FDO builds use -X{gcc/clang}-only

[s-kanev]

IIRC, these are supported by the internal crosstool, but my upstream gcc 5.2.1 and clang 3.6.2 don't like either one:

gcc: error: unrecognized command line option ‘-Xgcc-only=-fprofile-generate=/path’
gcc: error: unrecognized command line option ‘-Xclang-only=-fprofile-instr-generate=/path’
clang: warning: argument unused during compilation: '-Xclang-only=-fprofile-instr-generate=/path'
clang: warning: argument unused during compilation: '-Xgcc-only=-fprofile-generate=/path'

This is obviously low-priority because folks who care enough about setting up an FDO build process can customize the toolchain to include compiler-specific flags. But it would be nice if --fdo_{instrument/optimize} "just work" externally.

https://github.com/bazelbuild/bazel/blob/master/src/main/java/com/google/devtools/build/lib/rules/cpp/CppConfiguration.java#L721

[ulfjack]

We need to update our CROSSTOOL file and add features for fdo_instrument and fdo_optimize; that'll make the error go away.

You can do that in your own client, or send a patch - just copy the corresponding sections from:
third_party/bazel/src/main/java/com/google/devtools/build/lib/rules/cpp/CppConfiguration.java
to:
tools/cpp/CROSSTOOL.tpl (I think, maybe also to tools/cpp/CROSSTOOL)

and then update the flags to remove the -Xgcc-only prefixes. Even better to add code to cc_configure to auto-detect gcc / clang and add the correct flags.

[meteorcloudy]

I've moved these features from CppConfiguration.java to CROSSTOOL.tpl and using cc_configure to add corresponding flags, FDO build works fine on Linux with gcc. But on OS X, building with clang, I still get this error:
clang: error: unknown argument: '-fprofile-instr-generate=/Users/pcloudy/fdo_output'

It turns out the fdo options of clang actually work like this:
http://clang.llvm.org/docs/UsersManual.html#profiling-with-instrumentation.

Clang doesn't accept -fprofile-instr-generate=, but accepts -fprofile-generate= and -fprofile-use=， which is compatible with gcc. But only in version 3.7 or higher.

I guess I'll just use the same flag for both gcc and clang, and when people get errors with clang, they have to update it. Is it the right thing to do?

[ulfjack]

Yes. It would be even better if we could detect the clang version, but I'm not sure it's worth the effort. Maybe we can have an attribute on cc_configure to allow people to override the behavior as a workaround?

[benjaminp]

I posted https://bazel-review.googlesource.com/c/bazel/+/17470 to make fdo work out of the box for gcc and clang.

[meteorcloudy]

@benjaminp Great! Thanks for fixing this!