[DAGCombiner] Add ComputeNumSignBits vector demanded elements support to ASHR and INSERT_VECTOR_ELT (reapplied)

Reapplied r299221 after fix for nondeterminism in ThinLTO builder (rL301599), with extra check for implicit truncation of inserted element.

llvm-svn: 301644

1 files changed, 39 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 05ac837430a..2ab2e2fb44f 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -2938,7 +2938,7 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
     return std::max(Tmp, Tmp2);
 
   case ISD::SRA:
-    Tmp = ComputeNumSignBits(Op.getOperand(0), Depth+1);
+    Tmp = ComputeNumSignBits(Op.getOperand(0), DemandedElts, Depth+1);
     // SRA X, C   -> adds C sign bits.
     if (ConstantSDNode *C = isConstOrConstSplat(Op.getOperand(1))) {
       APInt ShiftVal = C->getAPIntValue();
@@ -3101,6 +3101,44 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
     // result. Otherwise it gives either negative or > bitwidth result
     return std::max(std::min(KnownSign - rIndex * BitWidth, BitWidth), 0);
   }
+  case ISD::INSERT_VECTOR_ELT: {
+    SDValue InVec = Op.getOperand(0);
+    SDValue InVal = Op.getOperand(1);
+    SDValue EltNo = Op.getOperand(2);
+    unsigned NumElts = InVec.getValueType().getVectorNumElements();
+
+    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.
+      unsigned EltIdx = CEltNo->getZExtValue();
+
+      // If we demand the inserted element then get its sign bits.
+      Tmp = UINT_MAX;
+      if (DemandedElts[EltIdx]) {
+        // TODO - handle implicit truncation of inserted elements.
+        if (InVal.getScalarValueSizeInBits() != VTBits)
+          break;
+        Tmp = ComputeNumSignBits(InVal, Depth + 1);
+      }
+
+      // If we demand the source vector then get its sign bits, and determine
+      // the minimum.
+      APInt VectorElts = DemandedElts;
+      VectorElts.clearBit(EltIdx);
+      if (!!VectorElts) {
+        Tmp2 = ComputeNumSignBits(InVec, VectorElts, Depth + 1);
+        Tmp = std::min(Tmp, Tmp2);
+      }
+    } else {
+      // Unknown element index, so ignore DemandedElts and demand them all.
+      Tmp = ComputeNumSignBits(InVec, Depth + 1);
+      Tmp2 = ComputeNumSignBits(InVal, Depth + 1);
+      Tmp = std::min(Tmp, Tmp2);
+    }
+    assert(Tmp <= VTBits && "Failed to determine minimum sign bits");
+    return Tmp;
+  }
   case ISD::EXTRACT_VECTOR_ELT: {
     SDValue InVec = Op.getOperand(0);
     SDValue EltNo = Op.getOperand(1);