https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110080
Bug ID: 110080
Summary: [13/14 Regression] Missed Dead Code Elimination at -Os
when using __builtin_unreachable since
r13-6945-g429a7a88438
Product: gcc
Version: 14.0
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: ---
Given:
void foo(void);
static unsigned char a = 131;
static int *b;
static int **c = &b;
static void d(int e, unsigned f) {
int *g;
if (e){
for (; a; ++a)
for (e = 0; 0;)
;
g = &e;
int **h = &g;
if (**h) {
foo();
}
}
*c = &e;
}
int main() { d(4 & a, a); }
gcc trunk/13.1/12.3 at -Os generate:
main:
testb $4, a(%rip)
je .L3
movb $0, a(%rip)
.L3:
leaq -4(%rsp), %rax
movq %rax, b(%rip)
xorl %eax, %eax
ret
a:
.byte -125
If I include a __builtin_unreachable() to help the compiler:
void foo(void);
static unsigned char a = 131;
static int *b;
static int **c = &b;
static void d(int e, unsigned f) {
int *g;
if (f != 131) {
__builtin_unreachable(); // <- THIS
}
if (!e){
for (; a; ++a)
for (e = 0; 0;)
;
g = &e;
int **h = &g;
if (**h) {
foo();
}
}
*c = &e;
}
int main() { d(4 & a, a); }
gcc-12.3 at -Os generates better code:
main:
leaq -4(%rsp), %rax
movb $0, a(%rip)
movq %rax, b(%rip)
xorl %eax, %eax
ret
a:
.byte -125
But gcc-13.1/trunk at -Os generate worse code:
main:
subq $24, %rsp
movb a(%rip), %al
movl %eax, %edx
andl $4, %edx
movl %edx, 12(%rsp)
xorl %edx, %edx
.L2:
testb %al, %al
je .L8
incl %eax
movb $1, %dl
jmp .L2
.L8:
testb %dl, %dl
je .L4
movb $0, a(%rip)
jmp .L5
.L4:
cmpl $0, 12(%rsp)
je .L5
call foo
.L5:
leaq 12(%rsp), %rax
movq %rax, b(%rip)
xorl %eax, %eax
addq $24, %rsp
ret
a:
.byte -125
https://godbolt.org/z/zvqshjEj3
Started with r13-6945-g429a7a88438
