[SelectionDAG] ComputeNumSignBits - Use getValidShiftAmountConstant for shift opcodes

getValidShiftAmountConstant handles out of bounds shift amounts for us, allowing us to remove the local handling.

Author: RKSimon

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -3606,25 +3606,18 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
     Tmp = VTBits - SrcVT.getScalarSizeInBits();
     return ComputeNumSignBits(Src, DemandedSrcElts, Depth+1) + Tmp;
   }
-
   case ISD::SRA:
-    Tmp = ComputeNumSignBits(Op.getOperand(0), DemandedElts, Depth+1);
+    Tmp = ComputeNumSignBits(Op.getOperand(0), DemandedElts, Depth + 1);
     // SRA X, C   -> adds C sign bits.
-    if (ConstantSDNode *C =
-            isConstOrConstSplat(Op.getOperand(1), DemandedElts)) {
-      APInt ShiftVal = C->getAPIntValue();
-      ShiftVal += Tmp;
-      Tmp = ShiftVal.uge(VTBits) ? VTBits : ShiftVal.getZExtValue();
-    }
+    if (const APInt *ShAmt = getValidShiftAmountConstant(Op, DemandedElts))
+      Tmp = std::min<uint64_t>(Tmp + ShAmt->getZExtValue(), VTBits);
     return Tmp;
   case ISD::SHL:
-    if (ConstantSDNode *C =
-            isConstOrConstSplat(Op.getOperand(1), DemandedElts)) {
-      // shl destroys sign bits.
-      Tmp = ComputeNumSignBits(Op.getOperand(0), DemandedElts, Depth+1);
-      if (C->getAPIntValue().uge(VTBits) ||      // Bad shift.
-          C->getAPIntValue().uge(Tmp)) break;    // Shifted all sign bits out.
-      return Tmp - C->getZExtValue();
+    if (const APInt *ShAmt = getValidShiftAmountConstant(Op, DemandedElts)) {
+      // shl destroys sign bits, ensure it doesn't shift out all sign bits.
+      Tmp = ComputeNumSignBits(Op.getOperand(0), DemandedElts, Depth + 1);
+      if (ShAmt->ult(Tmp))
+        return Tmp - ShAmt->getZExtValue();
     }
     break;
   case ISD::AND: