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[LLVM][DecoderEmitter] Add option to use function table in decodeToMCInst (#144814)

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Add option `use-fn-table-in-decode-to-mcinst` to use a table of function
pointers instead of a switch case in the generated `decodeToMCInst`
function.

When the number of switch cases in this function is large, the generated
code takes a long time to compile in release builds. Using a table of
function pointers instead improves the compile time significantly (~3x
speedup in compiling the code in a downstream target). This option will
allow targets to opt into this mode if they desire for better build
times.

Tested with `check-llvm-mc` with the option enabled by default.
This commit is contained in:
Rahul Joshi
2025-06-24 18:49:05 -07:00
committed by GitHub
parent f608716b09
commit ed5f8f238d
2 changed files with 153 additions and 24 deletions
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84
llvm/test/TableGen/DecoderEmitterFnTable.td Normal file
View File

@@ -0,0 +1,84 @@
// RUN: llvm-tblgen -gen-disassembler -use-fn-table-in-decode-to-mcinst -I %p/../../include %s | FileCheck %s
include "llvm/Target/Target.td"
def archInstrInfo : InstrInfo { }
def arch : Target {
let InstructionSet = archInstrInfo;
}
let Namespace = "arch" in {
def R0 : Register<"r0">;
def R1 : Register<"r1">;
def R2 : Register<"r2">;
def R3 : Register<"r3">;
}
def Regs : RegisterClass<"Regs", [i32], 32, (add R0, R1, R2, R3)>;
class TestInstruction : Instruction {
let Size = 1;
let OutOperandList = (outs);
field bits<8> Inst;
field bits<8> SoftFail = 0;
}
// Define instructions to generate 4 cases in decodeToMCInst.
// Lower 2 bits define the number of operands. Each register operand
// needs 2 bits to encode.
// An instruction with no inputs. Encoded with lower 2 bits = 0 and upper
// 6 bits = 0 as well.
def Inst0 : TestInstruction {
let Inst = 0x0;
let InOperandList = (ins);
let AsmString = "Inst0";
}
// An instruction with a single input. Encoded with lower 2 bits = 1 and the
// single input in bits 2-3.
def Inst1 : TestInstruction {
bits<2> r0;
let Inst{1-0} = 1;
let Inst{3-2} = r0;
let InOperandList = (ins Regs:$r0);
let AsmString = "Inst1";
}
// An instruction with two inputs. Encoded with lower 2 bits = 2 and the
// inputs in bits 2-3 and 4-5.
def Inst2 : TestInstruction {
bits<2> r0;
bits<2> r1;
let Inst{1-0} = 2;
let Inst{3-2} = r0;
let Inst{5-4} = r1;
let InOperandList = (ins Regs:$r0, Regs:$r1);
let AsmString = "Inst2";
}
// An instruction with three inputs. Encoded with lower 2 bits = 3 and the
// inputs in bits 2-3 and 4-5 and 6-7.
def Inst3 : TestInstruction {
bits<2> r0;
bits<2> r1;
bits<2> r2;
let Inst{1-0} = 3;
let Inst{3-2} = r0;
let Inst{5-4} = r1;
let Inst{7-6} = r2;
let InOperandList = (ins Regs:$r0, Regs:$r1, Regs:$r2);
let AsmString = "Inst3";
}
// CHECK-LABEL: DecodeStatus decodeFn0(DecodeStatus S, InsnType insn, MCInst &MI, uint64_t Address, const MCDisassembler *Decoder, bool &DecodeComplete)
// CHECK-LABEL: DecodeStatus decodeFn1(DecodeStatus S, InsnType insn, MCInst &MI, uint64_t Address, const MCDisassembler *Decoder, bool &DecodeComplete)
// CHECK-LABEL: DecodeStatus decodeFn2(DecodeStatus S, InsnType insn, MCInst &MI, uint64_t Address, const MCDisassembler *Decoder, bool &DecodeComplete)
// CHECK-LABEL: DecodeStatus decodeFn3(DecodeStatus S, InsnType insn, MCInst &MI, uint64_t Address, const MCDisassembler *Decoder, bool &DecodeComplete)
// CHECK-LABEL: decodeToMCInst(unsigned Idx, DecodeStatus S, InsnType insn, MCInst &MI, uint64_t Address, const MCDisassembler *Decoder, bool &DecodeComplete)
// CHECK: static constexpr DecodeFnTy decodeFnTable[]
// CHECK-NEXT: decodeFn0,
// CHECK-NEXT: decodeFn1,
// CHECK-NEXT: decodeFn2,
// CHECK-NEXT: decodeFn3,
// CHECK: return decodeFnTable[Idx](S, insn, MI, Address, Decoder, DecodeComplete)

