[llvm] [DAG] Add initial version of SelectionDAG::computeKnownFPClass (PR #188790)

[Xinlong Chen via llvm-commits]

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@@ -6045,6 +5988,82 @@ bool SelectionDAG::isBaseWithConstantOffset(SDValue Op) const {
          (Op.isAnyAdd() || isADDLike(Op));
 }
 
+KnownFPClass SelectionDAG::computeKnownFPClass(SDValue Op,
+                                               FPClassTest InterestedClasses,
+                                               unsigned Depth) const {
+  EVT VT = Op.getValueType();
+  APInt DemandedElts = VT.isFixedLengthVector()
+                           ? APInt::getAllOnes(VT.getVectorNumElements())
+                           : APInt(1, 1);
+  return computeKnownFPClass(Op, DemandedElts, InterestedClasses, Depth);
+}
+
+KnownFPClass SelectionDAG::computeKnownFPClass(SDValue Op,
+                                               const APInt &DemandedElts,
+                                               FPClassTest InterestedClasses,
+                                               unsigned Depth) const {
+  KnownFPClass Known;
+
+  if (auto *CFP = dyn_cast<ConstantFPSDNode>(Op)) {
+    Known.KnownFPClasses = CFP->getValueAPF().classify();
+    Known.SignBit = CFP->getValueAPF().isNegative();
+    return Known;
+  }
+
+  if (Depth >= MaxRecursionDepth)
+    return Known;
+
+  if (Op.getOpcode() == ISD::UNDEF)
+    return Known;
+
+  EVT VT = Op.getValueType();
+  assert((!VT.isFixedLengthVector() ||
+          DemandedElts.getBitWidth() == VT.getVectorNumElements()) &&
+         "Unexpected vector size");
+
+  if (!DemandedElts)
+    return Known;
+
+  unsigned Opcode = Op.getOpcode();
+  switch (Opcode) {
+  case ISD::POISON: {
+    Known.KnownFPClasses = fcNone;
+    Known.SignBit = false;
+    break;
+  }
+  case ISD::BUILD_VECTOR: {
+    assert(!VT.isScalableVector());
+    bool First = true;
+    for (unsigned I = 0, E = Op.getNumOperands(); I != E; ++I) {
+      if (!DemandedElts[I])
+        continue;
+
+      if (First) {
----------------
Xinlong-Chen wrote:

@RKSimon yes, the First flag pattern here follows the same approach used in `GISelValueTracking::computeKnownFPClass` for `G_BUILD_VECTOR` handling.

Since KnownFPClass defaults to unknown and `|=` only widens, we need the first demanded element as the starting point.

https://github.com/llvm/llvm-project/pull/188790