https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107772
Bug ID: 107772
Summary: [missed optimization] function prologue generated even
though it's only needed in an unlikely path
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: avi at scylladb dot com
Target Milestone: ---
Consider
int g(int);
void f(int* b, int* e) {
while (b != e) {
if (__builtin_expect(*b != 0, false)) [[unlikely]] {
*b = g(*b);
}
++b;
}
}
If we believe the __builtin_expect and/or unlikely annotations (had both for
extra safety), the loop usually does nothing. So we would expect any register
saving and restoring to be pushed to the unlikely section. Yet (-O3):
f(int*, int*):
cmp rdi, rsi
je .L10
push rbp
mov rbp, rsi
push rbx
mov rbx, rdi
sub rsp, 8
.L4:
mov edi, DWORD PTR [rbx]
test edi, edi
jne .L14
.L3:
add rbx, 4
cmp rbp, rbx
jne .L4
add rsp, 8
pop rbx
pop rbp
ret
.L14:
call g(int)
mov DWORD PTR [rbx], eax
jmp .L3
.L10:
ret
I count 8 instructions that could/should have been pushed to .L14.
