https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113080
Bug ID: 113080
Summary: Missed optimization of loop invariant 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 maybe there is a missed optimization for loop invariant
elimination.
Loop invariant: a+b.
https://godbolt.org/z/zYb7Ej47a
int a,b,n;
int w;
void fun1(int t){
for(int i=0;i<100;i++){
a+=w;
b-=w;
t+=a+b;
}
n=t;
}
GCC -O3 -fwrapv (Loop part of the code):
<bb 3> [local count: 1063004408]:
# t_20 = PHI <t_15(3), t_10(D)(2)>
# ivtmp_1 = PHI <ivtmp_7(3), 100(2)>
# ivtmp.32_34 = PHI <ivtmp.32_35(3), ivtmp.32_26(2)>
# ivtmp.33_25 = PHI <_31(3), ivtmp.33_22(2)>
# DEBUG i => NULL
# DEBUG t => NULL
b_lsm.18_24 = (int) ivtmp.33_25;
a_lsm.17_21 = (int) ivtmp.32_34;
# DEBUG i => NULL
# DEBUG t => NULL
# DEBUG BEGIN_STMT
# DEBUG BEGIN_STMT
# DEBUG BEGIN_STMT
_6 = a_lsm.17_21 + b_lsm.18_24;
t_15 = _6 + t_20;
# DEBUG t => t_15
# DEBUG BEGIN_STMT
# DEBUG i => NULL
# DEBUG t => t_15
# DEBUG BEGIN_STMT
ivtmp_7 = ivtmp_1 + 4294967295;
ivtmp.32_35 = _27 + ivtmp.32_34;
_14 = (unsigned int) w.1_2;
_31 = ivtmp.33_25 - _14;
if (ivtmp_7 != 0)
goto <bb 3>; [98.99%]
else
goto <bb 4>; [1.01%]
fun1(int):
mov r9d, DWORD PTR a[rip]
mov esi, DWORD PTR w[rip]
mov eax, 100
mov r10d, DWORD PTR b[rip]
mov ecx, r9d
mov edx, r10d
.L2:
lea r8d, [rcx+rdx]
add ecx, esi
sub edx, esi
add edi, r8d
sub eax, 1
jne .L2
imul eax, esi, -99
add r9d, esi
mov DWORD PTR n[rip], edi
sub r9d, eax
sub eax, esi
add eax, r10d
mov DWORD PTR a[rip], r9d
mov DWORD PTR b[rip], eax
ret
Expected code (Clang):
fun1(int): # @fun1(int)
mov eax, dword ptr [rip + a]
mov ecx, dword ptr [rip + b]
imul edx, dword ptr [rip + w], 100
lea esi, [rcx + rax]
add eax, edx
imul esi, esi, 100
sub ecx, edx
add esi, edi
mov dword ptr [rip + a], eax
mov dword ptr [rip + b], ecx
mov dword ptr [rip + n], esi
ret
The following code is optimized by gcc as expected in terms of loop invariants,
which may help with this issue:
int a,b,n;
int w;
void fun2(int t){
n=t;
for(int i=0;i<100;i++){
a+=w;
b-=w;
n+=a+b;
}
}
Thank you very much for your time and effort! We look forward to hearing from
you.
