Hi everyone,
In the x86_64 assembly dumps, I frequently come across combinations such
as the following:
cmpl %ebx,%edx
cmovll %ebx,%eax
cmovnll %edx,%eax
This is essentially the tertiary C operator "x = cond ? trueval :
falseval", or in Pascal "if (cond) then x := trueval else x :=
falseval;". However, because the CMOV instructions have exact opposite
conditions, is it better to optimise it into this?
movl %ebx,%eax
cmpl %ebx,%edx
cmovnll %edx,%eax
It's smaller, but is it actually faster (or the same speed)? At the
very least, the two CMOV instructions depend on the CMP instruction
being completed, but I'm not sure if the second CMOV depends on the
first one being evaluated (because of %eax). With the second block of
code, the MOV and CMP instructions can execute simultaneously.
My educated guess tells me that MOV/CMP/CMOV(~c) is faster than
CMP/CMOVc/CMOV(~c), but I haven't been able to find an authoritive
source on this yet.
Gareth aka. Kit
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