https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114701
Bug ID: 114701
Summary: Missed optimization of loop invariant
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 maybe there is a missed optimization of loop invariant.
Loop invariant: 100/(b+d).
https://godbolt.org/z/jKs5qohMs
int a, b;
void func() {
int d = 0;
for (int i = 0; i < 1000; i++) {
b += 1;
d += -1;
a += 100 / (b + d);
}
}
GCC -O3 -fwrapv:
func():
mov r9d, DWORD PTR b[rip]
mov edi, DWORD PTR a[rip]
xor ecx, ecx
lea r8d, [r9+1]
.L2:
mov esi, r8d
mov eax, 100
xor edx, edx
sub esi, ecx
sub ecx, 1
add esi, ecx
idiv esi
add edi, eax
cmp ecx, -1000
jne .L2
add r9d, 1000
mov DWORD PTR a[rip], edi
mov DWORD PTR b[rip], r9d
ret
Expected code (Clang -O3 -fwrapv -fno-tree-vectorize):
func(): # @func()
mov ecx, dword ptr [rip + b]
mov esi, dword ptr [rip + a]
mov eax, 100
xor edx, edx
idiv ecx
add eax, eax
mov edx, 1000
lea edi, [rax + rax]
.LBB0_1: # =>This Inner Loop Header: Depth=1
add esi, eax
add esi, edi
add esi, edi
add edx, -10
jne .LBB0_1
add ecx, 1000
mov dword ptr [rip + b], ecx
mov dword ptr [rip + a], esi
ret
Thank you very much for your time and effort! We look forward to hearing from
you.
