RWT Analyzes Bulldozer Benchmarks

I can't help but read the article title as a reference to a certain triple rainbow.

Reading now!

 

That was on purpose : P

David

 

I've tried to imagine what a bad case would be for BD.

I suppose it would be code that didn't use FMA, shuffled a lot (cutting FP throughput in half), had two threads slamming the write pipe with scattered writes that didn't coalesce in the write coalescing cache, and potentially wasn't blocked optimally for the smaller L1.

 

But what does the FP shuffle actually do? Sorry, but i really am clueless on this one.

 

It moves values around within a SIMD register(s).
The XBAR unit can go further in how it can permute vectors than what AVX is able, but it also takes up one of the two FP issue ports.
This could save instruction usage by having a permute move values around within and between vectors in a single operation, instead of having to use multiple less generic shuffles to achieve the same end.
That's in XOP, however, so it may be a very useful instruction that will not get used as much as it could.

 

Thank you