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

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

On Tue, Nov 30, 2021 at 9:51 AM Aldy Hernandez <aldyh@redhat.com> wrote:
>
> On Tue, Nov 30, 2021 at 8:37 AM Richard Biener
> <richard.guenther@gmail.com> wrote:
> >
> > On Mon, Nov 29, 2021 at 4:24 PM Aldy Hernandez <aldyh@redhat.com> wrote:
> > >
> > > On Mon, Nov 29, 2021 at 3:48 PM Richard Biener
> > > <richard.guenther@gmail.com> wrote:
> > > >
> > > > 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)
> > >
> > > Ughh, I think you're both right.
> > >
> > > We should fix this upstream AND we should test for the presence of the
> > > division by 0 in the optimized dump.
> > >
> > > Of course doing both opens a can of worms.  The division by zero can
> > > be cleaned up by (at least) DCE, DSE, and the code sinking passes.
> > > I've fixed all 3 in the attached (untested) patch.  Dunno what y'all
> > > want to do at this point.
> >
> > I think you need to add -fno-delete-dead-exceptions to the testcase.
> > The sinking
> > bug looks real, but just
> >
> >          && (cfun->can_delete_dead_exceptions
> >                 || !stmt_could_throw_p (cfun, stmt))
> >
> > is needed there.  That change is OK.
>
> Did you mean the entire patch (as attached) is OK, or just the sink part?

The DCE and DSE parts are wrong and not needed.  The remaining pieces
are OK.

Thanks,
Richard.

> Thanks.
> Aldy