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Message-ID: <20210814224512.GG37904@port70.net>
Date: Sun, 15 Aug 2021 00:45:12 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: Damian McGuckin <damianm@....com.au>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH #2] Properly simplified nextafter()
* Damian McGuckin <damianm@....com.au> [2021-08-14 14:07:34 +1000]:
> On Fri, 13 Aug 2021, Stefan Kanthak wrote:
> > > It may be possible to reduce the number of such ops too; not sure. But
> > > there's no way to eliminate them.
>
> Replacing the second FORCE'd expression with
>
> FORCE((ux.f + x) * (0x1.0p-52 * 0.25));
>
> eliminates one floating point OP, assuming the optimiser does the right
> thing to
>
> epsilon / 4
>
> in the expression. Some preliminary testing seems to suggest that the same
> exceptions get raised.
>
> > It's definitely possible to do a strength reduction and get rid of the
> > multiplications.
>
> But then how is the exception still raised, or are we talking soft FP?
that does not work, i think.
but the subnormal result case is not worth optimizing.
(or rather, i prefer to optimize cold code paths for size)
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