https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102402
Bug ID: 102402
Summary: Seemingly suboptimal optimization of jmp/cmovcc for
conditionally loading constants
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
#include <stdint.h>
struct MusicPlayerTrack
{
uint8_t flags;
uint8_t modT;
};
void ClearModM(struct MusicPlayerTrack *track, uint8_t modT)
{
if (track->modT == 0)
track->flags |= 3;
else
track->flags |= 12;
}
This is optimized weirdly by GCC. Leaving it as-is gives this AMD64 assembly:
ClearModM:
movzx edx, BYTE PTR [rdi]
mov eax, edx
or eax, 12
cmp BYTE PTR [rdi+1], 0
jne .L3
mov eax, edx
or eax, 3
.L3:
mov BYTE PTR [rdi], al
ret
Whereas changing the `if` to `if (modT == 0)` gives this:
ClearModM:
movzx eax, BYTE PTR [rdi]
mov edx, eax
or eax, 12
or edx, 3
test sil, sil
cmove eax, edx
mov BYTE PTR [rdi], al
ret
It seems to me that this should be better than the first output, though of
course this could be the other way considering how finicky cmovcc seems to be,
but it seems to me like at least one should be preferred above the other.
Note that this also occurs on IA-32, so the issue seems unrelated to whether
modT is in a register or in memory. Perhaps it's about whether it's a function
argument ?
