[SelectionDAG]: Add more cases for UDIV, SDIV, SRA, and SRL

Author: AZero13

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -5443,19 +5443,59 @@ bool SelectionDAG::isKnownNeverZero(SDValue Op, unsigned Depth) const {
     if (ValKnown.isNegative())
       return true;
     // If max shift cnt of known ones is non-zero, result is non-zero.
-    APInt MaxCnt = computeKnownBits(Op.getOperand(1), Depth + 1).getMaxValue();
+    const KnownBits Shift = computeKnownBits(Op.getOperand(1), Depth + 1);
+    APInt MaxCnt = Shift.getMaxValue();
     if (MaxCnt.ult(ValKnown.getBitWidth()) &&
         !ValKnown.One.lshr(MaxCnt).isZero())
       return true;
+    // Similar to udiv but we try to see if we can turn it into a division
+    const KnownBits One =
+        KnownBits::makeConstant(APInt(ValKnown.getBitWidth(), 1));
+
+    std::optional<bool> uge =
+        KnownBits::uge(ValKnown, KnownBits::shl(One, Shift));
+    if (uge && *uge)
+      return true;
     break;
   }
-  case ISD::UDIV:
-  case ISD::SDIV:
+  case ISD::UDIV: {
+    if (Op->getFlags().hasExact())
+      return isKnownNeverZero(Op.getOperand(0), Depth + 1);
+    KnownBits Op0 = computeKnownBits(Op.getOperand(0), Depth + 1);
+    KnownBits Op1 = computeKnownBits(Op.getOperand(1), Depth + 1);
+    // True if Op0 u>= Op1
+
+    std::optional<bool> uge = KnownBits::uge(Op0, Op1);
+    if (uge && *uge)
+      return true;
+    break;
+  }
+  case ISD::SDIV: {
     // div exact can only produce a zero if the dividend is zero.
-    // TODO: For udiv this is also true if Op1 u<= Op0
     if (Op->getFlags().hasExact())
       return isKnownNeverZero(Op.getOperand(0), Depth + 1);
+    KnownBits Op0 = computeKnownBits(Op.getOperand(0), Depth + 1);
+    KnownBits Op1 = computeKnownBits(Op.getOperand(1), Depth + 1);
+    if (Op0.isNegative() && Op1.isStrictlyPositive())
+      return true;
+
+    if (Op0.isStrictlyPositive() && Op1.isNegative())
+      return true;
+
+    // For negative numbers, the comparison is reversed. Op0 <= Op1
+    if (Op0.isNegative() && Op1.isNegative()) {
+      std::optional<bool> sle = KnownBits::sle(Op0, Op1);
+      if (sle && *sle)
+        return true;
+    }
+
+    if (Op0.isStrictlyPositive() && Op1.isStrictlyPositive()) {
+      std::optional<bool> uge = KnownBits::uge(Op0, Op1);
+      if (uge && *uge)
+        return true;
+    }
     break;
+  }
 
   case ISD::ADD:
     if (Op->getFlags().hasNoUnsignedWrap())

llvm/lib/Support/KnownBits.cpp

@@ -1009,6 +1009,9 @@ KnownBits KnownBits::udiv(const KnownBits &LHS, const KnownBits &RHS,
 
   Known.Zero.setHighBits(LeadZ);
   Known = divComputeLowBit(Known, LHS, RHS, Exact);
+  std::optional<bool> uge = KnownBits::uge(LHS, RHS);
+  if (uge && *uge)
+    Known.makeGE(APInt(BitWidth, 1));
 
   assert(!Known.hasConflict() && "Bad Output");
   return Known;