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mSSM

Reputation: 608

What are _mm256_testc_pd, _mm256_testz_pd, _mm256_testnzc_pd for?

I am trying to understand the _mm256_testc_pd, _mm256_testz_pd, and _mm256_testnzc_pd intrinsics, and I have a hard time understanding them.

To analyze _mm256_testc_pd, I have identified the following cases (a is the first, b the second __mm256d argument`):

In other words, a value of ZF=1 tells me that either a) b is entirely positive, or b) that for those doubles in b that are not positive, there is a matching double in a that is negative. A value of CF=1 tells me that either c) b is entirely positive, or d) that for those doubles in b that are not positive, there is a matching double in in !a that is negative.

Have I understood this correctly? I am a bit confused by this. What's the point of this check? What would I use these intrinsics for?

Upvotes: 2

Views: 549

Answers (1)

Paul R
Paul R

Reputation: 212979

The most common use case is testing the result of a compare operation, where the result elements from the comparison are either all 1 or all 0 (so testing the sign bit is sufficient) - this enables you to implement predicates such as "all equal" or "any greater than" etc, using the _mm*_testz_p* intrinsic.

I've never found a use for any of the other variants.

Upvotes: 2

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