled
-header file.
-
-### specified for precompiled header but not for current compile
-
-**Problem**: MSVS prints out a warning like this (that we treat as an error):
-`warning C4651: '/D_FOOBAR' specified for precompiled header but not for current
-compile`
-
-**Solution**: This means compilation flags have changed from when the
-precompiled header file was generated. The issue is that MSVS does not handle
-this correctly. As compilation flags may affect the precompiled header file, it
-should be rebuilt along with its dependents. The workaround for now is to do a
-full rebuild, or (if you want to try to be minimal) a rebuild of all projects
-previously built that use precompiled headers.