Re: [PATCH] [PR tree-optimization/107394] Canonicalize global franges as they are read back.

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

On Fri, Oct 28, 2022 at 8:48 AM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Fri, Oct 28, 2022 at 12:45 AM Jeff Law <jeffreyalaw@gmail.com> wrote:
> >
> >
> > On 10/25/22 15:01, Aldy Hernandez via Gcc-patches wrote:
> > > [Richi/Jakub/FP experts, does this sound like the right solution, or am I
> > > missing some subtle IPA/inlining issue?]
> > >
> > > The problem here is that we're inlining a global range with NANs into
> > > a function that has been tagged with __attribute__((optimize
> > > ("-ffinite-math-only"))).  As the global range is copied from
> > > SSA_NAME_RANGE_INFO, its NAN bits are copied, which then cause
> > > frange::verify_range() to fail a sanity check making sure no NANs
> > > creep in when !HONOR_NANS.
> > >
> > > I think what we should do is nuke the NAN bits as we're restoring the
> > > global range.  For that matter, if we use the frange constructor,
> > > everything except that NAN sign will be done automatically, including
> > > dropping INFs to the min/max representable range when appropriate.
> > >
> > >       PR tree-optimization/107394
> > >
> > > gcc/ChangeLog:
> > >
> > >       * value-range-storage.cc (frange_storage_slot::get_frange): Use
> > >       frange constructor.
> > >
> > > gcc/testsuite/ChangeLog:
> > >
> > >       * gcc.dg/tree-ssa/pr107394.c: New test.
> >
> > The other approach would be to disabling inlining in this case due to an
> > unsafe attribute mismatch, but we're not currently doing much sanity
> > checking in this space and it might be a huge can of worms.  I'm
> > inclined to ACK, but give Jakub and Richi until Monday to chime in first.
>
> We are actually quite careful in this regard but maybe our reasoning
> is wrong.  We are allowing inlining of -fno-finite-math-only into
> -ffinite-math-only code but not the other way around.
>
> On the actual patch I think that ranges with Inf/NaNs should be always
> treated as "valid", the optimization to trim them with certain options
> is optimization and thus optional.  So IMHO having verify_range ICE
> on NaNs isn't correct?

Just to make a point here - in functions with -ffinite-math-only in effect

volatile double x = __builtin_nan("");

will still have a literal NaN in the IL and that's not invalid GIMPLE.  You
cannot assume that no NaNs appear with -ffinite-math-only, you just
don't need to specially are about preserving them.

> That said, the patch is in line with what we do elsewhere at the moment,
> so I guess OK.
>
> Richard.
>
> >
> > jeff
> >