Failure to optimize?

Florian Weimer fweimer@redhat.com
Tue Jan 12 13:36:12 GMT 2021


* ☂Josh Chia (謝任中) via Gcc-help:

> I have a code snippet that I'm wondering why GCC didn't optimize the way I
> think it should:
> https://godbolt.org/z/1qKvax
>
> bar2() is a variant of bar1() that has been manually tweaked to avoid
> branches. I haven't done any benchmarks but, I would expect the branchless
> bar2() to perform better than bar1() but GCC does not automatically
> optimize bar1() to be like bar2(); the generated code for bar1() and bar2()
> are different and the generated code for bar1() contains a branch.

The optimization is probably valid for C99, but not for C11, where the
memory model prevents the compiler from introducing spurious writes:
Another thread may modify the variable concurrently, and if this happens
only if foo returns NULL, the original bar1 function does not contain a
data race, but the branchless version would.

Thanks,
Florian
-- 
Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn,
Commercial register: Amtsgericht Muenchen, HRB 153243,
Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill



More information about the Gcc-help mailing list