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From: BGB <cr88192@gmail.com>
Newsgroups: comp.arch
Subject: Re: Misc: Applications of small floating point formats.
Date: Fri, 2 Aug 2024 01:27:29 -0500
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On 8/2/2024 12:53 AM, Thomas Koenig wrote:
> EricP <ThatWouldBeTelling@thevillage.com> schrieb:
> 
>> With FP128 will there again be a significant difference in speed to
>> FP62 or FP32 (including transcendentals)? Seems there would be because not
>> every HW implementation is going to implement a full width multiplier.
> 
> The only major architecture I'm aware of that uses FP128, POWER,
> chose to use their decimal FP unit do do it on the side.
> 
> This makes multiplication _really_ slow, unfortunately.

FWIW: It is sufficiently overkill for most purposes, and sufficiently 
rarely used, that there isn't a strong reason not to just do Binary128 
or similar in software.

Except maybe in some special case one actually needs fast 128-bit 
floating point.

For most things though, Binary64 is sufficient (and many use-cases exist 
where Binary32 is not sufficient, so Binary64 seems mostly necessary for 
"general use").




At one point, some years back, I had looked into whether to do Binary128 
or to do something like the .NET Decimal format.

IIRC, .NET's format was something like:
   Three 32-bit words each holding 9 decimal digits;
   Another 32-bit word with an exponent.

Each 32-bit word representing a value as a linear integer between 
000000000 and 999999999.

In my evaluation, Binary128 won (both more accurate and faster for 
general computation).

Granted, both seem likely to be faster than a software implementation of 
Decimal128.

....


In my evaluations, I ended up prioritizing 128-bit integers:
   Cheaper to implement in hardware;
   Can be used to make 128-bit floating point faster;
   More often to be useful.

Though, the use of 128-bit integers is hindered for portable code, given 
seemingly none of the major compilers support them "in general".

....