From: Szabolcs Nagy <nsz@port70.net>
To: musl@lists.openwall.com
Subject: Re: [PATCH] math: optimize lrint on 32bit targets
Date: Mon, 23 Sep 2019 20:38:19 +0200 [thread overview]
Message-ID: <20190923183818.GE22009@port70.net> (raw)
In-Reply-To: <20190923174029.GN9017@brightrain.aerifal.cx>
* Rich Felker <dalias@libc.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).
next prev parent reply other threads:[~2019-09-23 18:38 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-21 15:52 Szabolcs Nagy
2019-09-22 20:43 ` Szabolcs Nagy
2019-09-23 14:24 ` Rich Felker
2019-09-23 14:54 ` Szabolcs Nagy
2019-09-23 16:08 ` Rich Felker
2019-09-23 17:40 ` Rich Felker
2019-09-23 18:38 ` Szabolcs Nagy [this message]
2019-09-23 20:42 ` Rich Felker
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20190923183818.GE22009@port70.net \
--to=nsz@port70.net \
--cc=musl@lists.openwall.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this public inbox
https://git.vuxu.org/mirror/musl/
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).