[llvm] [RISCV] Use DecoderMethod and a template function to reduce the amount of boilerplate for decodings a register class. NFC (PR #177296)

[Alexander Richardson via llvm-commits]

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@@ -78,44 +78,23 @@ LLVMInitializeRISCVDisassembler() {
                                          createRISCVDisassembler);
 }
 
-static DecodeStatus DecodeGPRRegisterClass(MCInst &Inst, uint32_t RegNo,
-                                           uint64_t Address,
-                                           const MCDisassembler *Decoder) {
-  bool IsRVE = Decoder->getSubtargetInfo().hasFeature(RISCV::FeatureStdExtE);
-
-  if (RegNo >= 32 || (IsRVE && RegNo >= 16))
-    return MCDisassembler::Fail;
-
-  MCRegister Reg = RISCV::X0 + RegNo;
-  Inst.addOperand(MCOperand::createReg(Reg));
-  return MCDisassembler::Success;
-}
-
-static DecodeStatus DecodeGPRF16RegisterClass(MCInst &Inst, uint32_t RegNo,
+template <unsigned FirstReg, unsigned NumRegsInClass, bool CheckRVE = false>
+static DecodeStatus DecodeSimpleRegisterClass(MCInst &Inst, uint32_t RegNo,
                                               uint64_t Address,
                                               const MCDisassembler *Decoder) {
-  bool IsRVE = Decoder->getSubtargetInfo().hasFeature(RISCV::FeatureStdExtE);
+  bool IsRVE =
+      CheckRVE && Decoder->getSubtargetInfo().hasFeature(RISCV::FeatureStdExtE);
 
-  if (RegNo >= 32 || (IsRVE && RegNo >= 16))
+  if (RegNo >= NumRegsInClass || (IsRVE && RegNo >= 16))
     return MCDisassembler::Fail;
 
-  MCRegister Reg = RISCV::X0_H + RegNo;
+  MCRegister Reg = FirstReg + RegNo;
   Inst.addOperand(MCOperand::createReg(Reg));
   return MCDisassembler::Success;
 }
 
-static DecodeStatus DecodeGPRF32RegisterClass(MCInst &Inst, uint32_t RegNo,
-                                              uint64_t Address,
-                                              const MCDisassembler *Decoder) {
-  bool IsRVE = Decoder->getSubtargetInfo().hasFeature(RISCV::FeatureStdExtE);
-
-  if (RegNo >= 32 || (IsRVE && RegNo >= 16))
-    return MCDisassembler::Fail;
-
-  MCRegister Reg = RISCV::X0_W + RegNo;
-  Inst.addOperand(MCOperand::createReg(Reg));
-  return MCDisassembler::Success;
-}
+constexpr auto DecodeGPRRegisterClass =
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arichardson wrote:

The reason this was not just done in tablegen is that it's called elsewhere as well, right? 

https://github.com/llvm/llvm-project/pull/177296