[llvm-dev] ShuffleKind SK_ExtractSubvector

[Jonas Paulsson via llvm-dev]

Hi,

I am a little unsure about the semantics of the ShuffleKind 
SK_ExtractSubvector. It seems a subvector is to be extracted, starting 
from a given index of a given subtype.

First of all, if index 0 is passed, I suppose this would mean a noop?

But what about calls like the one made of LoopVectorizer for 
Instruction::PHI in getInstructionCost():

return TTI.getShuffleCost(TargetTransformInfo::SK_ExtractSubvector,
                          VectorTy, VF - 1, VectorTy);

Here the highest index is passed, which doesn't make sense to me. Nor 
does it make sense to pass the the same VectorTy in both parameters.

In BBVectorize, start index 0 is passed in one place, but then in 
another place start index of 'VF' is passed, which should even be 
outside possible indexes, or?

I guess this is confusing since there are those extra parameters and 
everything, but in the end it seems to me there is no code anywhere 
checking this particular ShuffleKind, or?

/Jonas