https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112277
Bug ID: 112277
Summary: Missed optimization of loop deletion because of missed
loopUnswitch and useless instruction elimination
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: 652023330028 at smail dot nju.edu.cn
Target Milestone: ---
Hello, we noticed that the loop in the code below is not necessary. But GCC
misses this loop deletion optimization, probably because of the missing in loop
Unswitch and useless instruction elimination annotated below.
https://godbolt.org/z/vMPGnhc9n
int b, c;
int n;
void test() {
for (int i = 0; i < 1000; i += b) {
if(c < 0){
c = n;
}
}
}
GCC -O3:
test():
mov ecx, DWORD PTR c[rip]
mov edi, DWORD PTR n[rip]
xor esi, esi
xor eax, eax
mov edx, DWORD PTR b[rip]
test ecx, ecx
jns .L3
.L2: #Loop
add eax, edx
mov esi, 1
mov ecx, edi #Loop invariants: edi
cmp eax, 999
jg .L13
test ecx, ecx #Loop invariants: ecx
js .L2 #Missed LoopUnswitch
.L3: #Useless instructions
add eax, edx
cmp eax, 999
jle .L3 #Useless instructions
test sil, sil
je .L1
.L13:
mov DWORD PTR c[rip], ecx
.L1:
ret
Expected code:
test():
mov eax, DWORD PTR c[rip]
test eax, eax
js .L4
ret
.L4:
mov eax, DWORD PTR n[rip]
mov DWORD PTR c[rip], eax
ret
Thank you very much for your time and effort! We look forward to hearing from
you.
