06cbaa02ca
R=andybons@chromium.org BUG=524256 Review URL: https://codereview.chromium.org/1319543002 Cr-Commit-Position: refs/heads/master@{#345360}
69 lines
2.9 KiB
Markdown
69 lines
2.9 KiB
Markdown
# Windows precompiled headers
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Using precompiled headers on Windows can speed builds up by around 25%.
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Precompiled headers are used by default when GYP generates project files for
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Visual Studio 2010.
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When using Visual Studio 2008, use of precompiled headers is off by default (see
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discussion below). To turn on precompiled headers in your client when using
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MSVS 2008, make sure your `~\.gyp\include.gypi` file looks something like this,
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then run `gclient runhooks` to update the solution files generated by GYP:
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{
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'variables': {
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'chromium_win_pch': 1,
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}
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}
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Since [r174228](http://src.chromium.org/viewvc/chrome?view=rev&revision=174228),
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default is using precompiled header for non `Official` build.
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## Discussion
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MSVS 2008 has some limitations in how well it handles precompiled headers.
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We've run into two issues:
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1. Using precompiled headers can push our official builders over the edge of
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the world, into the dangerous Kingdom of Oom (out of memory).
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1. When compilation flags are changed, instead of doing the right thing and
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rebuilding the precompiled headers and their dependents, MSVS prints a
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warning instead, saying the precompiled header file was built with
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different flags than the current file.
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Because of the above, we disabled use of precompiled headers by default, and
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required the `chromium_win_pch` flag discussed above to be set.
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We may be able to turn use of precompiled headers back on for Debug builds by
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default, by adding a workaround to MSVS's limitations to GYP, i.e. if it detects
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a change in compile flags it could blow away MSVS's output directory.
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## Troubleshooting
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Both of these apply to Visual Studio 2008 only.
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### Precompiled header is missing
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**Problem**: You didn't rebuild recently, and you want to build an individual
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source file (Ctrl+F7). MSVS complains that the precompiled header is missing.
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**Solution**: You could do a full build of the target your source file is in. If
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you'd like to avoid that, find the precompiled header generator file, located
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within a filter somewhere like `../../build/precompile.cc` in your project,
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individually build that file, then individually build the source file you
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intended to build. The `precompile.cc` file is the generator for the precompiled
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header file.
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### specified for precompiled header but not for current compile
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**Problem**: MSVS prints out a warning like this (that we treat as an error):
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`warning C4651: '/D_FOOBAR' specified for precompiled header but not for current
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compile`
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**Solution**: This means compilation flags have changed from when the
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precompiled header file was generated. The issue is that MSVS does not handle
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this correctly. As compilation flags may affect the precompiled header file, it
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should be rebuilt along with its dependents. The workaround for now is to do a
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full rebuild, or (if you want to try to be minimal) a rebuild of all projects
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previously built that use precompiled headers.
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