[llvm] [RISCV][GISel] Add support for G_IS_FPCLASS in F and D extensions (PR #72000)

[Min-Yih Hsu via llvm-commits]

================
@@ -948,6 +952,30 @@ bool RISCVInstructionSelector::selectFPCompare(MachineInstr &MI,
   return true;
 }
 
+bool RISCVInstructionSelector::selectIsFPClass(MachineInstr &MI,
+                                               MachineIRBuilder &MIB,
+                                               MachineRegisterInfo &MRI) const {
+  Register CheckResult = MI.getOperand(0).getReg();
+  Register Src = MI.getOperand(1).getReg();
+  int64_t MaskImm = MI.getOperand(2).getImm();
+  unsigned NewOpc = MRI.getType(Src).getSizeInBits() == 32 ? RISCV::FCLASS_S
+                                                           : RISCV::FCLASS_D;
+
+  Register FClassResult = MRI.createVirtualRegister(&RISCV::GPRRegClass);
+  // Insert FCLASS_S/D.
+  auto FClass = MIB.buildInstr(NewOpc, {FClassResult}, {Src});
+  if (!FClass.constrainAllUses(TII, TRI, RBI))
+    return false;
+  // Insert AND to check Src aginst the mask.
+  auto And = MIB.buildInstr(RISCV::ANDI, {CheckResult}, {FClassResult})
+                 .addImm(MaskImm);
+  if (!And.constrainAllUses(TII, TRI, RBI))
+    return false;
+
----------------
mshockwave wrote:

I think G_IS_FPCLASS's [doc](https://llvm.org/docs/GlobalISel/GenericOpcode.html#g-is-fpclass) states that any non-zero values is allowed to represent true: "...Returns non-zero (true) or zero (false). It’s target specific whether a true value is 1, ~0U, or some other non-zero value."

https://github.com/llvm/llvm-project/pull/72000