Any idea why GCC zeroes out eax before overwriting it?
It never fails to amaze me how smart compilers can be while also being so stupid sometimes.
It could be a bug; I don't see an immediate reason to zero eax in this fragment. Sometimes it's easy to miss the reasons why compilers zero apparently unused things though: it can be about breaking data dependencies or making sure flags are in a known state.
It is a bug, but in the CPU. Until recently, on intel cpu, the popcnt instruction had an implicit dependency on its output register, which can be a performance issue. Explicitly clearing the register (which is very fast and almost free), is an easy way to workaround it.
Oh, nice info. It's gone when using e.g. -march=znver1.
This thread [1] might give you some idea of the complexity involved in a similar issue.
OTOH, if you see RAX being zeroed before using EAX, it is likely to be about breaking false deps.
GCC: https://godbolt.org/z/JUzmD8
Clang: https://godbolt.org/z/AVqMGl