[SelectionDAG][X86] CombineBT - more aggressively determine demanded bits

This patch is in 2 parts:

1 - replace combineBT's use of SimplifyDemandedBits (hasOneUse only) with SelectionDAG::GetDemandedBits to more aggressively determine the lower bits used by BT.

2 - update SelectionDAG::GetDemandedBits to support ANY_EXTEND - if the demanded bits are only in the non-extended portion, then peek through and demand from the source value and then ANY_EXTEND that if we found a match.

Differential Revision: https://reviews.llvm.org/D35896

llvm-svn: 309486

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index da69ef3b231..a396f3368cf 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -1999,6 +1999,18 @@ SDValue SelectionDAG::GetDemandedBits(SDValue V, const APInt &Mask) {
       return V.getOperand(0);
     break;
   }
+  case ISD::ANY_EXTEND: {
+    SDValue Src = V.getOperand(0);
+    unsigned SrcBitWidth = Src.getScalarValueSizeInBits();
+    // Being conservative here - only peek through if we only demand bits in the
+    // non-extended source (even though the extended bits are technically undef).
+    if (Mask.getActiveBits() > SrcBitWidth)
+      break;
+    APInt SrcMask = Mask.trunc(SrcBitWidth);
+    if (SDValue DemandedSrc = GetDemandedBits(Src, SrcMask))
+      return getNode(ISD::ANY_EXTEND, SDLoc(V), V.getValueType(), DemandedSrc);
+    break;
+  }
   }
   return SDValue();
 }