[Bug c/103647] New: constant array comparison not always folded

[denis.campredon at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103647

            Bug ID: 103647
           Summary: constant array comparison not always folded
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: denis.campredon at gmail dot com
  Target Milestone: ---

On trunk and gcc 11, with -O2 the following functions are not optimized to 
xor eax, eax

------------
bool f() {
    char a[] = {'a', 'c'};
    char b[] = {'a', 'b'};
    return __builtin_memcmp(a, b, 2);
}

bool g() {
    char a[] = {'a', 'b'};
    char b[] = {'a', 'b', 'c'};
    return __builtin_memcmp(a, b, 2);
}
------------

.LC1:
        .string "ab"
.LC0:
        .string "ac"
f():
        movzx   eax, WORD PTR .LC1[rip]
        cmp     WORD PTR .LC0[rip], ax
        setne   al
        ret
g():
        cmp     WORD PTR .LC1[rip], 25185
        setne   al
        ret