[llvm] [DecoderEmitter] Eliminate `DecodeComplete` parameter to `decodeToMCInst` (PR #159130)

[Rahul Joshi via llvm-commits]

https://github.com/jurahul created https://github.com/llvm/llvm-project/pull/159130

Currently, the code generated by decoder emitter always set `DeocdeComplete` to false when returning Fail from `decodeToMCInst`. As a result, we can just use the return `DecodeStatus` from `decodeToMCInst` to decide whether to terminate the decoding and return or continue trying.

>From c91b80dc8ec593362a612213b71847138494628f Mon Sep 17 00:00:00 2001
From: Rahul Joshi <rjoshi at nvidia.com>
Date: Tue, 16 Sep 2025 09:50:08 -0700
Subject: [PATCH] [DecoderEmitter] Eliminate `DecodeComplete` parameter to
 `decodeToMCInst`

Currently, the code generated by decoder emitter always set
`DeocdeComplete` to false when returning Fail from `decodeToMCInst`. As
a result, we can just use the return `DecodeStatus` from
`decodeToMCInst` to decide whether to terminate the decoding and
return or continue trying.
---
 .../llvm/MC/MCDisassembler/MCDisassembler.h   | 10 +++----
 llvm/utils/TableGen/DecoderEmitter.cpp        | 26 ++++++-------------
 2 files changed, 13 insertions(+), 23 deletions(-)

diff --git a/llvm/include/llvm/MC/MCDisassembler/MCDisassembler.h b/llvm/include/llvm/MC/MCDisassembler/MCDisassembler.h
index cae2fbcac1fef..3d10380fa1d46 100644
--- a/llvm/include/llvm/MC/MCDisassembler/MCDisassembler.h
+++ b/llvm/include/llvm/MC/MCDisassembler/MCDisassembler.h
@@ -98,11 +98,11 @@ class LLVM_ABI MCDisassembler {
   /// from Success->SoftFail ->Fail can be done with a simple
   /// bitwise-AND:
   ///
-  ///   LEFT & TOP =  | Success       Unpredictable   Fail
-  ///   --------------+-----------------------------------
-  ///   Success       | Success       Unpredictable   Fail
-  ///   Unpredictable | Unpredictable Unpredictable   Fail
-  ///   Fail          | Fail          Fail            Fail
+  ///   LEFT & TOP =  | Success    SoftFail   Fail
+  ///   --------------+-----------------------------
+  ///   Success       | Success    SoftFail   Fail
+  ///   SoftFail      | SoftFail   SoftFail   Fail
+  ///   Fail          | Fail           Fail   Fail
   ///
   /// An easy way of encoding this is as 0b11, 0b01, 0b00 for
   /// Success, SoftFail, Fail respectively.
diff --git a/llvm/utils/TableGen/DecoderEmitter.cpp b/llvm/utils/TableGen/DecoderEmitter.cpp
index 43e1299563762..c6e6629108c9f 100644
--- a/llvm/utils/TableGen/DecoderEmitter.cpp
+++ b/llvm/utils/TableGen/DecoderEmitter.cpp
@@ -868,7 +868,7 @@ void DecoderEmitter::emitDecoderFunction(formatted_raw_ostream &OS,
       "InsnType, uint64_t>;\n";
   StringRef DecodeParams =
       "DecodeStatus S, InsnType insn, MCInst &MI, uint64_t Address, const "
-      "MCDisassembler *Decoder, bool &DecodeComplete";
+      "MCDisassembler *Decoder";
 
   // Print the name of the decode function to OS.
   auto PrintDecodeFnName = [&OS, BucketBitWidth](unsigned DecodeIdx) {
@@ -907,7 +907,6 @@ void DecoderEmitter::emitDecoderFunction(formatted_raw_ostream &OS,
   OS << "// Handling " << Decoders.size() << " cases.\n";
   PrintTemplate();
   OS << "decodeToMCInst(unsigned Idx, " << DecodeParams << ") {\n";
-  OS << "  DecodeComplete = true;\n";
 
