[llvm] [DAG] Add basic ISD::IS_FPCLASS constant/identity folds (PR #189944)

[Matt Arsenault via llvm-commits]

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@@ -16012,6 +16014,32 @@ SDValue DAGCombiner::visitAssertAlign(SDNode *N) {
   return SDValue();
 }
 
+SDValue DAGCombiner::visitIS_FPCLASS(SDNode *N) {
+  SDValue Src = N->getOperand(0);
+  FPClassTest Mask = static_cast<FPClassTest>(N->getConstantOperandVal(1));
+  EVT VT = N->getValueType(0);
+  SDLoc DL(N);
+
+  KnownFPClass Known = DAG.computeKnownFPClass(Src, Mask);
+
+  // Clear test bits we know must be false from the source value.
+  // fp_class (nnan x), qnan|snan|other -> fp_class (nnan x), other
+  // fp_class (ninf x), ninf|pinf|other -> fp_class (ninf x), other
+  if ((Mask & Known.KnownFPClasses) != Mask)
+    return DAG.getNode(
+        ISD::IS_FPCLASS, DL, VT, Src,
+        DAG.getTargetConstant(Mask & Known.KnownFPClasses, DL, MVT::i32),
+        N->getFlags());
+
+  // If none of the tests which can return false are possible, fold to true.
+  // fp_class (nnan x), ~(qnan|snan) -> true
+  // fp_class (ninf x), ~(ninf|pinf) -> true
+  if (Mask == Known.KnownFPClasses)
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arsenm wrote:

Shouldn't this be a subset test instead of equality? KnownFPClasses as fcNone should also fold to false? 

https://github.com/llvm/llvm-project/pull/189944