Failure to optimize?

Florian Weimer
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:
> 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.

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