[clang] [llvm] [ValueTracking] use KnownBits to compute fpclass from bitcast (PR #97762)

[Alex MacLean via cfe-commits]

================
@@ -5805,6 +5805,37 @@ void computeKnownFPClass(const Value *V, const APInt &DemandedElts,
 
     break;
   }
+  case Instruction::BitCast: {
+    const Value *Src;
+    if (!match(Op, m_ElementWiseBitCast(m_Value(Src))) ||
+        !Src->getType()->isIntOrIntVectorTy())
+      break;
+
+    const Type *Ty = Op->getType()->getScalarType();
+    KnownBits Bits(Ty->getScalarSizeInBits());
+    computeKnownBits(Src, DemandedElts, Bits, Depth + 1, Q);
+
+    // Transfer information from the sign bit.
+    if (Bits.isNonNegative())
+      Known.signBitMustBeZero();
+    else if (Bits.isNegative())
+      Known.signBitMustBeOne();
+
+    if (Ty->isIEEE()) {
+      // IEEE floats are NaN when all bits of the exponent plus at least one of
+      // the fraction bits are 1. This means:
+      //   - If we assume unknown bits are 0 and the value is NaN, it will
+      //     always be NaN
+      //   - If we assume unknown bits are 1 and the value is not NaN, it can
+      //     never be NaN
+      if (APFloat(Ty->getFltSemantics(), Bits.One).isNaN())
+        Known.KnownFPClasses = fcNan;
+      else if (!APFloat(Ty->getFltSemantics(), ~Bits.Zero).isNaN())
+        Known.knownNot(fcNan);
----------------
AlexMaclean wrote:

Okay, I've added `inf` and also `zero` since those are both relatively simple, but I've left normal / subnormal  to the side for now.

https://github.com/llvm/llvm-project/pull/97762