https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108992
Bug ID: 108992
Summary: Regression: Branch direction canonicalization leads to
pointless tail duplication / CSE/sinking by inverting
branch
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: nok.raven at gmail dot com
Target Milestone: ---
There are two regressions, in GCC 7 and in GCC 8. If you invert branch manually
(replace 'cond' with '!cond') - the results were the same previously.
// Regressed since GCC 7: https://godbolt.org/z/h4brz7zG9
void use(int *);
void use2(int *);
void foo(bool cond, int * p)
{
if (cond) {
use(p);
}
use2(p);
}
// GCC 6
foo(bool, int*):
test dil, dil
push rbx
mov rbx, rsi
je .L2
mov rdi, rsi
call use(int*)
.L2:
mov rdi, rbx
pop rbx
jmp use2(int*)
// GCC 7
foo(bool, int*):
test dil, dil
jne .L8
mov rdi, rsi
jmp use2(int*)
.L8:
sub rsp, 24
mov rdi, rsi
mov QWORD PTR [rsp+8], rsi
call use2(int*)
mov rsi, QWORD PTR [rsp+8]
add rsp, 24
mov rdi, rsi
jmp use2(int*)
// Regressed since GCC 8: https://godbolt.org/z/MjxqTnbKa
void use(int *);
void use2(int *);
void foo(int * p, bool cond)
{
if (cond) {
use(p);
}
use2(p);
}
// GCC 7
foo(int*, bool):
test sil, sil
push rbx
mov rbx, rdi
je .L2
call use(int*)
.L2:
mov rdi, rbx
pop rbx
jmp use2(int*)
// GCC 8
foo(int*, bool):
push rbx
mov rbx, rdi
test sil, sil
jne .L5
mov rdi, rbx
pop rbx
jmp use2(int*)
.L5:
call use(int*)
mov rdi, rbx
pop rbx
jmp use2(int*)
