https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68000
--- Comment #3 from Tim Haines <thaines.astro at gmail dot com> ---
(In reply to Andrew Pinski from comment #1)
> Note I think the trunk already has improved code generation.
Here is the codegen from the latest trunk build using the same options as
before.
foo_manual_hoist:
movzx eax, BYTE PTR [rdi]
mov edx, 0
inc eax
cmp al, BYTE PTR [rdi+1]
cmove eax, edx
ret
foo:
movzx edx, BYTE PTR [rdi]
movzx ecx, BYTE PTR [rdi+1]
mov eax, edx
inc edx
cmp edx, ecx
je .L6
inc eax
ret
.L6:
xor eax, eax
ret
foo_if:
movzx eax, BYTE PTR [rdi]
mov edx, 0
inc eax
cmp al, BYTE PTR [rdi+1]
cmove eax, edx
ret
Changing from a cmove to a cmp/jmp doesn't change the instruction latency
(although I couldn't find the latency for je on intel. I assume it's 1 like on
AMD), but now the branch predictor will be invoked- bringing possible pipeline
hazards. I don't mean to be overly critical, but I wouldn't consider this to be
an improvement to the previous code- especially since the other two versions of
the function use cmove. As Marc noted, we are still missing CSE here.
NB: The structure offsets are different here because the assembly I originally
posted was poorly anonymized by me. Mea culpa!
