On Thu, Nov 14, 2013 at 05:49:33PM -0500, Ehsan Akhgari wrote:
> I've started to work on a project in my spare time to switch us to use
> unified builds for C/C++ compilation. The way that unified builds work is
> by using the UNIFIED_SOURCES instead of the SOURCES variable in moz.build
> files. With that, the build system creates files such as:
>
> // Unified_cpp_path_0.cpp
> #include "Source1.cpp"
> #include "Source2.cpp"
> // ...
>
> And compiles them instead of the individual source files.
>
> The advantage of this is that it speeds up the compilation (I've measured
> between 6-15x speed improvement depending on the code in question
> locally.) But there are also trade-offs with this approach. One trade-off
> is that the source code might require changes before it can be compiled in
> this way, due to things like name clashes, etc. The other one is that if
> you change one .cpp file which is built in unified mode, we would spend
> more time compiling it because we'll be compiling the unified .cpp file
> which includes more than what you have changed. It's hard to come up with
> numbers for this trade-off, but assuming that the linking step takes way
> longer than the C++ compilation in the "touch one .cpp file" scenario, and
> also that changes to headers will potentially trigger multiple .cpp
> rebuilds in the same directory, I think doing unified builds in more parts
> of the tree is a reasonable choice.
>
> I'm going to continue working on this as time permits, if you're interested
> in helping out, or if you have questions, please get in touch.
Well, it didn't take long. A build with unified sources disabled already
fails:
layout/style/nsStyleUtil.h:62:12: error: member access into incomplete
type 'nsAString_internal'
aResult.AppendFloat(aNumber);
^
../../dist/include/nsXPCOM.h:20:12: note: forward declaration
of 'nsAString_internal'
DECL_CLASS(nsAString);
Mike
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