https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109588
Bug ID: 109588
Summary: [13 Regression] Missed Dead Code Elimination when
using __builtin_unreachable
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: theodort at inf dot ethz.ch
Target Milestone: ---
void foo(void);
static int a, b;
static int *c = &b;
static int **d, **e = &c;
int main() {
if (b) {
int *f = &a;
if (a)
*e = f = *d;
if (f == &a)
; else foo();
}
}
both gcc trunk and 12.2 at -O3 generate:
main:
movl b(%rip), %eax
testl %eax, %eax
je .L2
cmpl $0, a(%rip)
je .L2
movq 0, %rax
ud2
.L2:
xorl %eax, %eax
ret
Annotating a dead branch with __builting_unreachable():
void foo(void);
static int a, b;
static int *c = &b;
static int **d, **e = &c;
int main() {
if (b) {
int *f = &a;
if (a)
*e = f = *d;
if (f == &a)
__builtin_unreachable(); else foo();
}
}
gcc trunk -O3 now generates:
main:
movl b(%rip), %ecx
testl %ecx, %ecx
je .L5
cmpl $0, a(%rip)
je .L3
movq 0, %rax
ud2
.L3:
pushq %rax
call foo
xorl %eax, %eax
popq %rdx
ret
.L5:
xorl %eax, %eax
ret
but gcc 12 generates:
main:
movl b(%rip), %eax
testl %eax, %eax
je .L2
movq 0, %rax
ud2
.L2:
xorl %eax, %eax
ret
