[PATCH] D39368: DAG: Add computeKnownBitsForFrameIndex

[Justin Bogner via llvm-commits]

Matt Arsenault via Phabricator <reviews at reviews.llvm.org> writes:
> arsenm created this revision.
> Herald added subscribers: tpr, wdng.
>
> Some of the AMDGPU stack addressing modes require knowing the sign bit
> is zero. We used to accomplish this by custom lowering frame indexes,
> and then putting an AssertZext around a TargetFrameIndex. This
> required specifically looking for the AssextZext + frame index pattern
> which was moderately disgusting. The same could probably be
> accomplished with a target specific node, but would still require
> special handling of frame indexes.

Adding this hook seems fine, but please add a documentation comment
explaining what its for to the default implementation.

>
> https://reviews.llvm.org/D39368
>
> Files:
>   include/llvm/Target/TargetLowering.h
>   lib/CodeGen/SelectionDAG/SelectionDAG.cpp
>
> Index: lib/CodeGen/SelectionDAG/SelectionDAG.cpp
> ===================================================================
> --- lib/CodeGen/SelectionDAG/SelectionDAG.cpp
> +++ lib/CodeGen/SelectionDAG/SelectionDAG.cpp
> @@ -2854,8 +2854,9 @@
>      if (unsigned Align = InferPtrAlignment(Op)) {
>        // The low bits are known zero if the pointer is aligned.
>        Known.Zero.setLowBits(Log2_32(Align));
> -      break;
>      }
> +
> +    TLI->computeKnownBitsForFrameIndex(Op, Known, DemandedElts, *this, Depth);
>      break;
>  
>    default:
> Index: include/llvm/Target/TargetLowering.h
> ===================================================================
> --- include/llvm/Target/TargetLowering.h
> +++ include/llvm/Target/TargetLowering.h
> @@ -2678,6 +2678,14 @@
>                                               const SelectionDAG &DAG,
>                                               unsigned Depth = 0) const;
>  
> +  virtual void computeKnownBitsForFrameIndex(const SDValue Op,
> +                                             KnownBits &Known,
> +                                             const APInt &DemandedElts,
> +                                             const SelectionDAG &DAG,
> +                                             unsigned Depth = 0) const {
> +    return;
> +  }
> +
>    /// This method can be implemented by targets that want to expose additional
>    /// information about sign bits to the DAG Combiner. The DemandedElts
>    /// argument allows us to only collect the minimum sign bits that are shared