[llvm] [SelectionDAG] Use `KnownBits` to determine if an operand may be NaN. (PR #188606)

[via llvm-commits]

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@@ -6251,6 +6193,38 @@ bool SelectionDAG::isKnownNeverNaN(SDValue Op, const APInt &DemandedElts,
                                                Depth);
     }
 
+    // Try to infer NaN from known bits, but only for detecting signaling or
+    // nonsignaling NaNs
+    EVT VT = Op.getValueType();
+    if (!SNaN && VT.isFloatingPoint()) {
+      const fltSemantics &FltSem = VT.getFltSemantics();
+      if (FltSem.precision > 0) {
+        KnownBits Known = computeKnownBits(Op, DemandedElts);
+        unsigned Mantissa = FltSem.precision - 1;
+        unsigned Exponent = FltSem.sizeInBits - FltSem.precision;
+        KnownBits KnownMan = Known.extractBits(Mantissa, 0);
+        KnownBits KnownExp = Known.extractBits(Exponent, Mantissa);
+
+        switch (FltSem.nanEncoding) {
+        case fltNanEncoding::IEEE: {
+          if (!KnownExp.getMaxValue().isAllOnes() || KnownMan.isZero())
+            return true;
+          break;
+        }
+        case fltNanEncoding::AllOnes: {
+          if (!KnownExp.getMaxValue().isAllOnes() ||
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zGoldthorpe wrote:

The pattern enforces that it *may* be all ones. If the expression is false, then it is known that it can never be NaN.

https://github.com/llvm/llvm-project/pull/188606