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From: "rguenther at suse dot de" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/104356] [12 Regression] divide by zero trap
 is being removed now when it should not be in some cases
Date: Thu, 03 Feb 2022 11:08:00 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104356

--- Comment #8 from rguenther at suse dot de <rguenther at suse dot de> ---
On Thu, 3 Feb 2022, ebotcazou at gcc dot gnu.org wrote:

> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104356
>=20
> Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:
>=20
>            What    |Removed                     |Added
> -------------------------------------------------------------------------=
---
>              Status|UNCONFIRMED                 |NEW
>      Ever confirmed|0                           |1
>    Last reconfirmed|                            |2022-02-03
>                  CC|                            |ebotcazou at gcc dot gnu=
.org
>=20
> --- Comment #7 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> > Hmm, but doesn't ada enable -fdelete-dead-exceptions?  That is, I'm not=
 sure
> > we make division by zero well-defined with -fnon-call-exceptions - the
> > transform assumes the exception cannot happen (because undefinedness) a=
nd
> > removes the exceptional path.
>=20
> Yes, the division by zero can be optimized away in Ada if the result of t=
he
> operation is not used later, so we would need to add a pragma Volatile to=
 the
> gnat.dg/div_zero.adb testcase:
>=20
>   D : Integer :=3D Zero;
>   pragma Volatile (D);
>=20
> for strict semantics, but this usually does not matter at -O0 when the re=
sult
> is assigned to a user variable:
>=20
>   D :=3D 1 / D;

So for Ada it would be valid to optimize it as

  tem =3D D;
  if (tem !=3D 0)
    D :=3D 1 / tem;
  else
    D =3D tem;

basically carrying out the division conditionally only?
(I've tried hard to preserve all volatile loads / stores, if not
volatile that can be elided)

Does Ada define what value D obtains when D is zero or does it
only allow the divison and the exceptional case to be optimized
together but not separate?  So optimization to

  tem =3D D;
  if (tem !=3D 0)
    D :=3D 1 / tem;
  else
    1/0;

and then optimizing the unused 1/0 "exceptional case" only is not
allowed?=