[llvm] [SelectionDAG]: Deduce known bits from SMIN and SMAX (PR #85722)

[via llvm-commits]

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@@ -5360,10 +5368,20 @@ 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:
+    if (computeKnownBits(Op.getOperand(1), Depth + 1).isStrictlyPositive() ||
+        computeKnownBits(Op.getOperand(0), Depth + 1).isStrictlyPositive())
+      return true;
+    return isKnownNeverZero(Op.getOperand(1), Depth + 1) &&
+           isKnownNeverZero(Op.getOperand(0), Depth + 1);
   case ISD::SMIN:
+    if (computeKnownBits(Op.getOperand(1), Depth + 1).isNegative() ||
+        computeKnownBits(Op.getOperand(0), Depth + 1).isNegative())
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goldsteinn wrote:

Both here and above, think you should cache the result of `computeKnownBits` so you can check if the `KnownBits` is trivially non-zero and potentially avoid 1/2 recursive calls through `isKnownNeverZero`.

https://github.com/llvm/llvm-project/pull/85722