https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108751
Bug ID: 108751
Summary: Removing dead code results in worse optimization at
-Os
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: ---
I found this case where slight changes in the program that, in theory, should
not affect the output (or affect it trivially) cause the compiler to generate
worse code:
static int a = 0;
static int b = 1;
int main() {
char c = 0;
for (;;) {
if (c)
break;
for (; a; a++) { // a is 0, this loop is dead
if (b) // this is always true
continue;
else
return 2; // this program will never return 2
}
c = 10;
}
return 3;
}
compiled with gcc-trunk -Os:
main:
.L2:
movl a(%rip), %eax
testl %eax, %eax
je .L6
incl %eax
movl %eax, a(%rip)
jmp .L2
.L6:
movl $3, %eax
ret
Clearly, the compiler has figured out that "return 2;" will never be executed.
But if I remove it from the source:
static int a = 0;
static int b = 1;
int main() {
char c = 0;
for (;;) {
if (c)
break;
for (; a; a++) {
if (b)
continue;
//else
// return 2;
}
c = 10;
}
return 3;
}
and compile with gcc-trunk -Os again:
main:
movl a(%rip), %eax
xorl %edx, %edx
.L2:
testl %eax, %eax
jne .L4
testb %dl, %dl
je .L7
xorl %eax, %eax
movl %eax, a(%rip)
jmp .L7
.L4:
incl %eax
movb $1, %dl
jmp .L2
.L7:
movl $3, %eax
ret
the generated code is worse.
The same thing happens if the return value is changed:
static int a = 0;
static int b = 1;
int main() {
char c = 0;
for (;;) {
if (c)
break;
for (; a; a++) {
if (b)
continue;
else
return 2;
}
c = 10;
}
return 1; // changed from return 3
}
gcc-trunk -Os:
main:
movl a(%rip), %eax
xorl %edx, %edx
.L2:
testl %eax, %eax
jne .L4
testb %dl, %dl
je .L7
xorl %eax, %eax
movl %eax, a(%rip)
jmp .L7
.L4:
incl %eax
movb $1, %dl
jmp .L2
.L7:
movl $1, %eax
ret
and if we constant propagate b:
static int a = 0;
int main() {
char c = 0;
for (;;) {
if (c)
break;
for (; a; a++) {
if (1) // this was if (b) before
continue;
else
return 2;
}
c = 10;
}
return 1;
}
gcc-trunk -Os:
main:
movl a(%rip), %eax
xorl %edx, %edx
.L2:
testl %eax, %eax
jne .L12
testb %dl, %dl
je .L7
xorl %eax, %eax
movl %eax, a(%rip)
jmp .L7
.L12:
incl %eax
movb $1, %dl
jmp .L2
.L7:
movl $1, %eax
ret
