[TableGen][DecoderEmitter][RISCV] Always handle bits<0> (#159951)

Previously, `bits<0>` only had effect if `ignore-non-decodable-operands`
wasn't specified. Handle it even if the option was specified. This
should allow for a smoother transition to the option removed.

The change revealed a couple of inaccuracies in RISCV compressed
instruction definitions.
* `C_ADDI4SPN` has `bits<5> rs1` field, but `rs1` is not encoded. It
should be `bits<0>`.
* `C_ADDI16SP` has `bits<5> rd` in the base class, but it is unused
since `Inst{11-7}` is overwritten with constant bits.
We should instead set `rd = 2` and `Inst{11-7} = rd`. There are a couple
of alternative fixes, but this one is the shortest.

Author: s-barannikov

llvm/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp

@@ -206,6 +206,14 @@ static DecodeStatus DecodeSPRegisterClass(MCInst &Inst,
   return MCDisassembler::Success;
 }
 
+static DecodeStatus DecodeSPRegisterClass(MCInst &Inst, uint64_t RegNo,
+                                          uint32_t Address,
+                                          const MCDisassembler *Decoder) {
+  assert(RegNo == 2);
+  Inst.addOperand(MCOperand::createReg(RISCV::X2));
+  return MCDisassembler::Success;
+}
+
 static DecodeStatus DecodeGPRX5RegisterClass(MCInst &Inst,
                                              const MCDisassembler *Decoder) {
   Inst.addOperand(MCOperand::createReg(RISCV::X5));

llvm/lib/Target/RISCV/RISCVInstrInfoC.td

@@ -298,7 +298,7 @@ def C_ADDI4SPN : RVInst16CIW<0b000, 0b00, (outs GPRC:$rd),
                              (ins SP:$rs1, uimm10_lsb00nonzero:$imm),
                              "c.addi4spn", "$rd, $rs1, $imm">,
                              Sched<[WriteIALU, ReadIALU]> {
-  bits<5> rs1;
+  bits<0> rs1;
   let Inst{12-11} = imm{5-4};
   let Inst{10-7} = imm{9-6};
   let Inst{6} = imm{2};
@@ -404,8 +404,8 @@ def C_ADDI16SP : RVInst16CI<0b011, 0b01, (outs SP:$rd_wb),
                             "c.addi16sp", "$rd, $imm">,
                  Sched<[WriteIALU, ReadIALU]> {
   let Constraints = "$rd = $rd_wb";
+  let rd = 2;
   let Inst{12} = imm{9};
-  let Inst{11-7} = 2;
   let Inst{6} = imm{4};
   let Inst{5} = imm{6};
   let Inst{4-3} = imm{8-7};
@@ -965,4 +965,3 @@ let Predicates = [HasStdExtCOrZcd, HasStdExtD] in {
   def : CompressPat<(FSD FPR64:$rs2, SPMem:$rs1, uimm9_lsb000:$imm),
                     (C_FSDSP FPR64:$rs2, SPMem:$rs1, uimm9_lsb000:$imm)>;
 } // Predicates = [HasStdExtCOrZcd, HasStdExtD]
-

llvm/utils/TableGen/Common/InstructionEncoding.cpp

@@ -316,6 +316,14 @@ static void addOneOperandFields(const Record *EncodingDef,
     else
       OpInfo.addField(I, J - I, Offset);
   }
+
+  if (!OpInfo.InitValue && OpInfo.fields().empty()) {
+    // We found a field in InstructionEncoding record that corresponds to the
+    // named operand, but that field has no constant bits and doesn't contribute
+    // to the Inst field. For now, treat that field as if it didn't exist.
+    // TODO: Remove along with IgnoreNonDecodableOperands.
+    OpInfo.HasNoEncoding = true;
+  }
 }
 
 void InstructionEncoding::parseFixedLenOperands(const BitsInit &Bits) {

llvm/utils/TableGen/DecoderEmitter.cpp

@@ -696,8 +696,6 @@ static void emitBinaryParser(raw_ostream &OS, indent Indent,
 
   // Special case for 'bits<0>'.
   if (OpInfo.Fields.empty() && !OpInfo.InitValue) {
-    if (IgnoreNonDecodableOperands)
-      return;
     assert(!OpInfo.Decoder.empty());
     // The operand has no encoding, so the corresponding argument is omitted.
     // This avoids confusion and allows the function to be overloaded if the