[llvm] ed5f8f2 - [LLVM][DecoderEmitter] Add option to use function table in decodeToMCInst (#144814)
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Tue Jun 24 18:49:08 PDT 2025
Author: Rahul Joshi
Date: 2025-06-24T18:49:05-07:00
New Revision: ed5f8f238d68811ad5eabcdb0248bcd65ab81960
URL: https://github.com/llvm/llvm-project/commit/ed5f8f238d68811ad5eabcdb0248bcd65ab81960
DIFF: https://github.com/llvm/llvm-project/commit/ed5f8f238d68811ad5eabcdb0248bcd65ab81960.diff
LOG: [LLVM][DecoderEmitter] Add option to use function table in decodeToMCInst (#144814)
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.
Added:
llvm/test/TableGen/DecoderEmitterFnTable.td
Modified:
llvm/utils/TableGen/DecoderEmitter.cpp
Removed:
################################################################################
diff --git a/llvm/test/TableGen/DecoderEmitterFnTable.td b/llvm/test/TableGen/DecoderEmitterFnTable.td
new file mode 100644
index 0000000000000..7bed18c19a513
--- /dev/null
+++ b/llvm/test/TableGen/DecoderEmitterFnTable.td
@@ -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)
diff --git a/llvm/utils/TableGen/DecoderEmitter.cpp b/llvm/utils/TableGen/DecoderEmitter.cpp
index 2e8ff2aa47d96..af25975f7c7ec 100644
--- a/llvm/utils/TableGen/DecoderEmitter.cpp
+++ b/llvm/utils/TableGen/DecoderEmitter.cpp
@@ -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 << ": ");
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