[llvm] [DAG] computeKnownFPClass - add ISD::SINT_TO_FP/UINT_TO_FP handling (PR #190539)

[Simon Pilgrim via llvm-commits]

================
@@ -6103,6 +6103,34 @@ KnownFPClass SelectionDAG::computeKnownFPClass(SDValue Op,
     Known = computeKnownFPClass(Op.getOperand(0), InterestedClasses, Depth + 1);
     break;
   }
+  case ISD::SINT_TO_FP:
+  case ISD::UINT_TO_FP: {
+    // Cannot produce nan
+    Known.knownNot(fcNan);
+
+    // Integers cannot be subnormal
+    Known.knownNot(fcSubnormal);
+
+    // sitofp and uitofp turn into +0.0 for zero.
+    Known.knownNot(fcNegZero);
+
+    // UIToFP is always non-negative regardless of known bits.
+    if (Opcode == ISD::UINT_TO_FP)
+      Known.signBitMustBeZero();
+
+    if (!(InterestedClasses & (fcPosZero | fcNormal | fcInf)))
+      break;
----------------
RKSimon wrote:

I realise this is useful for compile time performance (and it shold stay) - but if there's any chance that KnownFPClass::sitofp/uitofp are used on their own we should make sure that they mask the Nan/Subnormal/NegZero cases again.

https://github.com/llvm/llvm-project/pull/190539