Re: [eigen] Use of vec_min/max on ppc

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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:

Also, check these issues for further discussion on the topic:


On 17/01/2020 16.50, Everton Rufino Constantino1 wrote:
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

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