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Date: Mon, 23 Sep 2019 20:38:19 +0200
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] math: optimize lrint on 32bit targets

* Rich Felker <dalias@...c.org> [2019-09-23 13:40:29 -0400]:
> On Sun, Sep 22, 2019 at 10:43:35PM +0200, Szabolcs Nagy wrote:
> > +long lrint(double x)
> > +{
> > +	uint32_t abstop = asuint64(x)>>32 & 0x7fffffff;
> > +	uint64_t sign = asuint64(x) & (1ULL << 63);
> > +
> > +	if (abstop < 0x41dfffff) {
> > +		/* |x| < 0x7ffffc00, no overflow */
> > +		double_t toint = asdouble(asuint64(1/EPS) | sign);
> > +		double_t y = x + toint - toint;
> > +		return (long)y;
> > +	}
> > +	return lrint_slow(x);
> > +}
> >  #else
> >  long lrint(double x)
> >  {
> 
> This code should be considerably faster than calling rint on 64-bit
> archs too, no? I wonder if it should be something like (untested,
> written inline here):

yeah i'd expect it to be a bit faster, but e.g. a
target may prefer sign?-1/EPS:1/EPS to 1/EPS|sign,
and you need a threshold check even if there is no
inexact overflow issue:

> long lrint(double x)
> {
> 	uint32_t abstop = asuint64(x)>>32 & 0x7fffffff;
> 	uint64_t sign = asuint64(x) & (1ULL << 63);
> 
> #if LONG_MAX < 1U<<53 && defined(FE_INEXACT)
> 	if (abstop >= 0x41dfffff) return lrint_slow(x);

#else
	if (abstop >= 0x43300000) return (long)x;
	/* |x| < 2^52 <= 1/EPS */

> #endif
> 	/* |x| < 0x7ffffc00, no overflow */
> 	double_t toint = asdouble(asuint64(1/EPS) | sign);
> 	double_t y = x + toint - toint;
> 	return (long)y;
> }

i can try to benchmark this (although on x86_64 and
aarch64 there is single instruction lrint so i can
only benchmark machines where this is not relevant).

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