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[LLVM][MC][DecoderEmitter] Fail fatally if Insn and decoder table bitwidths mismatch #156734

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Merged
merged 4 commits into from
Sep 4, 2025
Merged

[LLVM][MC][DecoderEmitter] Fail fatally if Insn and decoder table bitwidths mismatch #156734

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300a2f8
[LLVM][MC][DecoderEmitter] Encode BitWidth at the start of the table
jurahul Sep 3, 2025
6b3caa7
Make mismatch a fatal error
jurahul Sep 3, 2025
1535051
Use assert instead of llvm_unreachable
jurahul Sep 3, 2025
34be441
Fix lit test
jurahul Sep 3, 2025
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22 changes: 18 additions & 4 deletions llvm/test/TableGen/DecoderEmitterBitwidthSpecialization.td
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Original file line number Diff line number Diff line change
Expand Up @@ -105,7 +105,8 @@ def Inst3 : Instruction16Bit<3> {
// When we specialize per bitwidth, we emit 2 decodeToMCInst functions and
// DecodeIdx is assigned per bit width.

// CHECK-SPECIALIZE-NO-TABLE-LABEL: DecoderTable8[25]
// CHECK-SPECIALIZE-NO-TABLE-LABEL: DecoderTable8[26]
// CHECK-SPECIALIZE-NO-TABLE: /* 0 */ 8, // Bitwidth 8
// CHECK-SPECIALIZE-NO-TABLE: DecodeIdx: 0
// CHECK-SPECIALIZE-NO-TABLE: DecodeIdx: 1
// CHECK-SPECIALIZE-NO-TABLE: };
Expand All @@ -116,7 +117,8 @@ def Inst3 : Instruction16Bit<3> {
// CHECK-SPECIALIZE-NO-TABLE: case 0
// CHECK-SPECIALIZE-NO-TABLE: case 1

// CHECK-SPECIALIZE-NO-TABLE-LABEL: DecoderTable16[25]
// CHECK-SPECIALIZE-NO-TABLE-LABEL: DecoderTable16[26]
// CHECK-SPECIALIZE-NO-TABLE: /* 0 */ 16, // Bitwidth 16
// CHECK-SPECIALIZE-NO-TABLE: DecodeIdx: 0
// CHECK-SPECIALIZE-NO-TABLE: DecodeIdx: 1
// CHECK-SPECIALIZE-NO-TABLE: };
Expand All @@ -127,11 +129,17 @@ def Inst3 : Instruction16Bit<3> {
// CHECK-SPECIALIZE-NO-TABLE: case 0
// CHECK-SPECIALIZE-NO-TABLE: case 1

// CHECK-SPECIALIZE-NO-TABLE-LABEL: template <typename InsnType>
// CHECK-SPECIALIZE-NO-TABLE-NEXT: decodeInstruction
// CHECK-SPECIALIZE-NO-TABLE: uint32_t BitWidth = decodeULEB128AndIncUnsafe(Ptr);
// CHECK-SPECIALIZE-NO-TABLE-NEXT: assert(InsnBitWidth<InsnType> == BitWidth &&

// -----------------------------------------------------------------------------
// Per bitwidth specialization with function table.

// 8 bit deccoder table, functions, and function table.
// CHECK-SPECIALIZE-TABLE-LABEL: DecoderTable8[25]
// CHECK-SPECIALIZE-TABLE-LABEL: DecoderTable8[26]
// CHECK-SPECIALIZE-TABLE: /* 0 */ 8, // Bitwidth 8
// CHECK-SPECIALIZE-TABLE: DecodeIdx: 0
// CHECK-SPECIALIZE-TABLE: DecodeIdx: 1
// CHECK-SPECIALIZE-TABLE: };
Expand All @@ -153,7 +161,8 @@ def Inst3 : Instruction16Bit<3> {
// CHECK-SPECIALIZE-TABLE-NEXT: };

// 16 bit deccoder table, functions, and function table.
// CHECK-SPECIALIZE-TABLE-LABEL: DecoderTable16[25]
// CHECK-SPECIALIZE-TABLE-LABEL: DecoderTable16[26]
// CHECK-SPECIALIZE-TABLE: /* 0 */ 16, // Bitwidth 16
// CHECK-SPECIALIZE-TABLE: DecodeIdx: 0
// CHECK-SPECIALIZE-TABLE: DecodeIdx: 1
// CHECK-SPECIALIZE-TABLE: };
Expand All @@ -173,3 +182,8 @@ def Inst3 : Instruction16Bit<3> {
// CHECK-SPECIALIZE-TABLE-NEXT: decodeFn_16bit_0,
// CHECK-SPECIALIZE-TABLE-NEXT: decodeFn_16bit_1,
// CHECK-SPECIALIZE-TABLE-NEXT: };

// CHECK-SPECIALIZE-TABLE-LABEL: template <typename InsnType>
// CHECK-SPECIALIZE-TABLE-NEXT: decodeInstruction
// CHECK-SPECIALIZE-TABLE: uint32_t BitWidth = decodeULEB128AndIncUnsafe(Ptr);
// CHECK-SPECIALIZE-TABLE-NEXT: assert(InsnBitWidth<InsnType> == BitWidth &&
29 changes: 26 additions & 3 deletions llvm/utils/TableGen/DecoderEmitter.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -611,7 +611,11 @@ class DecoderTableBuilder {
DecoderTableInfo &TableInfo)
: Target(Target), Encodings(Encodings), TableInfo(TableInfo) {}

void buildTable(const FilterChooser &FC) const {
void buildTable(const FilterChooser &FC, unsigned BitWidth) const {
// 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);
assert(TableInfo.FixupStack.empty() && "Fixup stack phasing error!");
}
Expand Down Expand Up @@ -774,6 +778,15 @@ unsigned DecoderEmitter::emitTable(formatted_raw_ostream &OS,
OS << "Skip to: " << Index;
};

// The first entry when specializing decoders per bitwidth is the bitwidth.
// This will be used for additional checks in `decodeInstruction`.
if (SpecializeDecodersPerBitwidth) {
OS << "/* 0 */";
OS.PadToColumn(14);
emitULEB128(I, OS);
OS << " // Bitwidth " << BitWidth << '\n';
}

unsigned OpcodeMask = 0;

while (I != E) {
Expand Down Expand Up @@ -2108,9 +2121,19 @@ static DecodeStatus decodeInstruction(const uint8_t DecodeTable[], MCInst &MI,
if (HasCheckPredicate)
OS << " const FeatureBitset &Bits = STI.getFeatureBits();\n";
OS << " using namespace llvm::MCD;\n";
OS << " const uint8_t *Ptr = DecodeTable;\n";

if (SpecializeDecodersPerBitwidth) {
// Fail with a fatal error if decoder table's bitwidth does not match
// `InsnType` bitwidth.
OS << R"(
[[maybe_unused]] uint32_t BitWidth = decodeULEB128AndIncUnsafe(Ptr);
assert(InsnBitWidth<InsnType> == BitWidth &&
"Table and instruction bitwidth mismatch");
)";
}

OS << R"(
const uint8_t *Ptr = DecodeTable;
uint64_t CurFieldValue = 0;
DecodeStatus S = MCDisassembler::Success;
while (true) {
Expand Down Expand Up @@ -2554,7 +2577,7 @@ 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);
TableBuilder.buildTable(FC, BitWidth);

// Print the table to the output stream.
OpcodeMask |= emitTable(OS, TableInfo.Table, DecoderNamespace, HwModeID,
Expand Down