Re: [PATCH] Remove can_throw_non_call_exceptions special case from operator_div::wi_fold.

[Richard Biener <richard.guenther@gmail.com>]

On Mon, Nov 29, 2021 at 3:39 PM Jeff Law <jeffreyalaw@gmail.com> wrote:
>
>
>
> On 11/29/2021 7:00 AM, Aldy Hernandez via Gcc-patches wrote:
> > As discussed in the PR.  The code makes no difference, so whatever test
> > we added this special case for has been fixed or is being papered over.
> > I think we should fix any fall out upstream.
> >
> > [Unless Andrew can remember why we added this and it still applies.]
> >
> > Tested on x86-64 Linux.
> >
> > OK for trunk?
> >
> >       PR 103451
> >
> > gcc/ChangeLog:
> >
> >       * range-op.cc (operator_div::wi_fold): Remove
> >       can_throw_non_call_exceptions special case.
> >
> > gcc/testsuite/ChangeLog:
> >
> >       * gcc.dg/pr103451.c: New test.
> I'll defer to Andrew, but it seems wrong to me.  The whole point is to
> set the result to varying so that we don't know the result and never
> remove the division which is critical for -fnon-call-exceptions.

But that has nothing to do with computing the value range for
the result which is only accessible when the stmt does _not_ throw ...

That is, if we compute non-VARYING here and because of that
remove the stmt then _that's_ the place to fix (IMO)

>
> > ---
> >   gcc/range-op.cc                 |  7 -------
> >   gcc/testsuite/gcc.dg/pr103451.c | 17 +++++++++++++++++
> >   2 files changed, 17 insertions(+), 7 deletions(-)
> >   create mode 100644 gcc/testsuite/gcc.dg/pr103451.c
> >
> > diff --git a/gcc/range-op.cc b/gcc/range-op.cc
> > index bbf2924f815..6fe5f1cb4e0 100644
> > --- a/gcc/range-op.cc
> > +++ b/gcc/range-op.cc
> > @@ -1832,13 +1832,6 @@ operator_div::wi_fold (irange &r, tree type,
> >         return;
> >       }
> >
> > -  // If flag_non_call_exceptions, we must not eliminate a division by zero.
> > -  if (cfun->can_throw_non_call_exceptions)
> > -    {
> > -      r.set_varying (type);
> > -      return;
> > -    }
> > -
> >     // If we're definitely dividing by zero, there's nothing to do.
> >     if (wi_zero_p (type, divisor_min, divisor_max))
> >       {
> > diff --git a/gcc/testsuite/gcc.dg/pr103451.c b/gcc/testsuite/gcc.dg/pr103451.c
> > new file mode 100644
> > index 00000000000..b83646d0b83
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.dg/pr103451.c
> > @@ -0,0 +1,17 @@
> > +// { dg-do compile }
> > +// { dg-options "-O2 -w" }
> ISTM that what you want to test for is that the division by zero remains
> in the IL for -fnon-call-exceptions.
>
> jeff
>