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Sep 10, 2025
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[TableGen][DecoderEmitter] Report all decoding conflicts #157847

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[TableGen][DecoderEmitter] Report all decoding conflicts
jurahul Sep 10, 2025
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jurahul Sep 10, 2025
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jurahul Sep 10, 2025
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50 changes: 43 additions & 7 deletions llvm/test/TableGen/FixedLenDecoderEmitter/conflict.td
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Original file line number Diff line number Diff line change
Expand Up @@ -9,27 +9,63 @@ def MyTarget : Target { let InstructionSet = MyTargetISA; }
def R0 : Register<"r0"> { let Namespace = "MyTarget"; }
def GPR32 : RegisterClass<"MyTarget", [i32], 32, (add R0)>;

def X0 : Register<"x0"> { let Namespace = "MyTarget"; }
def GPR64 : RegisterClass<"MyTarget", [i64], 64, (add X0)>;

class I<dag OOps, dag IOps, list<dag> Pat>
: Instruction {
let Namespace = "MyTarget";
let OutOperandList = OOps;
let InOperandList = IOps;
let Pattern = Pat;
let Size = 4;
bits<32> Inst;
bits<32> SoftFail;
}

// Assume there is a 2 bit encoding for the dst and src register.
def A : I<(outs GPR32:$dst), (ins GPR32:$src1), []> {
let Size = 4;
let Inst{31...0} = 0;
bits<2> dst;
bits<2> src1;
let Inst{31...4} = 0;
let Inst{1...0} = dst;
let Inst{3...2} = src1;
}

def B : I<(outs GPR32:$dst), (ins GPR32:$src1), []> {
let Size = 4;
let Inst{31...0} = 0;
bits<2> dst;
bits<2> src1;
let Inst{31...4} = 0;
let Inst{1...0} = dst;
let Inst{3...2} = src1;
}

def C : I<(outs GPR64:$dst), (ins GPR64:$src1), []> {
bits<2> dst;
bits<2> src1;
let Inst{31...4} = 1;
let Inst{1...0} = dst;
let Inst{3...2} = src1;
}

def D : I<(outs GPR64:$dst), (ins GPR64:$src1), []> {
bits<2> dst;
bits<2> src1;
let Inst{31...4} = 1;
let Inst{1...0} = dst;
let Inst{3...2} = src1;
}

// CHECK: Decoding Conflict:
// CHECK: ................................
// CHECK: 0000000000000000000000000000....
// CHECK: 0000000000000000000000000000____ A
// CHECK: 0000000000000000000000000000____ B

// CHECK: Decoding Conflict:
// CHECK: ................................
// CHECK: 00000000000000000000000000000000
// CHECK: 00000000000000000000000000000000 A
// CHECK: 00000000000000000000000000000000 B
// CHECK: 0000000000000000000000000001....
// CHECK: 0000000000000000000000000001____ C
// CHECK: 0000000000000000000000000001____ D

// CHECK: Decoding conflict encountered
16 changes: 7 additions & 9 deletions llvm/test/TableGen/FixedLenDecoderEmitter/var-len-conflict-2.td
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Expand Up @@ -10,16 +10,14 @@ class I : Instruction {

// Check pretty printing of decoding conflicts.

// CHECK: {{^}}Decoding Conflict:
// CHECK-NEXT: {{^}} ........
// CHECK-NEXT: {{^}} ..............11
// CHECK-NEXT: {{^}} .......0......11
// CHECK-NEXT: {{^}} _______0______11 I16_0
// CHECK-NEXT: {{^}} _______0______11 I16_1
// CHECK-NEXT: {{^}} _______0_______0______11 I24_0
// CHECK: Decoding Conflict:
// CHECK-NEXT: ................
// CHECK-NEXT: ..............11
// CHECK-NEXT: .......0......11
// CHECK-NEXT: _______0______11 I16_0
// CHECK-NEXT: _______0______11 I16_1
// CHECK-NEXT: _______0_______0______11 I24_0

def I8_0 : I { dag Inst = (descend (operand "$op", 6), 0b00); }
def I8_1 : I { dag Inst = (descend (operand "$op", 6), 0b01); }
def I16_0 : I { dag Inst = (descend (operand "$op2", 7), 0b0,
(operand "$op", 6), 0b11); }
def I16_1 : I { dag Inst = (descend (operand "$op2", 7), 0b0,
Expand Down
29 changes: 24 additions & 5 deletions llvm/utils/TableGen/DecoderEmitter.cpp
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Expand Up @@ -517,6 +517,9 @@ class FilterChooser {
/// The "field value" here refers to the encoding bits in the filtered range.
std::map<uint64_t, std::unique_ptr<const FilterChooser>> FilterChooserMap;

