[Bug target/96875] New: Aliased pointers to union members result in different output with optimisation level.

[iains at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96875

            Bug ID: 96875
           Summary: Aliased pointers to union members result in different
                    output with optimisation level.
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: iains at gcc dot gnu.org
  Target Milestone: ---

For my 0.02 GBP this is ill-conditioned code, but apparently either the try is
still out - or it has been decided to consider it OK.

struct s1 { double d; };
struct s2 { double d; };
union u { struct s1 x; struct s2 y; };

__attribute__((__noinline__))
double
f(struct s1 *a, struct s2 *b)
{
  a->d = 1.0;
  return b->d + 1.0;
}

int
main ()
{
  union u X;
  return f (&X.x, &X.y) == 2.0;
}

====
O1:
f:
.LFB0:
        .cfi_startproc
        fmov    d0, 1.0e+0
        str     d0, [x0]
        ldr     d1, [x1]
        fadd    d0, d1, d0
        ret
        .cfi_endproc

this is the "expected" result.
=====
O2:
f:
.LFB0:
        .cfi_startproc
        ldr     d1, [x1]
        fmov    d0, 1.0e+0
        str     d0, [x0]
        fadd    d0, d1, d0
        ret
        .cfi_endproc

this is some random answer.

====

I'd hazard a guess that something decides that pointers to different types
can't alias, so it's OK to move things around... but the user code bends over
backwards to force the different entities to alias.