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[TableGen][DecoderEmitter] Simplify FilterChooser::getIslands() (NFC) #159218

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Sep 17, 2025
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[TableGen][DecoderEmitter] Simplify FilterChooser::getIslands() (NFC) #159218

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87 changes: 35 additions & 52 deletions llvm/utils/TableGen/DecoderEmitter.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -300,6 +300,12 @@ class DecoderEmitter {

namespace {

struct EncodingIsland {
unsigned StartBit;
unsigned NumBits;
uint64_t FieldVal;
};

/// Filter - Filter works with FilterChooser to produce the decoding tree for
/// the ISA.
///
Expand Down Expand Up @@ -425,12 +431,6 @@ class FilterChooser {
/// Set to true if decoding conflict was encountered.
bool HasConflict = false;

struct Island {
unsigned StartBit;
unsigned NumBits;
uint64_t FieldVal;
};

public:
/// Constructs a top-level filter chooser.
FilterChooser(ArrayRef<InstructionEncoding> Encodings,
Expand Down Expand Up @@ -483,12 +483,6 @@ class FilterChooser {
return FilterBits.Zero[Idx] || FilterBits.One[Idx];
}

// Calculates the island(s) needed to decode the instruction.
// This returns a list of undecoded bits of an instructions, for example,
// Inst{20} = 1 && Inst{3-0} == 0b1111 represents two islands of yet-to-be
// decoded bits in order to verify that the instruction matches the Opcode.
std::vector<Island> getIslands(const KnownBits &EncodingBits) const;

/// Scans the well-known encoding bits of the encodings and, builds up a list
/// of candidate filters, and then returns the best one, if any.
std::unique_ptr<Filter> findBestFilter(ArrayRef<bitAttr_t> BitAttrs,
Expand Down Expand Up @@ -948,48 +942,36 @@ void FilterChooser::dumpStack(raw_ostream &OS, indent Indent,
// This returns a list of undecoded bits of an instructions, for example,
// Inst{20} = 1 && Inst{3-0} == 0b1111 represents two islands of yet-to-be
// decoded bits in order to verify that the instruction matches the Opcode.
std::vector<FilterChooser::Island>
FilterChooser::getIslands(const KnownBits &EncodingBits) const {
std::vector<Island> Islands;
static std::vector<EncodingIsland> getIslands(const KnownBits &EncodingBits,
const KnownBits &FilterBits) {
std::vector<EncodingIsland> Islands;
uint64_t FieldVal;
unsigned StartBit;

// 0: Init
// 1: Water (the bit value does not affect decoding)
// 2: Island (well-known bit value needed for decoding)
unsigned State = 0;

bool OnIsland = false;
unsigned FilterWidth = FilterBits.getBitWidth();
for (unsigned i = 0; i != FilterWidth; ++i) {
bool IsKnown = EncodingBits.Zero[i] || EncodingBits.One[i];
bool Filtered = isPositionFiltered(i);
switch (State) {
default:
llvm_unreachable("Unreachable code!");
case 0:
case 1:
if (Filtered || !IsKnown) {
State = 1; // Still in Water
for (unsigned I = 0; I != FilterWidth; ++I) {
bool IsKnown = EncodingBits.Zero[I] || EncodingBits.One[I];
bool IsFiltered = FilterBits.Zero[I] || FilterBits.One[I];
if (!IsFiltered && IsKnown) {
if (OnIsland) {
// Accumulate island bits.
FieldVal |= static_cast<uint64_t>(EncodingBits.One[I])
<< (I - StartBit);
} else {
State = 2; // Into the Island
StartBit = i;
FieldVal = static_cast<uint64_t>(EncodingBits.One[i]);
// Onto an island.
StartBit = I;
FieldVal = static_cast<uint64_t>(EncodingBits.One[I]);
OnIsland = true;
}
break;
case 2:
if (Filtered || !IsKnown) {
State = 1; // Into the Water
Islands.push_back({StartBit, i - StartBit, FieldVal});
} else {
State = 2; // Still in Island
FieldVal |= static_cast<uint64_t>(EncodingBits.One[i])
<< (i - StartBit);
}
break;
} else if (OnIsland) {
// Into the water.
Islands.push_back({StartBit, I - StartBit, FieldVal});
OnIsland = false;
}
}
// If we are still in Island after the loop, do some housekeeping.
if (State == 2)

if (OnIsland)
Islands.push_back({StartBit, FilterWidth - StartBit, FieldVal});

return Islands;
Expand Down Expand Up @@ -1136,17 +1118,17 @@ void DecoderTableBuilder::emitSingletonTableEntry(
KnownBits EncodingBits = Encoding.getMandatoryBits();

// Look for islands of undecoded bits of the singleton.
std::vector<FilterChooser::Island> Islands = FC.getIslands(EncodingBits);
std::vector<EncodingIsland> Islands = getIslands(EncodingBits, FC.FilterBits);

// Emit the predicate table entry if one is needed.
emitPredicateTableEntry(EncodingID);

// Check any additional encoding fields needed.
for (const FilterChooser::Island &Ilnd : reverse(Islands)) {
for (const EncodingIsland &Island : reverse(Islands)) {
TableInfo.Table.insertOpcode(OPC_CheckField);
TableInfo.Table.insertULEB128(Ilnd.StartBit);
TableInfo.Table.insertUInt8(Ilnd.NumBits);
TableInfo.Table.insertULEB128(Ilnd.FieldVal);
TableInfo.Table.insertULEB128(Island.StartBit);
TableInfo.Table.insertUInt8(Island.NumBits);
TableInfo.Table.insertULEB128(Island.FieldVal);
}

// Check for soft failure of the match.
Expand Down Expand Up @@ -1182,7 +1164,8 @@ FilterChooser::findBestFilter(ArrayRef<bitAttr_t> BitAttrs, bool AllowMixed,
KnownBits EncodingBits = Encoding.getMandatoryBits();

// Look for islands of undecoded bits of any instruction.
std::vector<Island> Islands = getIslands(EncodingBits);
std::vector<EncodingIsland> Islands =
getIslands(EncodingBits, FilterBits);
if (!Islands.empty()) {
// Found an instruction with island(s). Now just assign a filter.
return std::make_unique<Filter>(
Expand Down