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From: "jankowski938 at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/108740] New: two identical functions but the code generated
 differs. Why?
Date: Thu, 09 Feb 2023 11:17:38 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D108740

            Bug ID: 108740
           Summary: two identical functions but the code generated
                    differs. Why?
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jankowski938 at gmail dot com
  Target Milestone: ---

I know it is UB but I just wonder why two identical functions generate
different code:

```
#define OPPOSITE(c) (*((typeof(x) *)&(x)))


int foo(volatile int x)
{
    OPPOSITE(x) =3D OPPOSITE(x) + OPPOSITE(x);
    return x;
}

int bar(volatile int x)
{
    OPPOSITE(x) =3D OPPOSITE(x) + OPPOSITE(x);
    return x;
}
```

x86-65 gcc 12.2 -Wall -Wextra -Os=20
```
foo:
        mov     DWORD PTR [rsp-4], edi
        mov     eax, DWORD PTR [rsp-4]
        mov     edx, DWORD PTR [rsp-4]
        add     eax, edx
        mov     DWORD PTR [rsp-4], eax
        mov     eax, DWORD PTR [rsp-4]
        ret
bar:
        mov     DWORD PTR [rsp-4], edi
        mov     eax, DWORD PTR [rsp-4]
        add     eax, eax
        ret
```

ARM-eabi-none 11.2.1 -Wall -Wextra -O3 (same -|Os)
```
foo:
        sub     sp, sp, #8
        str     r0, [sp, #4]
        ldr     r3, [sp, #4]
        ldr     r2, [sp, #4]
        add     r3, r3, r2
        str     r3, [sp, #4]
        ldr     r0, [sp, #4]
        add     sp, sp, #8
        bx      lr
bar:
        sub     sp, sp, #8
        str     r0, [sp, #4]
        ldr     r0, [sp, #4]
        lsl     r0, r0, #1
        add     sp, sp, #8
        bx      lr
```

https://godbolt.org/z/7eMbPcdqs=20


I know that is UB in C11 onwards but I would expect both to be exactly the
same.=