[PATCH 2/3] SelectionDAG: Add KnownBits and SignBits computation for EXTRACT_ELEMENT

[Matt Arsenault]

> On Nov 18, 2014, at 11:06 AM, Jan Vesely <jan.vesely at rutgers.edu> wrote:
> 
> v2: use getZExtValue
>    add missing break
>    codestyle
> 
> Signed-off-by: Jan Vesely <jan.vesely at rutgers.edu>
> ---
> lib/CodeGen/SelectionDAG/SelectionDAG.cpp | 22 ++++++++++++++++++++++
> 1 file changed, 22 insertions(+)
> 
> diff --git a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
> index 33620e7..09e0239 100644
> --- a/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
> +++ b/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
> @@ -2323,6 +2323,19 @@ void SelectionDAG::computeKnownBits(SDValue Op, APInt &KnownZero,
>     KnownZero = APInt::getHighBitsSet(BitWidth, Leaders);
>     break;
>   }
> +  case ISD::EXTRACT_ELEMENT: {
> +    computeKnownBits(Op.getOperand(0), KnownZero, KnownOne, Depth+1);
> +    const unsigned Index =
> +      cast<ConstantSDNode>(Op.getOperand(1))->getZExtValue();
> +    const unsigned BitWidth = Op.getValueType().getSizeInBits();
> +
> +    KnownZero = KnownZero.getHiBits(KnownZero.getBitWidth() - Index * BitWidth);
> +    KnownOne = KnownOne.getHiBits(KnownOne.getBitWidth() - Index * BitWidth);
> +
> +    KnownZero = KnownZero.trunc(BitWidth);
> +    KnownOne = KnownOne.trunc(BitWidth);
> +    break;
> +  }
>   case ISD::FrameIndex:
>   case ISD::TargetFrameIndex:
>     if (unsigned Align = InferPtrAlignment(Op)) {
> @@ -2522,6 +2535,15 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, unsigned Depth) const{
>     // FIXME: it's tricky to do anything useful for this, but it is an important
>     // case for targets like X86.
>     break;
> +  case ISD::EXTRACT_ELEMENT: {
> +    const int KnownSign = ComputeNumSignBits(Op.getOperand(0), Depth+1);
> +    const int BitWidth = Op.getValueType().getSizeInBits();
> +    const int Items =
> +      Op.getOperand(0).getValueType().getSizeInBits() / BitWidth;
> +    const int rIndex = Items - 1 -
> +      cast<ConstantSDNode>(Op.getOperand(1))->getZExtValue();
> +    return std::max(std::min(KnownSign - rIndex * BitWidth, BitWidth), 0);
> +  }
>   }
> 
>   // If we are looking at the loaded value of the SDNode.
> -- 
> 1.9.3
> 
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits


These need tests. Do these really work? I remember trying to do this at some point, but then gave up because computeKnownBits doesn’t understand vectors very well

-Matt