| Re: [eigen] Use of vec_min/max on ppc |
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- To: eigen@xxxxxxxxxxxxxxxxxxx
- Subject: Re: [eigen] Use of vec_min/max on ppc
- From: Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 17 Jan 2020 17:44:48 +0100
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Hi!
Which IEEE754 version are you referring to?
In IEEE754-1985, there was no min/max, then the 2008 draft suggested
introducing minNum/maxNum which propagate numbers, and the 2019 draft
suggests replacing those by
{min,max}imum{,Number,Magnitude,MagnitudeNumber} which either propagate
NaNs or numbers. (All to the best of my knowledge, please correct me, if
I'm wrong).
Eigen follows the `std::min`/`std::max` convention of propagating the
first argument, if one argument is NaN. SSE/AVX propagates the first,
which is why we change the order for those.
I'm really no expert on Altivec/PPC, so no idea if there is a more
efficient instruction consistent with `std::min/max`.
For reference: Here is the commit which introduced the lines you are
referring to:
https://gitlab.com/libeigen/eigen/commit/e91e314347c14774206307a91d1b427e49f9b3e2
Also, check these issues for further discussion on the topic:
https://gitlab.com/libeigen/eigen/issues/564
https://gitlab.com/libeigen/eigen/issues/1373
https://gitlab.com/libeigen/eigen/issues/1494
Cheers,
Christoph
On 17/01/2020 16.50, Everton Rufino Constantino1 wrote:
Hi,
looking at the implementation of pmin and pmax for Packet4f on
Altivec/PacketMath.h I came across the following statement to justify not using
VSX's intrinsics:
"// NOTE: about 10% slower than vec_min, but consistent with std::min and SSE regarding NaN"
I fail to understand the reasoning, afaik ppc and stdlib both are IEEE
compatible right? Can somebody please clarify the statement? I think sometime in
the past ppc had a non-IEEE mode that you could turn on or off but that's gone
for a while now.
Best regards,
Everton Constantino
<everton.constantino@xxxxxxx>
--
Dr.-Ing. Christoph Hertzberg
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