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Re: [PATCH] Fix ldbl-128ibm fma (Inf, Inf, finite) (bug 23272)
- From: Florian Weimer <fweimer at redhat dot com>
- To: Joseph Myers <joseph at codesourcery dot com>
- Cc: Tulio Magno Quites Machado Filho <tuliom at linux dot ibm dot com>, <libc-alpha at sourceware dot org>
- Date: Fri, 21 Sep 2018 14:51:25 +0200
- Subject: Re: [PATCH] Fix ldbl-128ibm fma (Inf, Inf, finite) (bug 23272)
- References: <email@example.com> <alpine.DEB.firstname.lastname@example.org> <email@example.com> <alpine.DEB.firstname.lastname@example.org>
* 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?