Hello friends, TL;DR: PGO on MSVC is now opt-in per-source-directory.
Today I landed a patch[1] that significantly lowered the memory usage of the Visual C++ linker during PGO builds, which has been a consistent pain point for us. The ever-wonderful Ehsan got in touch with a VC++ program manager at Microsoft who got him in touch with some engineers who offered some concrete proposals on how to mitigate our problem. The crux of the workaround is that only source files compiled for PGO usage (using -GL to enable link-time-codegen on MSVC) contribute significantly to the linker's memory usage. The Microsoft engineer's suggestion was to use a tool called pgomgr[2] to dump statistics about what code is actually being exercised in our profiling scenario (and thus most able to benefit from the optimizations) and identify code for which we could disable PGO. After a bit of data-wrangling (details are in the bug) we decided that the best solution would be to make PGO on MSVC an opt-in optimization, so I picked an arbitrary threshold and enabled PGO only in the directories that have significant amounts of code run in our profiling scenario. Any new directory that needs PGO enabled will need to have "MSVC_ENABLE_PGO := 1" in its Makefile to turn it on. A Try run of this patch showed promising results, significantly lowering the linker memory usage and not regressing any Talos results. As an additional bonus it seems to have significantly sped up the build time for Windows PGO builds. Longer-term this should buy us more time for a real solution, as new directories won't automatically contribute to the problem. We heard from Microsoft that they are working on a 64-to-32-bit cross-toolchain, which is the only real silver bullet for this problem, but they don't have any timeline to give us. Hopefully this work buys us enough time for that to materialize without having to expend significant effort working against our toolchain or disabling PGO. Yrs, -Ted 1. https://bugzilla.mozilla.org/show_bug.cgi?id=871712 2. http://msdn.microsoft.com/en-us/library/2kw46d8w%28v=vs.80%29.aspx _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
