[X86] Add X86ISD::VTRUNC to computeKnownBitsForTargetNode.

We have to take special care to avoid the cases where the result of the truncate would be padded with zero elements.

Ideally we'd just use ISD::TRUNCATE for these cases instead.

llvm-svn: 322454

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index f8e0a939983..c1cbb64eeb5 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -27837,6 +27837,18 @@ void X86TargetLowering::computeKnownBitsForTargetNode(const SDValue Op,
     Known.Zero.setBitsFrom(InBitWidth);
     break;
   }
+  case X86ISD::VTRUNC: {
+    // TODO: Add DemandedElts support.
+    SDValue N0 = Op.getOperand(0);
+    // We can only handle cases with the same number of elements. Otherwise
+    // the truncate fills with zero elements.
+    // TODO: Maybe we could just discard any 1s we found instead of skipping?
+    if (VT.getVectorNumElements() != N0.getValueType().getVectorNumElements())
+      break;
+    DAG.computeKnownBits(N0, Known, Depth+1);
+    Known = Known.trunc(BitWidth);
+    break;
+  }
   case X86ISD::CMOV: {
     DAG.computeKnownBits(Op.getOperand(1), Known, Depth+1);
     // If we don't know any bits, early out.