From: Szabolcs Nagy <nsz@port70.net>
To: musl@lists.openwall.com
Subject: Re: remquo - underlying logic
Date: Wed, 6 Dec 2017 11:37:09 +0100 [thread overview]
Message-ID: <20171206103708.GA15263@port70.net> (raw)
In-Reply-To: <alpine.LRH.2.02.1712061141550.18308@key0.esi.com.au>
* Damian McGuckin <damianm@esi.com.au> [2017-12-06 12:17:40 +1100]:
> While my exploration with floating point numbers was less than stellar,
> I did notice that when
>
> ex - ey < p (where p is the digits in the significant)
>
> you can use the
>
> fma
>
> routine to compute some appropriately rounded/truncated version of the
> quotient for both remquo and fmod. And this appears to not loose any
> precision for the obvious reasons.
>
> > From my limited testing, the speed gain for this extremely limited range
> of exponent difference is huge over the standard routine in MUSL. I will do
> some more testing and report in detail but it seems to be orders of
> magnitude.
>
> Somebody might want to comment on that sort of approach.
it's not clear to me how you use fma (x-(int)(x/y)*y ?),
but efficient fma instruction is not available on all
targets and the software implementation can be very slow.
and i suspect such approach would break fenv correctness.
assuming you rely on exact fma result you may need two
different implementation depending on FP_FAST_FMA
(which is currently missing in musl).
(musl is compiled with -std=c99 so x*y+z is not contracted
to fma(x,y,z) automatically when the instruction is
available, you have to add -ffp-contract=fast if that's
what you want, but it might break some code in musl that
relies on exact arithmetics, most math code should work
either way though.)
next prev parent reply other threads:[~2017-12-06 10:37 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-30 18:11 Damian McGuckin
2017-11-30 18:59 ` Szabolcs Nagy
2017-11-30 20:29 ` Damian McGuckin
2017-11-30 21:16 ` John Reiser
2017-11-30 21:17 ` Szabolcs Nagy
2017-12-04 8:10 ` Damian McGuckin
2017-12-06 1:17 ` Damian McGuckin
2017-12-06 10:37 ` Szabolcs Nagy [this message]
2017-12-07 1:09 ` Damian McGuckin
2017-12-08 0:42 ` Szabolcs Nagy
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