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

[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.

[Bug target/96875] 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

Iain Sandoe <iains at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |aarch64-linux-gnu,
                   |                            |aarch64-darwin
     Ever confirmed|0                           |1
                 CC|                            |fxcoudert at gcc dot gnu.org,
                   |                            |rsandifo at redhat dot com
           Keywords|                            |alias
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2020-08-31

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

[pinskia at gcc dot gnu.org]

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There is a defect report against the C standard about this case and a much
older gcc bugzilla filed too.

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

[pinskia at gcc dot gnu.org]

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

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|NEW                         |RESOLVED

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
This is an exact dup of bug 14319

*** This bug has been marked as a duplicate of bug 14319 ***