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From: Andrew Pinski <pinskia@gmail.com>
To: GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH] fold a * (a > 0 ? 1 : -1) to abs(a) and related optimizations
Date: Sat, 24 Jun 2017 20:53:00 -0000	[thread overview]
Message-ID: <CA+=Sn1mzbjWx04p5XF30eBSy0qrAfYXyKHrpwcnYi4SEP4EyDg@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.20.1706242143400.2551@stedding.saclay.inria.fr>

On Sat, Jun 24, 2017 at 12:47 PM, Marc Glisse <marc.glisse@inria.fr> wrote:
> On Sat, 24 Jun 2017, Andrew Pinski wrote:
>
>>> * if X is NaN, we may get a qNaN with the wrong sign bit. We probably
>>> don't
>>> care much though...
>>
>>
>> Ok, I changed it to when not honoring NANs.
>
>
> Note that I have no idea what guarantees we give in gcc. It is quite
> possible that your patch is fine without this change, I only wanted to raise
> the question in case someone knows.

So looking through, we do guarantee the sign of the NaNs except when
not honoring NaNs in the first place.  So the conversion from
a>0?1.0:-1.0 to copysign will be conditional on honoring NaNs.  But
the x*copysign(1.0,x) will only conditional on not honoring sNaNs.

>
> --
> Marc Glisse

  reply	other threads:[~2017-06-24 20:53 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-06-24  3:59 Andrew Pinski
2017-06-24  3:59 ` Andrew Pinski
2017-06-24  6:50 ` Marc Glisse
2017-06-24 18:51   ` Andrew Pinski
2017-06-24 19:47     ` Marc Glisse
2017-06-24 20:53       ` Andrew Pinski [this message]
2017-06-26 15:02   ` Joseph Myers

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