[NFCI][TableGen][DecoderEmitter] Cull Op handling when possible (#142974)

TryDecode/CheckPredicate/SoftFail MCD ops are not used by many targets.
Track the set of opcodes that were emitted and emit code for handling
TryDecode/CheckPredicate/SoftFail ops when decoding only if there were
emitted. This is purely eliminating dead code in the generated
`decodeInstruction` function.

This results in the following reduction in the size of the Disassembler
.so files with a release x86_64 release build on Linux:

```
Target                                                   Old Size        New Size  %  reduction
build/lib/libLLVMAArch64Disassembler.so.21.0git             256656          256656          0.00
build/lib/libLLVMAMDGPUDisassembler.so.21.0git              813000          808168          0.59
build/lib/libLLVMARCDisassembler.so.21.0git                  44816           43536          2.86
build/lib/libLLVMARMDisassembler.so.21.0git                 281744          278808          1.04
build/lib/libLLVMAVRDisassembler.so.21.0git                  36040           34496          4.28
build/lib/libLLVMBPFDisassembler.so.21.0git                  26248           23168         11.73
build/lib/libLLVMCSKYDisassembler.so.21.0git                 55960           53632          4.16
build/lib/libLLVMHexagonDisassembler.so.21.0git             115952          113416          2.19
build/lib/libLLVMLanaiDisassembler.so.21.0git                24360           21008         13.76
build/lib/libLLVMLoongArchDisassembler.so.21.0git            58584           56168          4.12
build/lib/libLLVMM68kDisassembler.so.21.0git                 57264           53880          5.91
build/lib/libLLVMMSP430Disassembler.so.21.0git               28896           28440          1.58
build/lib/libLLVMMipsDisassembler.so.21.0git                123128          120568          2.08
build/lib/libLLVMPowerPCDisassembler.so.21.0git              80656           78096          3.17
build/lib/libLLVMRISCVDisassembler.so.21.0git               154080          150200          2.52
build/lib/libLLVMSparcDisassembler.so.21.0git                42040           39568          5.88
build/lib/libLLVMSystemZDisassembler.so.21.0git              97056           94552          2.58
build/lib/libLLVMVEDisassembler.so.21.0git                   83944           81352          3.09
build/lib/libLLVMWebAssemblyDisassembler.so.21.0git          25280           25280          0.00
build/lib/libLLVMX86Disassembler.so.21.0git                2920624         2920624          0.00
build/lib/libLLVMXCoreDisassembler.so.21.0git                48320           44288          8.34
build/lib/libLLVMXtensaDisassembler.so.21.0git               42248           35840         15.17
```

llvm/utils/TableGen/DecoderEmitter.cpp

@@ -222,10 +222,11 @@ public:
   DecoderEmitter(const RecordKeeper &R, StringRef PredicateNamespace)
       : RK(R), Target(R), PredicateNamespace(PredicateNamespace) {}
 
-  // Emit the decoder state machine table.
-  void emitTable(formatted_raw_ostream &OS, DecoderTable &Table, indent Indent,
-                 unsigned BitWidth, StringRef Namespace,
-                 const EncodingIDsVec &EncodingIDs) const;
+  // Emit the decoder state machine table. Returns a mask of MCD decoder ops
+  // that were emitted.
+  unsigned emitTable(formatted_raw_ostream &OS, DecoderTable &Table,
+                     indent Indent, unsigned BitWidth, StringRef Namespace,
+                     const EncodingIDsVec &EncodingIDs) const;
   void emitInstrLenTable(formatted_raw_ostream &OS,
                          ArrayRef<unsigned> InstrLen) const;
   void emitPredicateFunction(formatted_raw_ostream &OS,
@@ -826,11 +827,12 @@ unsigned Filter::usefulness() const {
 //                              //
 //////////////////////////////////
 
-// Emit the decoder state machine table.
-void DecoderEmitter::emitTable(formatted_raw_ostream &OS, DecoderTable &Table,
-                               indent Indent, unsigned BitWidth,
-                               StringRef Namespace,
-                               const EncodingIDsVec &EncodingIDs) const {
+// Emit the decoder state machine table. Returns a mask of MCD decoder ops
+// that were emitted.
+unsigned DecoderEmitter::emitTable(formatted_raw_ostream &OS,
+                                   DecoderTable &Table, indent Indent,
+                                   unsigned BitWidth, StringRef Namespace,
+                                   const EncodingIDsVec &EncodingIDs) const {
   // We'll need to be able to map from a decoded opcode into the corresponding
   // EncodingID for this specific combination of BitWidth and Namespace. This
   // is used below to index into NumberedEncodings.
@@ -884,6 +886,8 @@ void DecoderEmitter::emitTable(formatted_raw_ostream &OS, DecoderTable &Table,
       OS << " (Fail)";
   };
 
