[Bug target/125180] New: [16 Regression] Conditions based on volatile reads may result in undersized accesses on x86

[leon.a at serenityos dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125180

            Bug ID: 125180
           Summary: [16 Regression] Conditions based on volatile reads may
                    result in undersized accesses on x86
           Product: gcc
           Version: 16.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: leon.a at serenityos dot org
  Target Milestone: ---

Conditions based on the result of a volatile read on x86 seem to
contract the read of the volatile and check starting in version 16,
this seems to loose the volatile semantics or otherwise fails to preserve the
size of the memory access.

Small reproducer:
```c
void foo();

void frobnicate(unsigned volatile* reg)
{
    if (*reg & 1)
        foo();
}
```

GCC 15.2 output:
```
frobnicate:
        mov     eax, DWORD PTR [rdi]
        test    al, 1
        jne     .L4
        ret
.L4:
        jmp     foo
```

GCC 16.1 output:
```
"frobnicate":
        test    BYTE PTR [rdi], 1
        jne     .L4
        ret
.L4:
        jmp     "foo"
```

Correct code after contraction
```
"frobnicate":
        test    DWORD PTR [rdi], 1
        jne     .L4
        ret
.L4:
        jmp     "foo"
```

Godbolt link
<https://godbolt.org/z/onKcP361G>