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

[Alex MacLean via cfe-commits]

================
@@ -5921,6 +5921,61 @@ 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);
+
+      // Build KnownBits representing Inf and check if it must be equal or
+      // unequal to this value.
+      auto InfKB = KnownBits::makeConstant(
+          APFloat::getInf(Ty->getFltSemantics()).bitcastToAPInt());
+      InfKB.Zero.clearSignBit();
+      if (const auto InfResult = KnownBits::eq(Bits, InfKB)) {
----------------
AlexMaclean wrote:

If that is the case the values will be:
```
Bits  = 1 11111111 00000000000000000000000
InfKB = ? 11111111 00000000000000000000000
```
These may or may not be equal so `std::nullopt` will be returned and no information will be added to the fpclass. I suppose we could handle this case but it will be constant folded anyway so I don't think it is really necessary. 


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