[Bug target/97161] New: [8/9/10/11 Regression] aarch64: Wrong code at -O2/-O3 since r8-4308-g13494fcb3

[acoplan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97161

            Bug ID: 97161
           Summary: [8/9/10/11 Regression] aarch64: Wrong code at -O2/-O3
                    since r8-4308-g13494fcb3
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: acoplan at gcc dot gnu.org
  Target Milestone: ---

For the following C program:

__attribute__ ((noinline))
void f(char *p, char x)
{
  if (x != 1)
    __builtin_abort();
}

union {
  char a;
  int b;
  short c;
} d;
short g, *e = &d.c;
int i;
int main()
{
  int *h = &d.b;
  g = *h = 1;
  *e |= i;
  f(0, d.a);
}

GCC generates wrong code (and abort is called) at -O2 on AArch64 after
r8-4308-g13494fcb363e8a901db7768a851a9eed1dea62e3.

The issue goes away with -fno-strict-aliasing, but I believe this program does
not violate the strict aliasing constraints: we're simply accessing each member
of the union by its given type.

GCC trunk (x86) and clang 10 (x86 and aarch64) at -O{0,1,2,3} all produce a
program that exits cleanly.

To reproduce, on AArch64:

$ gcc test.c
$ ./a.out
$ gcc test.c -O2
$ ./a.out
Aborted (core dumped)