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From: "absoler at smail dot nju.edu.cn" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/114236] New: introduce unnecessary store
 operation when unrolling a loop
Date: Tue, 05 Mar 2024 11:54:35 +0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: new
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 13.2.0
X-Bugzilla-Keywords: 
X-Bugzilla-Severity: normal
X-Bugzilla-Who: absoler at smail dot nju.edu.cn
X-Bugzilla-Status: UNCONFIRMED
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X-Bugzilla-Priority: P3
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114236

            Bug ID: 114236
           Summary: introduce unnecessary store operation when unrolling a
                    loop
           Product: gcc
           Version: 13.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: absoler at smail dot nju.edu.cn
  Target Milestone: ---

here's the code, compiled with gcc-13.2.0 -O2
```
// https://godbolt.org/z/5xWrr44hn
short g_16[2][1] =3D {{(-8L)},{(-8L)}};
int g_22 =3D (-1L);
int g_71 =3D 0xA62A9801L;
int *g_70 =3D &g_71;

int c;
void func_1() {
  int d;
  for (; c;)
    ;
  for (d =3D -1; d < 17; d++) {
    short *e =3D g_16[0];
    *g_70 =3D *e =3D g_22;
  }
}

```
```
func_1():
  401440:       mov    0x10762(%rip),%eax        # 411ba8 <c>
  401446:       test   %eax,%eax
  401448:       je     401450 <func_1+0x10>
  40144a:       jmp    40144a <func_1+0xa>
  40144c:       nopl   0x0(%rax)

  401450:       mov    0x2c09(%rip),%rsi        # 404060 <g_70>
  401457:       mov    $0x12,%eax
  40145c:       nopl   0x0(%rax)
  401460:       mov    0x2c06(%rip),%ecx        # 40406c <g_22>
  401466:       movswl %cx,%edx
  401469:       mov    %edx,(%rsi)

  40146b:       sub    $0x1,%eax
  40146e:       jne    401460 <func_1+0x20>
  401470:       mov    %cx,0x2bf9(%rip)        # 404070 <g_16>
  401477:       mov    %edx,(%rsi)
```
we can see that the store operation at 0x401477 is unnecessary and lead to =
19
times memory write, more one time than the loop.
And I found this behavior showing up from the `141t.lim2` file when I check=
 the
output files of `-fdump-tree-all`.=