Re: What happened to short float?

From	GOTHIER Nathan <nathan.gothier@gmail.com>
Newsgroups	comp.lang.c, comp.std.c
Subject	Re: What happened to short float?
Date	2018-02-08 09:54 +0100
Organization	A noiseless patient Spider
Message-ID	<20180208095429.606364963470edfaf0cc3206@gmail.com> (permalink)
References	(1 earlier) <ef0ada5f-6cbd-a060-fab8-894272f9b914@verizon.net> <p52ngk$evf$1@dont-email.me> <fdr6e4Fp1fcU1@mid.individual.net> <20180208012936.09fa549ce0fe0785da7cf560@gmail.com> <lnk1voz46u.fsf@kst-u.example.com>

Cross-posted to 2 groups.

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On Wed, 07 Feb 2018 16:59:53 -0800
Keith Thompson <kst-u@mib.org> wrote:

> Wouldn't you need extra code to convert between 16-bit and 32-bit
> formats?  For normal values you'd need to do some bit-twiddling.
> Special values like infinities, NaNs, subnormals, and so forth might
> require some extra work (I haven't looked into the details).

No. Supporting the short float type for computing purpose doesn't mean
you have to implement the half floating point format.

Since the IEEE 754 standard requires only the implementation of the
single precision float format, any compliant hardware could implement
the short float type as a single precision float. Hardware with a full
specific half float support could provide more performance.

> > typedef char half_t[2];
> 
> (I'd use uint16_t, or at least unsigned char[2] rather than char[2].)

I agree, the object should be stored at least as unsigned char[2].

> Sure, if you have a CPU instruction that does the conversion, it's
> easy.

Not really. The F16C instruction set for example only provides the
conversion to half precision float but not the arithmetic.
Consequently, the implementation should distinguish the storage format
from the computing format such as:

typedef unsigned char half_t[2];
typedef float sflt_t;

sflt_t *htosf(half_t *x);
half_t *sftoh(sflt_t *x);

> If you don't, it's not prohitively difficult, but it's not quite as
> obvious that it's worth the effort.

As already said, there's no need to implement the half precision
format. The short float type should only provide hardware acceleration
when available.

-- 
GOTHIER Nathan

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Thread

Re: What happened to short float? jacobnavia <jacob@jacob.remcomp.fr> - 2018-02-02 23:08 +0100
  Re: What happened to short float? Jakob Bohm <jb-usenet@wisemo.com> - 2018-02-05 07:57 +0100
    Re: What happened to short float? jameskuyper@verizon.net - 2018-02-05 05:31 -0800
  Re: What happened to short float? Tim Prince <tprince@intelretiree.com> - 2018-02-05 08:55 -0500
    Re: What happened to short float? jacobnavia <jacob@jacob.remcomp.fr> - 2018-02-05 15:46 +0100
    Re: What happened to short float? GOTHIER Nathan <nathan.gothier@gmail.com> - 2018-02-08 01:29 +0100
      Re: What happened to short float? Keith Thompson <kst-u@mib.org> - 2018-02-07 16:59 -0800
        Re: What happened to short float? Philipp Klaus Krause <pkk@spth.de> - 2018-02-08 09:08 +0100
          Re: What happened to short float? Keith Thompson <kst-u@mib.org> - 2018-02-08 09:27 -0800
        Re: What happened to short float? GOTHIER Nathan <nathan.gothier@gmail.com> - 2018-02-08 09:54 +0100
          Re: What happened to short float? Keith Thompson <kst-u@mib.org> - 2018-02-08 10:13 -0800
        Re: What happened to short float? jameskuyper@verizon.net - 2018-02-08 06:36 -0800
          Re: What happened to short float? GOTHIER Nathan <nathan.gothier@gmail.com> - 2018-02-08 17:04 +0100
            Re: What happened to short float? jameskuyper@verizon.net - 2018-02-08 08:56 -0800
              Re: What happened to short float? scott@slp53.sl.home (Scott Lurndal) - 2018-02-08 17:23 +0000
              Re: What happened to short float? GOTHIER Nathan <nathan.gothier@gmail.com> - 2018-02-08 18:52 +0100
                Re: What happened to short float? jameskuyper@verizon.net - 2018-02-08 11:27 -0800
                Re: What happened to short float? GOTHIER Nathan <nathan.gothier@gmail.com> - 2018-02-08 22:58 +0100
          Re: What happened to short float? Keith Thompson <kst-u@mib.org> - 2018-02-08 10:18 -0800
      Re: What happened to short float? Jakob Bohm <jb-usenet@wisemo.com> - 2018-02-08 18:59 +0100
        Re: What happened to short float? Robert Wessel <robertwessel2@yahoo.com> - 2018-02-08 16:22 -0600
          Re: What happened to short float? Jakob Bohm <jb-usenet@wisemo.com> - 2018-02-09 15:01 +0100

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