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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/114701] New: Missed optimization of loop
 invariant
Date: Fri, 12 Apr 2024 07:14:14 +0000
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X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114701

            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 invaria=
nt.

Loop invariant: 100/(b+d).

https://godbolt.org/z/jKs5qohMs

int a, b;
void func() {
  int d =3D 0;
  for (int i =3D 0; i < 1000; i++) {
    b +=3D 1;
    d +=3D -1;
    a +=3D 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:                                # =3D>This Inner Loop Header: Depth=
=3D1
        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 fr=
om
you.=