[patch, fortran] Fix common subexpression elimination with IEEE rounding (PR108329)

[Richard Biener]

On Sun, Jan 8, 2023 at 5:21 PM Thomas Koenig <tkoenig@netcologne.de> wrote:
>
> Hi Richard,
>
> >> Am 08.01.2023 um 14:31 schrieb Paul Richard Thomas via Fortran <fortran@gcc.gnu.org>:
> >>
> >> Hi Thomas,
> >>
> >> Following your off-line explanation that the seemingly empty looking
> >> assembly line forces an effective reload from memory, all is now clear.
> >
> > It’s not a full fix (for register vars) and it’s ‚superior‘ to the call itself only because asm handling is implemented in a rather stupid way in the Alias oracle.  So I don’t think this is a „fix“ at all.
>
> There are no register variables in Fortran, this is Fortran FE only,
> and it is a fix in the sense that correct code is no longer miscompiled.

It's a quite big hammer and the fact that it "works" is just luck and
the fact that the memory barrier implied by the ieee_set_rouding_mode
does not is because by-reference passed arguments are marked by
the frontend so they can be CSEd since memory barriers may not
affect them.

As said, the fact that this "works" is just because we're lazy on GIMPLE:

/* If the statement STMT may clobber the memory reference REF return true,
   otherwise return false.  */

bool
stmt_may_clobber_ref_p_1 (gimple *stmt, ao_ref *ref, bool tbaa_p)
{
...
  else if (gimple_code (stmt) == GIMPLE_ASM)
    return true;

> There's a FIXME in the code pointing to the relevant PR precisely
> because I think that this is less than elegant (as do you, obviously).
> Do you have other suggestions how to implement this?  If PR 34678
> is solved, this would probably provide a mechanism that we could
> simply re-use.

There is no reliable way to get this correct at the moment and if there
were good and easy ways to get this working they'd be implemented already.

Richard.

> Best regards
>
>         Thomas