https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109546
Bug ID: 109546
Summary: [13 Regression] Missed Dead Code Elimination when
using __builtin_unreachable
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: theodort at inf dot ethz.ch
Target Milestone: ---
cat input.c
void foo(void);
static int a, c;
static int *b = &a;
static int **d = &b;
void __assert_fail() __attribute__((__noreturn__));
int main() {
int *e = *d;
if (e == &a || e == &c);
else {
__assert_fail();
}
if (e == &a || e == &c);
else
foo();
}
both gcc 12 and trunk at -O3 generate the following code:
main:
movq b(%rip), %rax
cmpq $c, %rax
je .L2
cmpq $a, %rax
jne .L7
.L2:
xorl %eax, %eax
ret
.L7:
pushq %rax
xorl %eax, %eax
call __assert_fail
b:
.quad a
the call to foo is eliminated but the call to __assert_fail is missed, even
though
both if statement conditions are identical.
If add a __builtin_unreachable before the call to __assert_fail the opposite
happens:
void foo(void);
static int a, c;
static int *b = &a;
static int **d = &b;
void assert_fail() __attribute__((__noreturn__));
int main() {
int *e = *d;
if (e == &a || e == &c);
else {
__builtin_unreachable();
assert_fail();
}
if (e == &a || e == &c);
else
foo();
}
gcc-trunk -O3 generates:
main:
movq b(%rip), %rax
cmpq $c, %rax
je .L4
cmpq $a, %rax
je .L4
pushq %rax
call foo
xorl %eax, %eax
popq %rdx
ret
.L4:
xorl %eax, %eax
ret
b:
.quad a
the call to __assert_fail is now properly eliminated but now the call to foo is
missed.
This is a regression as gcc-12 generates:
main:
xorl %eax, %eax
ret
