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From: "652023330028 at smail dot nju.edu.cn" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/113426] New: Missed optimization of loop
 invariant elimination
Date: Tue, 16 Jan 2024 14:42:11 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113426

            Bug ID: 113426
           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 invari=
ant
elimination.

Loop invariant: a+b.

https://godbolt.org/z/Ebzd3dMPW

int a, b, c;
int n;
int w;
void func(){
    for(int i=3D0;i<100;i++){
        c=3Dn;
        a+=3Dw;
        b-=3Dw;
        n=3Da+b;
    }
}

GCC -O3:
func():
        mov     r9d, DWORD PTR a[rip]
        mov     r10d, DWORD PTR b[rip]
        mov     eax, 100
        mov     edi, DWORD PTR n[rip]
        mov     esi, DWORD PTR w[rip]
        mov     ecx, r10d
        mov     edx, r9d
.L2:
        mov     r8d, edi
        lea     edi, [rcx+rdx]=20=20
        sub     ecx, esi
        add     edx, esi
        sub     eax, 1
        jne     .L2
        imul    edx, esi, -99
        lea     eax, [rsi+r9]
        add     r9d, r10d
        mov     DWORD PTR c[rip], r8d
        mov     DWORD PTR n[rip], r9d
        sub     eax, edx
        mov     DWORD PTR a[rip], eax
        mov     eax, r10d
        sub     eax, esi
        add     eax, edx
        mov     DWORD PTR b[rip], eax
        ret

Expected code (Clang):
func():                               # @func()
        mov     eax, dword ptr [rip + a]
        mov     ecx, dword ptr [rip + b]
        lea     edx, [rcx + rax]
        imul    esi, dword ptr [rip + w], 100
        add     eax, esi
        sub     ecx, esi
        mov     dword ptr [rip + n], edx
        mov     dword ptr [rip + c], edx
        mov     dword ptr [rip + a], eax
        mov     dword ptr [rip + b], ecx
        ret

We analyze and find that in the optimization results of GCC, the calculatio=
n of
n is actually outside the loop (add r9d, r10d; mov DWORD PTR n[rip], r9d).=
=20
However, because of the effect of c=3Dn, a+b is calculated once each time t=
hrough
the loop.
Hope this analysis will help with this issue.

Thank you very much for your time and effort! We look forward to hearing fr=
om
you.=