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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/114823] New: Missed optimization of redundant
 loops
Date: Tue, 23 Apr 2024 10:11:55 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 14.0
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X-Bugzilla-Severity: normal
X-Bugzilla-Who: 652023330028 at smail dot nju.edu.cn
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114823

            Bug ID: 114823
           Summary: Missed optimization of redundant loops
           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, for the reduced code below, its second loop is redundant. There seem=
s to
be a missed optimization.

https://godbolt.org/z/McqeYdnfY

int a[1024];
int b[1024];

void func()
{
    for (int i =3D 0; i < 1024; i+=3D1) {
        a[i] =3D b[i] * 2;
    }

    for (int i =3D 0; i < 1024; i+=3D1) {
        a[i] =3D b[i] * 2;
    }
}

GCC -O3:
func:
        xor     eax, eax
.L2:
        movdqa  xmm0, XMMWORD PTR b[rax]
        add     rax, 16
        pslld   xmm0, 1
        movaps  XMMWORD PTR a[rax-16], xmm0
        cmp     rax, 4096
        jne     .L2
        xor     eax, eax
.L3:
        movdqa  xmm0, XMMWORD PTR b[rax]
        add     rax, 16
        pslld   xmm0, 1
        movaps  XMMWORD PTR a[rax-16], xmm0
        cmp     rax, 4096
        jne     .L3
        ret

Expected code:
func:
        xor     eax, eax
.L2:
        movdqa  xmm0, XMMWORD PTR b[rax]
        add     rax, 16
        pslld   xmm0, 1
        movaps  XMMWORD PTR a[rax-16], xmm0
        cmp     rax, 4096
        jne     .L2
        ret

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