[DAG] computeKnownBits - Move ISD::SRA handling into KnownBits::ashr

As discussed on D90527, we should be trying to move shift handling functionality into KnownBits to avoid code duplication in SelectionDAG/GlobalISel/ValueTracking.

Author: RKSimon

llvm/include/llvm/Support/KnownBits.h

@@ -278,6 +278,10 @@ struct KnownBits {
   /// NOTE: RHS (shift amount) bitwidth doesn't need to be the same as LHS.
   static KnownBits lshr(const KnownBits &LHS, const KnownBits &RHS);
 
+  /// Compute known bits for ashr(LHS, RHS).
+  /// NOTE: RHS (shift amount) bitwidth doesn't need to be the same as LHS.
+  static KnownBits ashr(const KnownBits &LHS, const KnownBits &RHS);
+
   /// Insert the bits from a smaller known bits starting at bitPosition.
   void insertBits(const KnownBits &SubBits, unsigned BitPosition) {
     Zero.insertBits(SubBits.Zero, BitPosition);

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -2979,13 +2979,10 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
       Known.Zero.setHighBits(ShMinAmt->getZExtValue());
     break;
   case ISD::SRA:
-    if (const APInt *ShAmt = getValidShiftAmountConstant(Op, DemandedElts)) {
-      Known = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
-      unsigned Shift = ShAmt->getZExtValue();
-      // Sign extend known zero/one bit (else is unknown).
-      Known.Zero.ashrInPlace(Shift);
-      Known.One.ashrInPlace(Shift);
-    }
+    Known = computeKnownBits(Op.getOperand(0), DemandedElts, Depth + 1);
+    Known2 = computeKnownBits(Op.getOperand(1), DemandedElts, Depth + 1);
+    Known = KnownBits::ashr(Known, Known2);
+    // TODO: Add minimum shift high known sign bits.
     break;
   case ISD::FSHL:
   case ISD::FSHR:

llvm/lib/Support/KnownBits.cpp

@@ -192,6 +192,23 @@ KnownBits KnownBits::lshr(const KnownBits &LHS, const KnownBits &RHS) {
   return Known;
 }
 
+KnownBits KnownBits::ashr(const KnownBits &LHS, const KnownBits &RHS) {
+  unsigned BitWidth = LHS.getBitWidth();
+  KnownBits Known(BitWidth);
+
+  if (RHS.isConstant() && RHS.getConstant().ult(BitWidth)) {
+    unsigned Shift = RHS.getConstant().getZExtValue();
+    Known = LHS;
+    Known.Zero.ashrInPlace(Shift);
+    Known.One.ashrInPlace(Shift);
+    return Known;
+  }
+
+  // TODO: Minimum shift amount high bits are known sign bits.
+  // TODO: No matter the shift amount, the leading sign bits will stay.
+  return Known;
+}
+
 KnownBits KnownBits::abs() const {
   // If the source's MSB is zero then we know the rest of the bits already.
   if (isNonNegative())