On Tue, 12 Jan 2021 at 13:37, Florian Weimer via Gcc-help <gcc-help@xxxxxxxxxxx> wrote: > > * ☂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. I'm not sure about the rules for C, but in C++ the compiler can assume there is no race, because the increment is not atomic. If there were another access to the variable then a non-atomic store would be a race even in the bar1 version.
