Instruction Frequency Analysis Pass For LLVM/Clang

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Introduction

In the realm of compiler development, understanding the frequency of instructions in a program can provide valuable insights into its performance and optimization potential. This article describes the development of a new analysis pass in the LLVM compiler framework that iterates over all functions in each program and classifies each instruction into predefined categories. The pass then creates a frequency table for each function, detailing the number of instructions in each category. This frequency table is emitted into a file with the same name as the source file but with a ‘.ic’ extension.

Instruction Categories

The instructions will be classified into the following categories:

  • Arithmetic: Includes instructions for addition, subtraction, multiplication, division, remainder, etc.
  • Logical: Includes bitwise operations like AND, OR, XOR, and shifting instructions.
  • Comparison: Includes integer and floating-point comparisons (e.g., icmp, fcmp).
  • Memory: Includes memory operations like load, store, allocation (alloca), and element access (getelementptr).
  • Control Flow: Includes branching (br, condbr), multiway branching (switch), and phi-nodes (phi).
  • Function Call: Includes function call instructions (call, invoke) and return instructions (ret).

Key Tasks

To implement the instruction frequency analysis pass, the following tasks need to be completed:

  • Modify Clang to accept a new command-line option: Modify the Clang compiler to accept a new command-line option (e.g., -emit-instr-freq) that enables the analysis pass.
  • Implement a new analysis pass in LLVM: Implement a new analysis pass in LLVM that iterates over all functions in the input program and classifies each instruction into one of the predefined categories.
  • Maintain a frequency table for each function: Maintain a frequency table for each function in the source file being compiled.
  • Emit the frequency table for each function: For each source file compiled, emit the frequency table for each function into a ‘<source_file>.ic’ file.
  • Validate the functionality: Validate the functionality against a set of popular open-source C/C++ projects (e.g., LLVM itself, SQLite, Git, Cmake).

Implementation

To implement the instruction frequency analysis pass, the following steps need to be followed:

Step 1: Modify Clang to accept a new command-line option

To enable the analysis pass, a new command-line option needs to be added to Clang. This can be done by modifying the clang source code to accept the new option.

// clang/lib/Driver/ToolArgs.cpp
void ClangTool::addEmitInstrFreqOption(ClangToolOptions &Opts) {
    Opts.addOption(
        opt::Option(
            "emit-instr-freq",
            "Enable instruction frequency analysis pass",
            opt::value<std::string>(),
            opt::value<std::string>()
        )
    );
}

Step 2: Implement a new analysis pass in LLVM

To implement the analysis pass, a new pass needs to be added to the LLVM pipeline. This can be done by creating a new pass class that inherits from the Pass class.

// llvm/lib/Passes/Analysis/InstrFreqAnalysisPass.cpp
class InstrFreqAnalysisPass : public Pass {
public:
    static char ID;
    InstrFreqAnalysisPass() : Pass(ID) {}

    bool runOnFunction(Function &F) override {
        // Classify instructions into categories
        for (auto &Inst : F) {
            // ...
        }

        // Create frequency table for each function
        FrequencyTable FT;
        for (auto &Inst : F) {
            FT.addClass(Inst);
        }

        // Emit frequency table for each function
        emitFrequencyTable(FT);
        return true;
    }

    void emitFrequencyTable(FrequencyTable &FT) {
        // ...
    }
};

Step 3: Maintain a frequency table for each function

To maintain a frequency table for each function, a new data structure needs to be created to store the frequency of each instruction category.

// llvm/lib/Passes/Analysis/FrequencyTable.h
class FrequencyTable {
public:
    void addClass(Instruction &Inst) {
        // ...
    }

    void emitTable() {
        // ...
    }
};

Step 4: Emit the frequency table for each function

To emit the frequency table for each function, a new function needs to be added to the analysis pass that writes the frequency table to a file.

// llvm/lib/Passes/Analysis/InstrFreqAnalysisPass.cpp
void InstrFreqAnalysisPass::emitFrequencyTable(FrequencyTable &FT) {
    // ...
    std::ofstream file("output.ic");
    FT.emitTable(file);
    file.close();
}

Step 5: Validate the functionality

To validate the functionality, the analysis pass needs to be tested against a set of popular open-source C/C++ projects.

