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[NFC][MC][DecoderEmitter] Refactor code related to EncodingField #156759

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Sep 4, 2025
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[NFC][MC][DecoderEmitter] Refactor code related to EncodingField #156759

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58 changes: 39 additions & 19 deletions llvm/utils/TableGen/DecoderEmitter.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -131,6 +131,14 @@ static void printKnownBits(raw_ostream &OS, const KnownBits &Bits,

namespace {

// Represents a span of bits in the instruction encoding that's based on a span
// of bits in an operand's encoding.
//
// Width is the width of the span.
// Base is the starting position of that span in the instruction encoding.
// Offset if the starting position of that span in the operand's encoding.
// That is, bits {Base + Width - 1, Base} in the instruction encoding form
// bits {Offset + Width - 1, Offset} in the operands encoding.
struct EncodingField {
unsigned Base, Width, Offset;
EncodingField(unsigned B, unsigned W, unsigned O)
Expand All @@ -146,15 +154,12 @@ struct OperandInfo {
OperandInfo(std::string D, bool HCD) : Decoder(D), HasCompleteDecoder(HCD) {}

void addField(unsigned Base, unsigned Width, unsigned Offset) {
Fields.push_back(EncodingField(Base, Width, Offset));
Fields.emplace_back(Base, Width, Offset);
}

unsigned numFields() const { return Fields.size(); }

typedef std::vector<EncodingField>::const_iterator const_iterator;

const_iterator begin() const { return Fields.begin(); }
const_iterator end() const { return Fields.end(); }
ArrayRef<EncodingField> fields() const { return Fields; }
};

/// Represents a parsed InstructionEncoding record or a record derived from it.
Expand Down Expand Up @@ -1100,17 +1105,17 @@ void DecoderTableBuilder::emitBinaryParser(raw_ostream &OS, indent Indent,
OS << ";\n";
}

for (const EncodingField &EF : OpInfo) {
for (const auto &[Base, Width, Offset] : OpInfo.fields()) {
OS << Indent;
if (UseInsertBits)
OS << "insertBits(tmp, ";
else
OS << "tmp = ";
OS << "fieldFromInstruction(insn, " << EF.Base << ", " << EF.Width << ')';
OS << "fieldFromInstruction(insn, " << Base << ", " << Width << ')';
if (UseInsertBits)
OS << ", " << EF.Offset << ", " << EF.Width << ')';
else if (EF.Offset != 0)
OS << " << " << EF.Offset;
OS << ", " << Offset << ", " << Width << ')';
else if (Offset != 0)
OS << " << " << Offset;
OS << ";\n";
}

Expand Down Expand Up @@ -1920,7 +1925,8 @@ static void debugDumpRecord(const Record &Rec) {
/// For an operand field named OpName: populate OpInfo.InitValue with the
/// constant-valued bit values, and OpInfo.Fields with the ranges of bits to
/// insert from the decoded instruction.
static void addOneOperandFields(const Record *EncodingDef, const BitsInit &Bits,
static void addOneOperandFields(const Record *EncodingDef,
const BitsInit &InstBits,
std::map<StringRef, StringRef> &TiedNames,
StringRef OpName, OperandInfo &OpInfo) {
// Some bits of the operand may be required to be 1 depending on the
Expand All @@ -1932,19 +1938,33 @@ static void addOneOperandFields(const Record *EncodingDef, const BitsInit &Bits,
if (OpBit->getValue())
OpInfo.InitValue |= 1ULL << I;

for (unsigned I = 0, J = 0; I != Bits.getNumBits(); I = J) {
// Find out where the variable bits of the operand are encoded. The bits don't
// have to be consecutive or in ascending order. For example, an operand could
// be encoded as follows:
//
// 7 6 5 4 3 2 1 0
// {1, op{5}, op{2}, op{1}, 0, op{4}, op{3}, ?}
//
// In this example the operand is encoded in three segments:
//
// Base Width Offset
// op{2...1} 4 2 1
// op{4...3} 1 2 3
// op{5} 6 1 5
//
for (unsigned I = 0, J = 0; I != InstBits.getNumBits(); I = J) {
const VarInit *Var;
unsigned Offset = 0;
for (; J != Bits.getNumBits(); ++J) {
const VarBitInit *BJ = dyn_cast<VarBitInit>(Bits.getBit(J));
if (BJ) {
Var = dyn_cast<VarInit>(BJ->getBitVar());
for (; J != InstBits.getNumBits(); ++J) {
const Init *BitJ = InstBits.getBit(J);
if (const auto *VBI = dyn_cast<VarBitInit>(BitJ)) {
Var = dyn_cast<VarInit>(VBI->getBitVar());
if (I == J)
Offset = BJ->getBitNum();
else if (BJ->getBitNum() != Offset + J - I)
Offset = VBI->getBitNum();
else if (VBI->getBitNum() != Offset + J - I)
break;
} else {
Var = dyn_cast<VarInit>(Bits.getBit(J));
Var = dyn_cast<VarInit>(BitJ);
}
if (!Var ||
(Var->getName() != OpName && Var->getName() != TiedNames[OpName]))
Expand Down