[CMake] Fix __builtin_thread_pointer check with LTO

[thesamesam]

With LTO, gcc's IPA passes might drop the foo() function and then the test will pass even on platforms where __builtin_thread_pointer is unavailable.

On PPC64, we get this as a result:

llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp:361:61: error: ‘__builtin_thread_pointer’ is not supported on this targ

Just mark the function in the CMake configure test with the 'used' attribute to avoid it being optimised out. The test then behaves correctly with -flto.

Reported-by: matoro

[MaskRay]

With LTO, gcc's IPA passes might drop the foo() function and then the test will pass even on platforms where __builtin_thread_pointer is unavailable.

Interesting. powerpc64le-linux-gnu-gcc -O2 -flto a.cc incorrectly passes in the absence of used. used makes the command fail as expected.

Suggest: add something like powerpc64le-linux-gnu-gcc -O2 -flto a.cc to the description.

[MaskRay]

Alternative: int main(void) { return (long)__builtin_thread_pointer(); } (if the return value doesn't matter)

gcc lto seems smart enough to optimize out int main(void) { return (long)__builtin_thread_pointer() && 0; } and not emit an error.

[thesamesam]

Thank you! I made that change to the commit message. I kept the approach the same though as I was worried about it optimising it better in future.