[SelectionDAG]: Deduce KnownNeverZero from SMIN and SMAX (#85722)

Author: AZero13

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -5362,10 +5362,44 @@ bool SelectionDAG::isKnownNeverZero(SDValue Op, unsigned Depth) const {
     return isKnownNeverZero(Op.getOperand(1), Depth + 1) ||
            isKnownNeverZero(Op.getOperand(0), Depth + 1);
 
-    // TODO for smin/smax: If either operand is known negative/positive
+    // For smin/smax: If either operand is known negative/positive
     // respectively we don't need the other to be known at all.
-  case ISD::SMAX:
-  case ISD::SMIN:
+  case ISD::SMAX: {
+    KnownBits Op1 = computeKnownBits(Op.getOperand(1), Depth + 1);
+    if (Op1.isStrictlyPositive())
+      return true;
+
+    KnownBits Op0 = computeKnownBits(Op.getOperand(0), Depth + 1);
+    if (Op0.isStrictlyPositive())
+      return true;
+
+    if (Op1.isNonZero() && Op0.isNonZero())
+      return true;
+
+    if (KnownBits::smax(Op0, Op1).isNonZero())
+      return true;
+
+    return isKnownNeverZero(Op.getOperand(1), Depth + 1) &&
+           isKnownNeverZero(Op.getOperand(0), Depth + 1);
+  }
+  case ISD::SMIN: {
+    KnownBits Op1 = computeKnownBits(Op.getOperand(1), Depth + 1);
+    if (Op1.isNegative())
+      return true;
+
+    KnownBits Op0 = computeKnownBits(Op.getOperand(0), Depth + 1);
+    if (Op0.isNegative())
+      return true;
+
+    if (Op1.isNonZero() && Op0.isNonZero())
+      return true;
+
+    if (KnownBits::smin(Op0, Op1).isNonZero())
+      return true;
+
+    return isKnownNeverZero(Op.getOperand(1), Depth + 1) &&
+           isKnownNeverZero(Op.getOperand(0), Depth + 1);
+  }
   case ISD::UMIN:
     return isKnownNeverZero(Op.getOperand(1), Depth + 1) &&
            isKnownNeverZero(Op.getOperand(0), Depth + 1);