93
llvm/utils/TableGen/DecoderEmitter.cpp
View File

@@ -83,6 +83,14 @@ static cl::opt<bool> LargeTable(
"in the table instead of the default 16 bits."),
cl::init(false), cl::cat(DisassemblerEmitterCat));
static cl::opt<bool> UseFnTableInDecodetoMCInst(
"use-fn-table-in-decode-to-mcinst",
cl::desc(
"Use a table of function pointers instead of a switch case in the\n"
"generated `decodeToMCInst` function. Helps improve compile time\n"
"of the generated code."),
cl::init(false), cl::cat(DisassemblerEmitterCat));
STATISTIC(NumEncodings, "Number of encodings considered");
STATISTIC(NumEncodingsLackingDisasm,
"Number of encodings without disassembler info");
@@ -1066,31 +1074,67 @@ void DecoderEmitter::emitPredicateFunction(formatted_raw_ostream &OS,
void DecoderEmitter::emitDecoderFunction(formatted_raw_ostream &OS,
DecoderSet &Decoders,
indent Indent) const {
// The decoder function is just a big switch statement based on the
// input decoder index.
OS << Indent << "template <typename InsnType>\n";
OS << Indent << "static DecodeStatus decodeToMCInst(DecodeStatus S,"
<< " unsigned Idx, InsnType insn, MCInst &MI,\n";
OS << Indent << " uint64_t "
<< "Address, const MCDisassembler *Decoder, bool &DecodeComplete) {\n";
Indent += 2;
OS << Indent << "DecodeComplete = true;\n";
// The decoder function is just a big switch statement or a table of function
// pointers based on the input decoder index.
// TODO: When InsnType is large, using uint64_t limits all fields to 64 bits
// It would be better for emitBinaryParser to use a 64-bit tmp whenever
// possible but fall back to an InsnType-sized tmp for truly large fields.
OS << Indent
<< "using TmpType = "
"std::conditional_t<std::is_integral<InsnType>::"
"value, InsnType, uint64_t>;\n";
OS << Indent << "TmpType tmp;\n";
OS << Indent << "switch (Idx) {\n";
OS << Indent << "default: llvm_unreachable(\"Invalid index!\");\n";
for (const auto &[Index, Decoder] : enumerate(Decoders)) {
OS << Indent << "case " << Index << ":\n";
OS << Decoder;
OS << Indent + 2 << "return S;\n";
StringRef TmpTypeDecl =
"using TmpType = std::conditional_t<std::is_integral<InsnType>::value, "
"InsnType, uint64_t>;\n";
StringRef DecodeParams =
"DecodeStatus S, InsnType insn, MCInst &MI, uint64_t Address, const "
"MCDisassembler *Decoder, bool &DecodeComplete";
if (UseFnTableInDecodetoMCInst) {
// Emit a function for each case first.
for (const auto &[Index, Decoder] : enumerate(Decoders)) {
OS << Indent << "template <typename InsnType>\n";
OS << Indent << "DecodeStatus decodeFn" << Index << "(" << DecodeParams
<< ") {\n";
Indent += 2;
OS << Indent << TmpTypeDecl;
OS << Indent << "[[maybe_unused]] TmpType tmp;\n";
OS << Decoder;
OS << Indent << "return S;\n";
Indent -= 2;
OS << Indent << "}\n\n";
}
}
OS << Indent << "// Handling " << Decoders.size() << " cases.\n";
OS << Indent << "template <typename InsnType>\n";
OS << Indent << "static DecodeStatus decodeToMCInst(unsigned Idx, "
<< DecodeParams << ") {\n";
Indent += 2;
OS << Indent << "DecodeComplete = true;\n";
if (UseFnTableInDecodetoMCInst) {
// Build a table of function pointers.
OS << Indent << "using DecodeFnTy = DecodeStatus (*)(" << DecodeParams
<< ");\n";
OS << Indent << "static constexpr DecodeFnTy decodeFnTable[] = {\n";
for (size_t Index : llvm::seq(Decoders.size()))
OS << Indent + 2 << "decodeFn" << Index << ",\n";
OS << Indent << "};\n";
OS << Indent << "if (Idx >= " << Decoders.size() << ")\n";
OS << Indent + 2 << "llvm_unreachable(\"Invalid index!\");\n";
OS << Indent
<< "return decodeFnTable[Idx](S, insn, MI, Address, Decoder, "
"DecodeComplete);\n";
} else {
OS << Indent << TmpTypeDecl;
OS << Indent << "TmpType tmp;\n";
OS << Indent << "switch (Idx) {\n";
OS << Indent << "default: llvm_unreachable(\"Invalid index!\");\n";
for (const auto &[Index, Decoder] : enumerate(Decoders)) {
OS << Indent << "case " << Index << ":\n";
OS << Decoder;
OS << Indent + 2 << "return S;\n";
}
OS << Indent << "}\n";
}
OS << Indent << "}\n";
Indent -= 2;
OS << Indent << "}\n";
}
@@ -1267,7 +1311,8 @@ std::pair<unsigned, bool> FilterChooser::getDecoderIndex(DecoderSet &Decoders,
// FIXME: emitDecoder() function can take a buffer directly rather than
// a stream.
raw_svector_ostream S(Decoder);
bool HasCompleteDecoder = emitDecoder(S, indent(4), Opc);
indent Indent(UseFnTableInDecodetoMCInst ? 2 : 4);
bool HasCompleteDecoder = emitDecoder(S, Indent, Opc);
// Using the full decoder string as the key value here is a bit
// heavyweight, but is effective. If the string comparisons become a
@@ -2371,7 +2416,7 @@ static DecodeStatus decodeInstruction(const uint8_t DecodeTable[], MCInst &MI,
<< " makeUp(insn, Len);";
}
OS << R"(
S = decodeToMCInst(S, DecodeIdx, insn, MI, Address, DisAsm, DecodeComplete);
S = decodeToMCInst(DecodeIdx, S, insn, MI, Address, DisAsm, DecodeComplete);
assert(DecodeComplete);
LLVM_DEBUG(dbgs() << Loc << ": OPC_Decode: opcode " << Opc
@@ -2393,7 +2438,7 @@ static DecodeStatus decodeInstruction(const uint8_t DecodeTable[], MCInst &MI,
MCInst TmpMI;
TmpMI.setOpcode(Opc);
bool DecodeComplete;
S = decodeToMCInst(S, DecodeIdx, insn, TmpMI, Address, DisAsm, DecodeComplete);
S = decodeToMCInst(DecodeIdx, S, insn, TmpMI, Address, DisAsm, DecodeComplete);
LLVM_DEBUG(dbgs() << Loc << ": OPC_TryDecode: opcode " << Opc
<< ", using decoder " << DecodeIdx << ": ");