/// Set to true if decoding conflict was encountered.
bool HasConflict = false;

struct Island {
unsigned StartBit;
unsigned NumBits;
Expand Down Expand Up @@ -600,18 +603,22 @@ class DecoderTableBuilder {
ArrayRef<InstructionEncoding> Encodings;
DecoderTableInfo &TableInfo;

bool HasConflict = false;

public:
DecoderTableBuilder(const CodeGenTarget &Target,
ArrayRef<InstructionEncoding> Encodings,
DecoderTableInfo &TableInfo)
: Target(Target), Encodings(Encodings), TableInfo(TableInfo) {}

void buildTable(const FilterChooser &FC, unsigned BitWidth) const {
/// Returns true if a decoding conflict was encountered.
bool buildTable(const FilterChooser &FC, unsigned BitWidth) {
// When specializing decoders per bit width, each decoder table will begin
// with the bitwidth for that table.
if (SpecializeDecodersPerBitwidth)
TableInfo.Table.insertULEB128(BitWidth);
emitTableEntries(FC);
return HasConflict;
}

private:
Expand All @@ -637,7 +644,7 @@ class DecoderTableBuilder {

void emitSingletonTableEntry(const FilterChooser &FC) const;

void emitTableEntries(const FilterChooser &FC) const;
void emitTableEntries(const FilterChooser &FC);
};

} // end anonymous namespace
Expand Down Expand Up @@ -1592,7 +1599,7 @@ void FilterChooser::doFilter() {
// Print out useful conflict information for postmortem analysis.
errs() << "Decoding Conflict:\n";
dump();
PrintFatalError("Decoding conflict encountered");
HasConflict = true;
}

void FilterChooser::dump() const {
Expand All @@ -1612,7 +1619,12 @@ void FilterChooser::dump() const {
}
}

void DecoderTableBuilder::emitTableEntries(const FilterChooser &FC) const {
void DecoderTableBuilder::emitTableEntries(const FilterChooser &FC) {
if (FC.HasConflict) {
HasConflict = true;
return;
}

DecoderTable &Table = TableInfo.Table;

// If there are other encodings that could match if those with all bits
Expand Down Expand Up @@ -2570,6 +2582,7 @@ template <typename T> constexpr uint32_t InsnBitWidth = 0;
DecoderTableBuilder TableBuilder(Target, Encodings, TableInfo);
unsigned OpcodeMask = 0;

bool HasConflict = false;
for (const auto &[BitWidth, BWMap] : EncMap) {
for (const auto &[Key, EncodingIDs] : BWMap) {
auto [DecoderNamespace, HwModeID] = Key;
Expand All @@ -2585,7 +2598,10 @@ template <typename T> constexpr uint32_t InsnBitWidth = 0;
// across all decoder tables.
// - predicates are shared across all decoder tables.
TableInfo.Table.clear();
TableBuilder.buildTable(FC, BitWidth);
HasConflict |= TableBuilder.buildTable(FC, BitWidth);
// Skip emitting table entries if a conflict has been detected.
if (HasConflict)
continue;

// Print the table to the output stream.
OpcodeMask |= emitTable(OS, TableInfo.Table, DecoderNamespace, HwModeID,
Expand All @@ -2600,6 +2616,9 @@ template <typename T> constexpr uint32_t InsnBitWidth = 0;
}
}

if (HasConflict)
PrintFatalError("Decoding conflict encountered");

// Emit the decoder function for the last bucket. This will also emit the
// single decoder function if SpecializeDecodersPerBitwidth = false.
if (!SpecializeDecodersPerBitwidth)
Expand Down