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| 1 # Introduction |
| 2 |
| 3 Using precompiled headers on Windows can speed builds up by around 25%. |
| 4 |
| 5 Precompiled headers are used by default when GYP generates project files for Vis
ual Studio 2010. |
| 6 |
| 7 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 MS
VS 2008, make sure your `~\.gyp\include.gypi` file looks something like this, th
en run `gclient runhooks` to update the solution files generated by GYP: |
| 8 |
| 9 ``` |
| 10 { |
| 11 'variables': { |
| 12 'chromium_win_pch': 1, |
| 13 } |
| 14 } |
| 15 ``` |
| 16 |
| 17 Since [r174228](http://src.chromium.org/viewvc/chrome?view=rev&revision=174228),
default is using precompiled header for non `Official` build. |
| 18 |
| 19 # Discussion |
| 20 |
| 21 MSVS 2008 has some limitations in how well it handles precompiled headers. We'v
e run into two issues: |
| 22 1. Using precompiled headers can push our official builders over the edge of t
he world, into the dangerous Kingdom of Oom (out of memory). |
| 23 1. When compilation flags are changed, instead of doing the right thing and re
building the precompiled headers and their dependents, MSVS prints a warning ins
tead, saying the precompiled header file was built with different flags than the
current file. |
| 24 |
| 25 Because of the above, we disabled use of precompiled headers by default, and req
uired the `chromium_win_pch` flag discussed above to be set. |
| 26 |
| 27 We may be able to turn use of precompiled headers back on for Debug builds by de
fault, 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. |
| 28 |
| 29 # Troubleshooting |
| 30 |
| 31 Both of these apply to Visual Studio 2008 only. |
| 32 |
| 33 |
| 34 --- |
| 35 |
| 36 |
| 37 **Problem**: You didn't rebuild recently, and you want to build an individual so
urce file (Ctrl+F7). MSVS complains that the precompiled header is missing. |
| 38 |
| 39 **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 w
ithin a filter somewhere like `../../build/precompile.cc` in your project, indiv
idually build that file, then individually build the source file you intended to
build. The `precompile.cc` file is the generator for the precompiled header fil
e. |
| 40 |
| 41 |
| 42 --- |
| 43 |
| 44 |
| 45 **Problem**: MSVS prints out a warning like this (that we treat as an error): `w
arning C4651: '/D_FOOBAR' specified for precompiled header but not for current c
ompile` |
| 46 |
| 47 **Solution**: This means compilation flags have changed from when the precompile
d header file was generated. The issue is that MSVS does not handle this correct
ly. As compilation flags may affect the precompiled header file, it should be re
built 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 buil
t that use precompiled headers. |
| 48 |
| 49 |
| 50 --- |
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