Re: [PATCH] Fix ldbl-128ibm fma (Inf, Inf, finite) (bug 23272)

[Florian Weimer <fweimer at redhat dot com>]

* Joseph Myers:

> On Fri, 21 Sep 2018, Florian Weimer wrote:
>
>> * Joseph Myers:
>> 
>> > On Wed, 13 Jun 2018, Tulio Magno Quites Machado Filho wrote:
>> >
>> >> This solution is mainly based on Joseph's fix at commit
>> >> ca121b117f2c9c97a4c121334481a96c94fef3a0.
>> >> 
>> >> It has extra differences because:
>> >>  - part of the non-finite arguments were already being treated;
>> >>  - when x and y are +-Inf and z if finite, an overflow can be
>> >>    generated.
>> >
>> > I'm not clear what the observed issues you are fixing are.  Is this patch 
>> > to avoid running into libgcc generating spurious overflows for the case 
>> > you mention, or is it intended to fix wrong results?
>> >
>> >> +    if (!isfinite (x) && !isfinite (y) && isfinite(z))
>> >> +      /* Compute the result as x * y to avoid an overflow.  */
>> >> +      return x * y;
>> >
>> > Why wouldn't this be needed if just one of x and y is not finite?  If x * 
>> > y + z generates a spurious overflow in some cases where x * y is infinite, 
>> > I'd expect that to apply equally to the case where the infinity comes from 
>> > (Inf * finite), not just from (Inf * Inf).
>> 
>> We still see a failures (see bug 23272).  How can we move this forward?
>
> Do you mean bug 23584?

Yes, sorry I used the wrong bug number.

> That answers the first question about the symptoms (both spurious
> overflow exceptions and bad results, apparently coming from __gcc_qadd
> mishandling certain cases involving infinities), but doesn't answer
> the second question about what happens when the infinite x * y comes
> from (Inf * finite) (I'd guess the same problem, in which case the
> proposed patch needs fixing further, but it looks like tests for that
> case also need adding to libm-test-fma.inc).

Thanks.  I have to admit this is a bit over my head.

Tulio, can you take care of this, or should someone else pick this up?

Thanks,
Florian