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

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Jun 17, 2025
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[NFCI][TableGen][DecoderEmitter] Cull Op handling when possible #142974

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d12a2d0
[NFCI][TableGen][DecoderEmitter] Cull `TryDecode` when possible
jurahul Jun 5, 2025
e3966c6
Generalize to support other opcodes
jurahul Jun 16, 2025
b8ef2c7
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jurahul Jun 17, 2025
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89 changes: 56 additions & 33 deletions llvm/utils/TableGen/DecoderEmitter.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -222,11 +222,11 @@ class DecoderEmitter {
DecoderEmitter(const RecordKeeper &R, StringRef PredicateNamespace)
: RK(R), Target(R), PredicateNamespace(PredicateNamespace) {}

// Emit the decoder state machine table. Return true if any `TryDecode` ops
// were generated.
bool 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,
Expand Down Expand Up @@ -827,11 +827,12 @@ unsigned Filter::usefulness() const {
// //
//////////////////////////////////

// Emit the decoder state machine table.
bool 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.
Expand Down Expand Up @@ -885,7 +886,7 @@ bool DecoderEmitter::emitTable(formatted_raw_ostream &OS, DecoderTable &Table,
OS << " (Fail)";
};

bool HasTryDecode = false;
unsigned OpcodeMask = 0;

while (I != E) {
assert(I < E && "incomplete decode table entry!");
Expand All @@ -895,6 +896,7 @@ bool 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) +
Expand Down Expand Up @@ -967,7 +969,6 @@ bool DecoderEmitter::emitTable(formatted_raw_ostream &OS, DecoderTable &Table,
case MCD::OPC_TryDecodeOrFail: {
bool IsFail = DecoderOp == MCD::OPC_TryDecodeOrFail;
bool IsTry = DecoderOp == MCD::OPC_TryDecode || IsFail;
HasTryDecode |= IsTry;
// Decode the Opcode value.
const char *ErrMsg = nullptr;
unsigned Opc = decodeULEB128(&*I, nullptr, EndPtr, &ErrMsg);
Expand Down Expand Up @@ -1032,7 +1033,7 @@ bool DecoderEmitter::emitTable(formatted_raw_ostream &OS, DecoderTable &Table,

OS << Indent << "};\n\n";

return HasTryDecode;
return OpcodeMask;
}

void DecoderEmitter::emitInstrLenTable(formatted_raw_ostream &OS,
Expand Down Expand Up @@ -2224,7 +2225,14 @@ 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,
bool HasTryDecode) {
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++;
Expand All @@ -2244,9 +2252,11 @@ 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;
Expand Down Expand Up @@ -2327,7 +2337,9 @@ 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;
Expand All @@ -2349,7 +2361,9 @@ 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);
Expand Down Expand Up @@ -2409,17 +2423,20 @@ static DecodeStatus decodeInstruction(const uint8_t DecodeTable[], MCInst &MI,
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_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;
}
case MCD::OPC_Fail: {
LLVM_DEBUG(dbgs() << Loc << ": OPC_Fail\n");
return MCDisassembler::Fail;
Expand Down Expand Up @@ -2619,7 +2636,7 @@ namespace {
}

DecoderTableInfo TableInfo;
bool HasTryDecode = false;
unsigned OpcodeMask = 0;
for (const auto &Opc : OpcMap) {
// Emit the decoder for this namespace+width combination.
ArrayRef<EncodingAndInst> NumberedEncodingsRef(NumberedEncodings.data(),
Expand All @@ -2645,23 +2662,29 @@ namespace {
TableInfo.Table.push_back(MCD::OPC_Fail);

// Print the table to the output stream.
HasTryDecode |= 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
// to let the decoder know how long the instructions are.
// 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, HasTryDecode);
emitDecodeInstruction(OS, IsVarLenInst, OpcodeMask);

OS << "\n} // namespace\n";
}
Expand Down
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