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From: "congli at smail dot nju.edu.cn" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/114206] New: GCC generates wrong-code
Date: Sat, 02 Mar 2024 09:06:24 +0000
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X-Bugzilla-Product: gcc
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X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114206

            Bug ID: 114206
           Summary: GCC generates wrong-code
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: congli at smail dot nju.edu.cn
  Target Milestone: ---

The program shown below presents a wrong code bug, where the correct results
should be "f(0, NULL) =3D 0" while `-Os -fno-tree-ccp -fno-tree-copy-prop
-fno-tree-forwprop -fno-tree-fre -fno-tree-vrp` prints "f(0, NULL) =3D 1".

```
#include <stdio.h>

int f(int t, const int *a) {
  const int b[4] =3D {0};

  if (t =3D=3D 0) {
    return f(1, b);
  } else {
    return b =3D=3D a;
  }
}

int main(void) {
  printf("f(0, NULL) =3D %d\n", f(0, NULL));
}
```

Compiler Explorer: https://gcc.godbolt.org/z/W164xWMrP=20

We checked the assembly, finding that it is weird that the compiler generat=
es a
`cmove` instruction. See explanations below:

```
f:
        leaq    -16(%rsp), %rax -> RAX =3D RSP-16
        testl   %edi, %edi      -> we called f(0, NULL); %edi =3D 0, ZF =3D=
 1
        cmove   %rax, %rsi      -> condition fulfilled; RSI=3DRAX=3DRSP-16;=
 weird
generation
        cmpq    %rax, %rsi      -> RSI=3DRAX; ZF=3D1
        sete    %al             -> AL =3D 1
        movzbl  %al, %eax       -> EAX =3D 1 (error)
        ret
```=