[SelectionDAG] Add knownbits support for EXTRACT_VECTOR_ELT opcodes (REAPPLIED)

Reapplied with fix for PR31323 - X86 SSE2 vXi16 multiplies for illegal types were creating CONCAT_VECTORS nodes with vector inputs that might not total the number of elements in the result type.

llvm-svn: 289232

1 files changed, 36 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 75b4e8b6d68..d80048c1396 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2631,6 +2631,42 @@ void SelectionDAG::computeKnownBits(SDValue Op, APInt &KnownZero,
     }
     break;
   }
+  case ISD::INSERT_VECTOR_ELT: {
+    SDValue InVec = Op.getOperand(0);
+    SDValue InVal = Op.getOperand(1);
+    SDValue EltNo = Op.getOperand(2);
+
+    ConstantSDNode *CEltNo = dyn_cast<ConstantSDNode>(EltNo);
+    if (CEltNo && CEltNo->getAPIntValue().ult(NumElts)) {
+      // If we know the element index, split the demand between the
+      // source vector and the inserted element.
+      KnownZero = KnownOne = APInt::getAllOnesValue(BitWidth);
+      unsigned EltIdx = CEltNo->getZExtValue();
+
+      // If we demand the inserted element then add its common known bits.
+      if (DemandedElts[EltIdx]) {
+        computeKnownBits(InVal, KnownZero2, KnownOne2, Depth + 1);
+        KnownOne &= KnownOne2.zextOrTrunc(KnownOne.getBitWidth());
+        KnownZero &= KnownZero2.zextOrTrunc(KnownZero.getBitWidth());;
+      }
+
+      // If we demand the source vector then add its common known bits, ensuring
+      // that we don't demand the inserted element.
+      APInt VectorElts = DemandedElts & ~(APInt::getOneBitSet(NumElts, EltIdx));
+      if (!!VectorElts) {
+        computeKnownBits(InVec, KnownZero2, KnownOne2, VectorElts, Depth + 1);
+        KnownOne &= KnownOne2;
+        KnownZero &= KnownZero2;
+      }
+    } else {
+      // Unknown element index, so ignore DemandedElts and demand them all.
+      computeKnownBits(InVec, KnownZero, KnownOne, Depth + 1);
+      computeKnownBits(InVal, KnownZero2, KnownOne2, Depth + 1);
+      KnownOne &= KnownOne2.zextOrTrunc(KnownOne.getBitWidth());
+      KnownZero &= KnownZero2.zextOrTrunc(KnownZero.getBitWidth());;
+    }
+    break;
+  }
   case ISD::BSWAP: {
     computeKnownBits(Op.getOperand(0), KnownZero2, KnownOne2, DemandedElts,
                      Depth + 1);