[llvm] [DirectX] Implement `llvm.is.fpclass` lowering for the fcNegZero FPClassTest and the `IsNaN`, `IsInf`, `IsFinite`, `IsNormal` DXIL ops (PR #138048)

[via llvm-commits]

================
@@ -273,6 +274,59 @@ static Value *expandExpIntrinsic(CallInst *Orig) {
   return Exp2Call;
 }
 
+static Value *expandIsFPClass(CallInst *Orig) {
+  Value *T = Orig->getArgOperand(1);
+  auto *TCI = dyn_cast<ConstantInt>(T);
+
+  // These FPClassTest cases have DXIL opcodes, so they will be handled in
+  // DXIL Op Lowering instead.
+  switch (TCI->getZExtValue()) {
+  case FPClassTest::fcInf:
+  case FPClassTest::fcNan:
+  case FPClassTest::fcNormal:
+  case FPClassTest::fcFinite:
+    return nullptr;
+  }
+
+  IRBuilder<> Builder(Orig);
+
+  Value *F = Orig->getArgOperand(0);
+  Type *FTy = F->getType();
+  unsigned FNumElem = 0; // 0 => F is not a vector
+
+  unsigned BitWidth; // Bit width of F or the ElemTy of F
+  Type *BitCastTy;   // An IntNTy of the same bitwidth as F or ElemTy of F
+
+  if (auto *FVecTy = dyn_cast<FixedVectorType>(FTy)) {
+    Type *ElemTy = FVecTy->getElementType();
+    FNumElem = FVecTy->getNumElements();
+    BitWidth = ElemTy->getPrimitiveSizeInBits();
+    BitCastTy = FixedVectorType::get(Builder.getIntNTy(BitWidth), FNumElem);
+  } else {
+    BitWidth = FTy->getPrimitiveSizeInBits();
+    BitCastTy = Builder.getIntNTy(BitWidth);
+  }
+
+  Value *FBitCast = Builder.CreateBitCast(F, BitCastTy);
+  switch (TCI->getZExtValue()) {
+  case FPClassTest::fcNegZero: {
+    Value *NegZero =
+        ConstantInt::get(Builder.getIntNTy(BitWidth), 1 << (BitWidth - 1));
+    Value *RetVal;
+    if (FNumElem) {
+      Value *NegZeroSplat = Builder.CreateVectorSplat(FNumElem, NegZero);
+      RetVal =
+          Builder.CreateICmpEQ(FBitCast, NegZeroSplat, "is.fpclass.negzero");
+    } else
+      RetVal = Builder.CreateICmpEQ(FBitCast, NegZero, "is.fpclass.negzero");
+    return RetVal;
+  }
+  default:
----------------
joaosaffran wrote:

If this switch is fully covered we don't need the default here, we can move it to after the switch like:

```cpp
switch (TCI->getZExtValue()) {
  case FPClassTest::fcNegZero: {
    ...
    return RetVal;
  }
}
report_fatal_error(Twine("Unsupported FPClassTest"),
                       /* gen_crash_diag=*/false);
```


https://github.com/llvm/llvm-project/pull/138048