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From: "zhongyunde at huawei dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/106323] New: [Suboptimal] memcmp(s1, s2, n) == 0 expansion on
 AArch64 compare to llvm
Date: Sat, 16 Jul 2022 09:55:02 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106323

            Bug ID: 106323
           Summary: [Suboptimal] memcmp(s1, s2, n) =3D=3D 0 expansion on
                    AArch64 compare to llvm
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhongyunde at huawei dot com
  Target Milestone: ---

test case, see detail https://gcc.godbolt.org/z/PM3jxEM9M

```
#include <string.h>

int src(char* s1, char* s2) {=20
  return memcmp(s1, s2, 3) =3D=3D 0;=20
}
```

* llvm doesn't emit branch with instruction cset
```
src:                                    // @src
        ldrh    w8, [x0]
        ldrh    w9, [x1]
        ldrb    w10, [x0, #2]
        ldrb    w11, [x1, #2]
        eor     w8, w8, w9
        eor     w9, w10, w11
        orr     w8, w8, w9
        cmp     w8, #0
        cset    w0, eq
        ret
```

* gcc
```
src:
        ldrh    w3, [x0]
        ldrh    w2, [x1]
        cmp     w3, w2
        beq     .L5
.L2:
        mov     w0, 1
        eor     w0, w0, 1
        ret
.L5:
        ldrb    w2, [x0, 2]
        ldrb    w0, [x1, 2]
        cmp     w2, w0
        bne     .L2
        mov     w0, 0
        eor     w0, w0, 1
        ret
```=