[X86] Use GetDemandedBits to simplify the operands of PMULDQ/PMULUDQ.

This is an alternative to what I attempted in D56057.

GetDemandedBits is a special version of SimplifyDemandedBits that allows simplifications even when the operand has other uses. GetDemandedBits will only do simplifications that allow a node to be bypassed. It won't create new nodes or alter any of the other users.

I had to add support for bypassing SIGN_EXTEND_INREG to GetDemandedBits.

Based on a patch that Simon Pilgrim sent me in email.

Fixes PR40142.

llvm-svn: 350059

1 files changed, 9 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index b4947de31c9..f38770b773f 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2118,6 +2118,15 @@ SDValue SelectionDAG::GetDemandedBits(SDValue V, const APInt &Mask) {
       return getNode(ISD::ANY_EXTEND, SDLoc(V), V.getValueType(), DemandedSrc);
     break;
   }
+  case ISD::SIGN_EXTEND_INREG:
+    EVT ExVT = cast<VTSDNode>(V.getOperand(1))->getVT();
+    unsigned ExVTBits = ExVT.getScalarSizeInBits();
+
+    // If none of the extended bits are demanded, eliminate the sextinreg.
+    if (Mask.getActiveBits() <= ExVTBits)
+      return V.getOperand(0);
+
+    break;
   }
   return SDValue();
 }