[llvm] [LLVM][DecoderEmitter] Add option to use lambdas in decodeToMCInst (PR #144814)
Rahul Joshi via llvm-commits
llvm-commits at lists.llvm.org
Wed Jun 18 16:48:41 PDT 2025
https://github.com/jurahul created https://github.com/llvm/llvm-project/pull/144814
Add option `use-lambda-in-decode-to-mcinst` to use a table of lambdas 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 lambdas 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.
>From 665180b709e207eef6f61e6f94d8d0cffc2a296c Mon Sep 17 00:00:00 2001
From: Rahul Joshi <rjoshi at nvidia.com>
Date: Tue, 3 Jun 2025 22:25:00 -0700
Subject: [PATCH] [LLVM][TableGen][DecoderEmitter] Add option to use lambdas in
decodeToMCInst
Add option `use-lambda-in-decode-to-mcinst` to use a table of lambdas
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
lambdas 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.
---
llvm/test/TableGen/DecoderEmitterLambda.td | 84 ++++++++++++++++++++++
llvm/utils/TableGen/DecoderEmitter.cpp | 41 +++++++++--
2 files changed, 118 insertions(+), 7 deletions(-)
create mode 100644 llvm/test/TableGen/DecoderEmitterLambda.td
diff --git a/llvm/test/TableGen/DecoderEmitterLambda.td b/llvm/test/TableGen/DecoderEmitterLambda.td
new file mode 100644
index 0000000000000..81bef10dc4a64
--- /dev/null
+++ b/llvm/test/TableGen/DecoderEmitterLambda.td
@@ -0,0 +1,84 @@
+// RUN: llvm-tblgen -gen-disassembler -use-lambda-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 atleast 3 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: decodeToMCInst
+// CHECK: decodeLambda0 =
+// CHECK: decodeLambda1 =
+// CHECK: decodeLambda2 =
+// CHECK: decodeLambda3 =
+// CHECK: decodeLambdaTable[] = {
+// CHECK-NEXT: decodeLambda0
+// CHECK-NEXT: decodeLambda1
+// CHECK-NEXT: decodeLambda2
+// CHECK-NEXT: decodeLambda3
+// CHECK: return decodeLambdaTable[Idx]();
diff --git a/llvm/utils/TableGen/DecoderEmitter.cpp b/llvm/utils/TableGen/DecoderEmitter.cpp
index 37814113b467a..ebaa08c3261f8 100644
--- a/llvm/utils/TableGen/DecoderEmitter.cpp
+++ b/llvm/utils/TableGen/DecoderEmitter.cpp
@@ -83,6 +83,13 @@ static cl::opt<bool> LargeTable(
"in the table instead of the default 16 bits."),
cl::init(false), cl::cat(DisassemblerEmitterCat));
+static cl::opt<bool> UseLambdaInDecodetoMCInst(
+ "use-lambda-in-decode-to-mcinst",
+ cl::desc("Use a table of lambdas 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");
@@ -1083,14 +1090,34 @@ void DecoderEmitter::emitDecoderFunction(formatted_raw_ostream &OS,
"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";
+
+ if (UseLambdaInDecodetoMCInst) {
+ // Emit one lambda for each case first.
+ for (const auto &[Index, Decoder] : enumerate(Decoders)) {
+ OS << Indent << "auto decodeLambda" << Index << " = [&]() {\n";
+ OS << Decoder;
+ OS << Indent + 2 << "return S;\n";
+ OS << Indent << "};\n";
+ }
+ // Build a table of lambdas.
+ OS << Indent
+ << "llvm::function_ref<DecodeStatus()> decodeLambdaTable[] = {\n";
+ for (size_t Index : llvm::seq(Decoders.size()))
+ OS << Indent + 2 << "decodeLambda" << Index << ",\n";
+ OS << Indent << "};\n";
+ OS << Indent << "if (Idx >= " << Decoders.size() << ")\n";
+ OS << Indent + 2 << "llvm_unreachable(\"Invalid index!\");\n";
+ OS << Indent << "return decodeLambdaTable[Idx]();\n";
+ } else {
+ 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";
}
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