   if (UseFnTableInDecodeToMCInst) {
     // Build a table of function pointers
@@ -921,8 +920,7 @@ void DecoderEmitter::emitDecoderFunction(formatted_raw_ostream &OS,
     OS << "  };\n";
     OS << "  if (Idx >= " << Decoders.size() << ")\n";
     OS << "    llvm_unreachable(\"Invalid decoder index!\");\n";
-    OS << "  return decodeFnTable[Idx](S, insn, MI, Address, Decoder, "
-          "DecodeComplete);\n";
+    OS << "  return decodeFnTable[Idx](S, insn, MI, Address, Decoder);\n";
   } else {
     OS << "  " << TmpTypeDecl;
     OS << "  TmpType tmp;\n";
@@ -1055,9 +1053,7 @@ static void emitBinaryParser(raw_ostream &OS, indent Indent,
   StringRef Decoder = OpInfo.Decoder;
   if (!Decoder.empty()) {
     OS << Indent << "if (!Check(S, " << Decoder
-       << "(MI, tmp, Address, Decoder))) { "
-       << (OpInfo.HasCompleteDecoder ? "" : "DecodeComplete = false; ")
-       << "return MCDisassembler::Fail; }\n";
+       << "(MI, tmp, Address, Decoder))) { return MCDisassembler::Fail; }\n";
   } else {
     OS << Indent << "MI.addOperand(MCOperand::createImm(tmp));\n";
   }
@@ -1069,9 +1065,7 @@ static void emitDecoder(raw_ostream &OS, indent Indent,
   StringRef DecoderMethod = Encoding.getDecoderMethod();
   if (!DecoderMethod.empty()) {
     OS << Indent << "if (!Check(S, " << DecoderMethod
-       << "(MI, insn, Address, Decoder))) { "
-       << (Encoding.hasCompleteDecoder() ? "" : "DecodeComplete = false; ")
-       << "return MCDisassembler::Fail; }\n";
+       << "(MI, insn, Address, Decoder))) { return MCDisassembler::Fail; }\n";
     return;
   }
 
@@ -1704,15 +1698,13 @@ static DecodeStatus decodeInstruction(const uint8_t DecodeTable[], MCInst &MI,
       unsigned DecodeIdx = decodeULEB128AndIncUnsafe(Ptr);
 
       MI.clear();
-      MI.setOpcode(Opc);
-      bool DecodeComplete;)";
+      MI.setOpcode(Opc);)";
   if (IsVarLenInst) {
     OS << "\n      unsigned Len = InstrLenTable[Opc];\n"
        << "      makeUp(insn, Len);";
   }
   OS << R"(
-      S = decodeToMCInst(DecodeIdx, S, insn, MI, Address, DisAsm, DecodeComplete);
-      assert(DecodeComplete);
+      S = decodeToMCInst(DecodeIdx, S, insn, MI, Address, DisAsm);
 
       LLVM_DEBUG(dbgs() << Loc << ": OPC_Decode: opcode " << Opc
                    << ", using decoder " << DecodeIdx << ": "
@@ -1729,18 +1721,16 @@ static DecodeStatus decodeInstruction(const uint8_t DecodeTable[], MCInst &MI,
       // Perform the decode operation.
       MCInst TmpMI;
       TmpMI.setOpcode(Opc);
-      bool DecodeComplete;
-      S = decodeToMCInst(DecodeIdx, S, insn, TmpMI, Address, DisAsm, DecodeComplete);
+      S = decodeToMCInst(DecodeIdx, S, insn, TmpMI, Address, DisAsm);
       LLVM_DEBUG(dbgs() << Loc << ": OPC_TryDecode: opcode " << Opc
                    << ", using decoder " << DecodeIdx << ": ");
 
-      if (DecodeComplete) {
+      if (S != MCDisassembler::Fail) {
         // Decoding complete.
         LLVM_DEBUG(dbgs() << (S != MCDisassembler::Fail ? "PASS\n" : "FAIL\n"));
         MI = TmpMI;
         return S;
       }
-      assert(S == MCDisassembler::Fail);
       if (ScopeStack.empty()) {
         LLVM_DEBUG(dbgs() << "FAIL, returning FAIL\n");
         return MCDisassembler::Fail;