+  unsigned OpcodeMask = 0;
+
   while (I != E) {
     assert(I < E && "incomplete decode table entry!");
 
@@ -892,6 +896,7 @@ void DecoderEmitter::emitTable(formatted_raw_ostream &OS, DecoderTable &Table,
     OS.PadToColumn(12);
 
     const uint8_t DecoderOp = *I++;
+    OpcodeMask |= (1 << DecoderOp);
     switch (DecoderOp) {
     default:
       PrintFatalError("Invalid decode table opcode: " + Twine((int)DecoderOp) +
@@ -1027,6 +1032,8 @@ void DecoderEmitter::emitTable(formatted_raw_ostream &OS, DecoderTable &Table,
   Indent -= 2;
 
   OS << Indent << "};\n\n";
+
+  return OpcodeMask;
 }
 
 void DecoderEmitter::emitInstrLenTable(formatted_raw_ostream &OS,
@@ -1045,19 +1052,13 @@ void DecoderEmitter::emitPredicateFunction(formatted_raw_ostream &OS,
   OS << Indent << "static bool checkDecoderPredicate(unsigned Idx, "
      << "const FeatureBitset &Bits) {\n";
   Indent += 2;
-  if (!Predicates.empty()) {
-    OS << Indent << "switch (Idx) {\n";
-    OS << Indent << "default: llvm_unreachable(\"Invalid index!\");\n";
-    unsigned Index = 0;
-    for (const auto &Predicate : Predicates) {
-      OS << Indent << "case " << Index++ << ":\n";
-      OS << Indent + 2 << "return (" << Predicate << ");\n";
-    }
-    OS << Indent << "}\n";
-  } else {
-    // No case statement to emit
-    OS << Indent << "llvm_unreachable(\"Invalid index!\");\n";
+  OS << Indent << "switch (Idx) {\n";
+  OS << Indent << "default: llvm_unreachable(\"Invalid index!\");\n";
+  for (const auto &[Index, Predicate] : enumerate(Predicates)) {
+    OS << Indent << "case " << Index << ":\n";
+    OS << Indent + 2 << "return (" << Predicate << ");\n";
   }
+  OS << Indent << "}\n";
   Indent -= 2;
   OS << Indent << "}\n\n";
 }
@@ -2217,8 +2218,15 @@ static void insertBits(InsnType &field, InsnType bits, unsigned startBit,
 
 // emitDecodeInstruction - Emit the templated helper function
 // decodeInstruction().
-static void emitDecodeInstruction(formatted_raw_ostream &OS,
-                                  bool IsVarLenInst) {
+static void emitDecodeInstruction(formatted_raw_ostream &OS, bool IsVarLenInst,
+                                  unsigned OpcodeMask) {
+  const bool HasTryDecode = OpcodeMask & ((1 << MCD::OPC_TryDecode) |
+                                          (1 << MCD::OPC_TryDecodeOrFail));
+  const bool HasCheckPredicate =
+      OpcodeMask &
+      ((1 << MCD::OPC_CheckPredicate) | (1 << MCD::OPC_CheckPredicateOrFail));
+  const bool HasSoftFail = OpcodeMask & (1 << MCD::OPC_SoftFail);
+
   OS << R"(
 static unsigned decodeNumToSkip(const uint8_t *&Ptr) {
   unsigned NumToSkip = *Ptr++;
@@ -2238,9 +2246,11 @@ static DecodeStatus decodeInstruction(const uint8_t DecodeTable[], MCInst &MI,
     OS << ",\n                                      "
           "llvm::function_ref<void(APInt &, uint64_t)> makeUp";
   }
-  OS << R"() {
-  const FeatureBitset &Bits = STI.getFeatureBits();
+  OS << ") {\n";
+  if (HasCheckPredicate)
+    OS << "  const FeatureBitset &Bits = STI.getFeatureBits();\n";
 
