[Bug tree-optimization/119946] New: memset/memcpy with a null argument should be folded into `if (size_arg) __builtin_unreachable();`

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119946

            Bug ID: 119946
           Summary: memset/memcpy with a null argument should be folded
                    into `if (size_arg) __builtin_unreachable();`
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
unsigned char my_func(int n)
{
  __builtin_memset((void*)0, 0, n);
  return n+1;
}
```

Since C23, the memset here is only undefined if `n!=0`, I think we should be
able to fold the memset into:
```
if (n != 0)
  __builtin_unreachable();
```

which is exactly what LLVM does starting in LLVM 20.

Thought maybe a trap instead would be better.  If anything this could be part
of path isolation.

Note I noticed this while working through the review of the patches of PR
67797.