https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114782
Bug ID: 114782
Summary: [Regression] Missed optimization of Induction Variable
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 for the following code, GCC may have missed an
optimization of inductive variables ('x'). It may be a regression from
gcc-4.8.1.
https://godbolt.org/z/je9sjzPj6
int a,b;
void func(int x){
for(int i=0;i<b;i++){
x=i*5;
a+=x;
b--;
}
}
GCC-trunk -O3:
func(int):
mov edx, DWORD PTR b[rip]
test edx, edx
jle .L1
mov ecx, DWORD PTR a[rip]
xor eax, eax
.L3:
lea esi, [rax+rax*4]
sub edx, 1
add eax, 1
add ecx, esi
cmp edx, eax
jg .L3
mov DWORD PTR a[rip], ecx
mov DWORD PTR b[rip], edx
.L1:
ret
Expected code:
GCC-4.7.4 -O3:
func(int):
mov esi, DWORD PTR b[rip]
test esi, esi
jle .L1
mov ecx, DWORD PTR a[rip]
lea edi, [rsi-1]
xor edx, edx
xor eax, eax
.L3:
mov esi, edi
add ecx, edx
add edx, 5
sub esi, eax
add eax, 1
cmp eax, esi
jl .L3
mov DWORD PTR a[rip], ecx
mov DWORD PTR b[rip], esi
.L1:
rep
ret
Thank you very much for your time and effort! We look forward to hearing from
you.
