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From: "Stefan Kanthak" <stefan.kanthak@nexgo.de>
To: "Rich Felker" <dalias@libc.org>
Cc: "Szabolcs Nagy" <nsz@port70.net>, <musl@lists.openwall.com>
Subject: Re: [musl] [PATCH #2] Properly simplified nextafter()
Date: Fri, 13 Aug 2021 20:30:00 +0200	[thread overview]
Message-ID: <DBCC9A601F614AA6AC6B7F6E4922C667@H270> (raw)
In-Reply-To: <20210813155917.GE13220@brightrain.aerifal.cx>

Rich Felker <dalias@libc.org> wrote:

> On Fri, Aug 13, 2021 at 02:04:51PM +0200, Stefan Kanthak wrote:
>> Szabolcs Nagy <nsz@port70.net> wrote on 2021-08-10 at 23:34:
>> 
>> >* Stefan Kanthak <stefan.kanthak@nexgo.de> [2021-08-10 08:23:46 +0200]:
>> >> <https://git.musl-libc.org/cgit/musl/plain/src/math/nextafter.c>
>> >> has quite some superfluous statements:
>> >> 
>> >> 1. there's absolutely no need for 2 uint64_t holding |x| and |y|;
>> >> 2. IEEE-754 specifies -0.0 == +0.0, so (x == y) is equivalent to
>> >>    (ax == 0) && (ay == 0): the latter 2 tests can be removed;
>> > 
>> > you replaced 4 int cmps with 4 float cmps (among other things).
>> > 
>> > it's target dependent if float compares are fast or not.
>> 
>> It's also target dependent whether the FP additions and multiplies
>> used to raise overflow/underflow are SLOOOWWW: how can you justify
>> them, especially for targets using soft-float?
> 
> On a target with fenv, raising the exception flags is mandatory and
> something must be done to make that happen. Performing the arithmetic
> is far faster than prodding at the status flag registers explicitly.

I did NOT propose the latter; I just questioned whether/why you use
two multiplications to detect underflow.

JFTR: FORCE_EVAL(dummy+0.0); raises both overflow and underflow,
      without multiplication.

> On a target without fenv (all the existing soft float targets), it
> should be valid for the compiler just not to emit them, since they
> have no side effects. I think we could make FORCE_EVAL expand to
> nothing on such targets (if the FE_* macros are not defined). This is
> probably worth doing.

Correct, that's the right thing to do there.

[ buggy compiler ]

> It may be possible to reduce the number of such ops too; not sure. But
> there's no way to eliminate them.

It's definitely possible to do a strength reduction and get rid of the
multiplications.

Stefan

  reply	other threads:[~2021-08-13 18:32 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-10  6:23 [musl] [PATCH] " Stefan Kanthak
2021-08-10 21:34 ` Szabolcs Nagy
2021-08-10 22:53   ` Stefan Kanthak
2021-08-11  2:40     ` Rich Felker
2021-08-11 15:44       ` Stefan Kanthak
2021-08-11 16:09         ` Rich Felker
2021-08-11 16:50           ` Stefan Kanthak
2021-08-11 17:57             ` Rich Felker
2021-08-11 22:16               ` Szabolcs Nagy
2021-08-11 22:43                 ` Stefan Kanthak
2021-08-12  0:59                   ` Rich Felker
2021-08-11  8:23     ` Szabolcs Nagy
2021-08-13 12:04   ` [musl] [PATCH #2] " Stefan Kanthak
2021-08-13 15:59     ` Rich Felker
2021-08-13 18:30       ` Stefan Kanthak [this message]
2021-08-14  4:07         ` Damian McGuckin
2021-08-14 22:45           ` Szabolcs Nagy
2021-08-14 23:46     ` Szabolcs Nagy
2021-08-15  7:04       ` Stefan Kanthak
2021-08-15  7:46         ` Ariadne Conill
2021-08-15 13:59           ` Rich Felker
2021-08-15 14:57             ` Ariadne Conill
2021-08-15  8:24         ` Damian McGuckin
2021-08-15 14:03           ` Rich Felker
2021-08-15 15:10             ` Damian McGuckin
2021-08-15 14:56         ` Szabolcs Nagy
2021-08-15 15:19           ` Stefan Kanthak
2021-08-15 15:48             ` Rich Felker
2021-08-15 16:29               ` Stefan Kanthak
2021-08-15 16:49                 ` Rich Felker
2021-08-15 20:52                   ` Stefan Kanthak
2021-08-15 21:48                     ` Rich Felker
2021-08-15 15:52             ` Ariadne Conill
2021-08-15 16:09               ` Rich Felker

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