From: "Stefan Kanthak" <stefan.kanthak@nexgo.de>
To: "Joseph Myers" <joseph@codesourcery.com>
Cc: "Szabolcs Nagy" <szabolcs.nagy@arm.com>,
"Adhemerval Zanella" <adhemerval.zanella@linaro.org>,
<libc-help@sourceware.org>, <libc-alpha@sourceware.org>
Subject: Re: [PATCH/2nd version] Re: nextafter() about an order of magnitude slower than trivial implementation
Date: Fri, 20 Aug 2021 22:19:20 +0200 [thread overview]
Message-ID: <2246A49E9E024DDBB9AF9C212B29712A@H270> (raw)
In-Reply-To: <alpine.DEB.2.22.394.2108201654040.407143@digraph.polyomino.org.uk>
"Joseph Myers" <joseph@codesourcery.com> wrote:
> On Fri, 20 Aug 2021, Stefan Kanthak wrote:
>
>> if(ax > 0x7ffull << 53) /* x is nan */
>> return x;
>> if(ay > 0x7ffull << 53) /* y is nan */
>> return y;
>
> How has this been tested? I'd have expected it to fail the nextafter
> tests in the glibc testsuite (libm-test-nextafter.inc), because they
> verify that sNaN arguments produce a qNaN result with the "invalid"
> exception raised, and this looks like it would just return an sNaN
> argument unchanged.
It doesn't look like it would, it REALLY does!
As I explicitly wrote, my changes avoid FP operations if possible.
<https://pubs.opengroup.org/onlinepubs/9699919799/functions/nextafter.html>
| If x or y is NaN, a NaN shall be returned.
I choose to return the argument NaN, what both POSIX and ISO C allow.
If my mind serves me well, one of the former editions even stated
"If an argument is NaN, this argument shall be returned". This may
but be influenced/spoiled by
<https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/nextafter-functions>
| If either x or y is a NAN, then the return value is one of the input NANs.
If that bothers you, change these 4 lines to either
if(ax > 0x7ffull << 53) /* x is nan */
return 0.0 + x;
if(ay > 0x7ffull << 53) /* y is nan */
return 0.0 + y;
(the addition has eventually to be guarded from optimisation) or
if(ax > 0x7ffull << 53 /* x is nan */
|| ay > 0x7ffull << 53) /* y is nan */
return x + y;
whatever you like best.
Stefan
next prev parent reply other threads:[~2021-08-20 20:31 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-16 16:03 Stefan Kanthak
2021-08-18 12:51 ` Szabolcs Nagy
2021-08-18 17:11 ` Stefan Kanthak
2021-08-19 11:20 ` Adhemerval Zanella
2021-08-19 17:57 ` [PATCH] " Stefan Kanthak
2021-08-20 9:52 ` [PATCH/2nd version] " Stefan Kanthak
2021-08-20 16:55 ` Joseph Myers
2021-08-20 20:19 ` Stefan Kanthak [this message]
2021-08-20 21:03 ` Joseph Myers
2021-08-23 12:50 ` Adhemerval Zanella
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