+  OS << R"(
   const uint8_t *Ptr = DecodeTable;
   uint64_t CurFieldValue = 0;
   DecodeStatus S = MCDisassembler::Success;
@@ -2321,7 +2331,9 @@ static DecodeStatus decodeInstruction(const uint8_t DecodeTable[], MCInst &MI,
         Ptr += NumToSkip;
       }
       break;
-    }
+    })";
+  if (HasCheckPredicate) {
+    OS << R"(
     case MCD::OPC_CheckPredicate:
     case MCD::OPC_CheckPredicateOrFail: {
       bool IsFail = DecoderOp == MCD::OPC_CheckPredicateOrFail;
@@ -2343,7 +2355,9 @@ static DecodeStatus decodeInstruction(const uint8_t DecodeTable[], MCInst &MI,
         Ptr += NumToSkip;
       }
       break;
-    }
+    })";
+  }
+  OS << R"(
     case MCD::OPC_Decode: {
       // Decode the Opcode value.
       unsigned Opc = decodeULEB128AndIncUnsafe(Ptr);
@@ -2364,7 +2378,9 @@ static DecodeStatus decodeInstruction(const uint8_t DecodeTable[], MCInst &MI,
                    << ", using decoder " << DecodeIdx << ": "
                    << (S != MCDisassembler::Fail ? "PASS\n" : "FAIL\n"));
       return S;
-    }
+    })";
+  if (HasTryDecode) {
+    OS << R"(
     case MCD::OPC_TryDecode:
     case MCD::OPC_TryDecodeOrFail: {
       bool IsFail = DecoderOp == MCD::OPC_TryDecodeOrFail;
@@ -2399,17 +2415,22 @@ static DecodeStatus decodeInstruction(const uint8_t DecodeTable[], MCInst &MI,
       // set before the decode attempt.
       S = MCDisassembler::Success;
       break;
-    }
-    case MCD::OPC_SoftFail: {
-      // Decode the mask values.
-      uint64_t PositiveMask = decodeULEB128AndIncUnsafe(Ptr);
-      uint64_t NegativeMask = decodeULEB128AndIncUnsafe(Ptr);
-      bool Failed = (insn & PositiveMask) != 0 || (~insn & NegativeMask) != 0;
-      if (Failed)
-        S = MCDisassembler::SoftFail;
-      LLVM_DEBUG(dbgs() << Loc << ": OPC_SoftFail: " << (Failed ? "FAIL\n" : "PASS\n"));
-      break;
-    }
+    })";
+  }
+  if (HasSoftFail) {
+    OS << R"(
+      case MCD::OPC_SoftFail: {
+        // Decode the mask values.
+        uint64_t PositiveMask = decodeULEB128AndIncUnsafe(Ptr);
+        uint64_t NegativeMask = decodeULEB128AndIncUnsafe(Ptr);
+        bool Failed = (insn & PositiveMask) != 0 || (~insn & NegativeMask) != 0;
+        if (Failed)
+          S = MCDisassembler::SoftFail;
+        LLVM_DEBUG(dbgs() << Loc << ": OPC_SoftFail: " << (Failed ? "FAIL\n" : "PASS\n"));
+        break;
+  })";
+  }
+  OS << R"(
     case MCD::OPC_Fail: {
       LLVM_DEBUG(dbgs() << Loc << ": OPC_Fail\n");
       return MCDisassembler::Fail;
@@ -2609,6 +2630,7 @@ namespace {
   }
 
   DecoderTableInfo TableInfo;
+  unsigned OpcodeMask = 0;
   for (const auto &Opc : OpcMap) {
     // Emit the decoder for this namespace+width combination.
     ArrayRef<EncodingAndInst> NumberedEncodingsRef(NumberedEncodings.data(),
@@ -2634,8 +2656,8 @@ namespace {
     TableInfo.Table.push_back(MCD::OPC_Fail);
 
     // Print the table to the output stream.
-    emitTable(OS, TableInfo.Table, indent(0), FC.getBitWidth(), Opc.first.first,
-              Opc.second);
+    OpcodeMask |= emitTable(OS, TableInfo.Table, indent(0), FC.getBitWidth(),
+                            Opc.first.first, Opc.second);
   }
 
   // For variable instruction, we emit a instruction length table
@@ -2643,14 +2665,20 @@ namespace {
   // You can see example usage in M68k's disassembler.
   if (IsVarLenInst)
     emitInstrLenTable(OS, InstrLen);
+
+  const bool HasCheckPredicate =
+      OpcodeMask &
+      ((1 << MCD::OPC_CheckPredicate) | (1 << MCD::OPC_CheckPredicateOrFail));
+
   // Emit the predicate function.
-  emitPredicateFunction(OS, TableInfo.Predicates, indent(0));
+  if (HasCheckPredicate)
+    emitPredicateFunction(OS, TableInfo.Predicates, indent(0));
 
   // Emit the decoder function.
   emitDecoderFunction(OS, TableInfo.Decoders, indent(0));
 
   // Emit the main entry point for the decoder, decodeInstruction().
-  emitDecodeInstruction(OS, IsVarLenInst);
+  emitDecodeInstruction(OS, IsVarLenInst, OpcodeMask);
 
   OS << "\n} // namespace\n";
 }