[Python-ideas] Fused multiply-add (FMA)

[Xavier Combelle]

I never said it was impossible, just very hard.


Le 17/01/2017 à 16:48, Stephan Houben a écrit :
> Hi Xavier,
>
> In this bright age of IEEE-754 compatible CPUs,
> it is certainly possible to achieve reproducible FP.
> I worked for a company whose software produced bit-identical
> results on various CPUs (x86, Sparc, Itanium) and OSes (Linux,
> Solaris, Windows).
>
> The trick is to closely RTFM for your CPU and compiler, in particular
> all those nice
> appendices related to "FPU control words" and "FP consistency models".
>
> For example, if the author of that article had done so, he might have
> learned
> about the "precision control" field of the x87 status register, which
> you can set
> so that all intermediate operations are always represented as 64-bits
> doubles.
> So no double roundings from double-extended precision.
>
> (Incidentally, the x87-internal double-extended precision is another
> fine example where 
> being "more precise on occasion"  usually does not help.)
>
> Frankly not very impressed with that article. 
> I could go in detail but that's off-topic, and I will try to fight
> the "somebody is *wrong* on the Internet" urge.
>
> Stephan
>
> 2017-01-17 16:04 GMT+01:00 Xavier Combelle <xavier.combelle at gmail.com
> <mailto:xavier.combelle at gmail.com>>:
>
>
>>     Generally speaking, there are two reasons why people may *not*
>>     want an FMA operation.
>>     1. They need their results to be reproducible across
>>     compilers/platforms. (the most common reason)
>>
>     The reproducibility of floating point calculation is very hard to
>     reach  a good survey of the problem is
>     https://randomascii.wordpress.com/2013/07/16/floating-point-determinism/
>     <https://randomascii.wordpress.com/2013/07/16/floating-point-determinism/>
>     it mention the fma problem but it only a part of a biggest picture
>
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