[PATCH] D25007: [DAG] Teach computeKnownBits and ComputeNumSignBits in SelectionDAG to look through EXTRACT_VECTOR_ELT.

[Michael Kuperstein via llvm-commits]

mkuper added inline comments.


> SelectionDAG.cpp:2460
> +    }
> +    computeKnownBits(Op.getOperand(0), KnownZero, KnownOne, Depth+1);
> +    if (BitWidth > EltBitWidth) {

Note that this isn't as good as it could be - computeKnownBits for a vector returns the "lowest common denominator" for all vector elements. If you know, statically, which element you're looking for,  you could, in theory, do better.
I don't see a clean way to do this right now, but it'd perhaps worth to have a TODO. (There's already one in the BUILD_VECTOR code... :-) )

> SelectionDAG.cpp:2741
> +    if (BitWidth == EltBitWidth)
> +      return ComputeNumSignBits(Op.getOperand(0), Depth+1);
> +    break;

I'm a bit surprised that ComputeNumSignBits does the right thing for vectors. In fact, I have a creeping suspicion it doesn't.
Do you have a test for that specifically?

https://reviews.llvm.org/D25007