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[clang] Enable clang build with LLVM_BUILD_INSTRUMENTED without setting LLVM_PROFTDATA
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Authored by alexshap on Oct 12 2017, 12:46 PM.

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Summary

At the moment if LLVM_BUILD_INSTRUMENTED is set to True
one has to set LLVM_PROFTDATA even if it's not necessary (because of message(FATAL_ERROR ...)).
Building instrumented Clang is useful even if one doesn't plan to use the target generate-profdata
(currently that target would use only llvm/tools/clang/utils/perf-training/cxx/hello_world.cpp as a source).
For example, one can run the instrumented version of Clang via a separate build system against a different codebase,
collect the profiles and merge them by lllvm-profdata later.

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rL LLVM

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alexshap created this revision.Oct 12 2017, 12:46 PM
vsk added a subscriber: vsk.Oct 12 2017, 5:20 PM

llvm-profdata is tightly coupled with the host compiler: while this setup may work if you get lucky, I don't think it's resilient to changes in libProfData. Also, using the instrumented llvm-profdata will be slow and create extra profiles.

As an alternative, have you considered building an llvm-profdata compatible with your host compiler in a separate step?

yeah, i agree, this is not a good idea. My thoughts were different - right now it's not particularly convenient that one has to specify LLVM_PROFDATA when it's not actually used by the build.
Maybe we can create the target "generate-profdata" only if LLVM_PROFDATA is set (but don't fail otherwise) ?

vsk added a comment.Oct 12 2017, 5:29 PM

yeah, i agree, this is not a good idea. My thoughts were different - right now it's not particularly convenient that one has to specify LLVM_PROFDATA when it's not actually used by the build.
Maybe we can create the target "generate-profdata" only if LLVM_PROFDATA is set (but don't fail otherwise) ?

+ 1, sgtm

alexshap updated this revision to Diff 118868.Oct 12 2017, 5:38 PM
alexshap updated this revision to Diff 118869.Oct 12 2017, 5:41 PM

Update the wording

vsk accepted this revision.Oct 12 2017, 5:57 PM

Thanks!

This revision is now accepted and ready to land.Oct 12 2017, 5:57 PM
This revision was automatically updated to reflect the committed changes.