// llvm/test/Analysis/InstrFreqAnalysisPass.cpp
TEST(InstrFreqAnalysisPass, Test1) {
    // ...
}

TEST(InstrFreqAnalysisPass, Test2) {
    // ...
}

Example Use Case

To use the instruction frequency analysis pass, the following command can be used:

clang -emit-instr-freq input.c -o output

This will enable the analysis pass and emit the frequency table for each function into a file named output.ic.

Expected Output

The expected output of the analysis pass is a frequency table for each function in the source file, detailing the number of instructions in each category.

Function,Arithmetic,Logical,Comparison,Memory,Control Flow,Function Call
add,1,0,0,1,0,1
main,0,0,1,5,3,3

Q: What is the Instruction Frequency Analysis Pass?

A: The Instruction Frequency Analysis Pass is a new analysis pass in the LLVM compiler framework that iterates over all functions in each program and classifies each instruction into predefined categories. The pass then creates a frequency table for each function, detailing the number of instructions in each category.

Q: What are the predefined categories for instructions?

A: The instructions will be classified into the following categories:

  • Arithmetic: Includes instructions for addition, subtraction, multiplication, division, remainder, etc.
  • Logical: Includes bitwise operations like AND, OR, XOR, and shifting instructions.
  • Comparison: Includes integer and floating-point comparisons (e.g., icmp, fcmp).
  • Memory: Includes memory operations like load, store, allocation (alloca), and element access (getelementptr).
  • Control Flow: Includes branching (br, condbr), multiway branching (switch), and phi-nodes (phi).
  • Function Call: Includes function call instructions (call, invoke) and return instructions (ret).

Q: How does the analysis pass work?

A: The analysis pass works as follows:

  1. Modify Clang to accept a new command-line option: Modify the Clang compiler to accept a new command-line option (e.g., -emit-instr-freq) that enables the analysis pass.
  2. Implement a new analysis pass in LLVM: Implement a new analysis pass in LLVM that iterates over all functions in the input program and classifies each instruction into one of the predefined categories.
  3. Maintain a frequency table for each function: Maintain a frequency table for each function in the source file being compiled.
  4. Emit the frequency table for each function: For each source file compiled, emit the frequency table for each function into a ‘<source_file>.ic’ file.

Q: What is the expected output of the analysis pass?

A: The expected output of the analysis pass is a frequency table for each function in the source file, detailing the number of instructions in each category.

Function,Arithmetic,Logical,Comparison,Memory,Control Flow,Function Call
add,1,0,0,1,0,1
main,0,0,1,5,3,3

Q: How can I use the Instruction Frequency Analysis Pass?

A: To use the Instruction Frequency Analysis Pass, you can use the following command:

clang -emit-instr-freq input.c -o output

This will enable the analysis pass and emit the frequency table for each function into a file named output.ic.

Q: What are the benefits of using the Instruction Frequency Analysis Pass?

A: The benefits of using the Instruction Frequency Analysis Pass include:

  • Improved code optimization: By understanding the frequency of instructions in a program, developers can optimize the code to reduce the number of instructions and improve performance.
  • Better code analysis: The analysis pass provides a detailed breakdown of the instructions in each function, which can be useful for code and debugging.
  • Enhanced code understanding: The frequency table provides a clear understanding of the instructions in each function, which can help developers understand the code better.

Q: Can I customize the Instruction Frequency Analysis Pass?

A: Yes, you can customize the Instruction Frequency Analysis Pass by modifying the analysis pass to include additional categories or instructions. You can also modify the frequency table to include additional information.

Q: Is the Instruction Frequency Analysis Pass compatible with all LLVM passes?

A: The Instruction Frequency Analysis Pass is designed to be compatible with all LLVM passes. However, you may need to modify the analysis pass to work with specific passes or optimizations.

Q: Can I use the Instruction Frequency Analysis Pass with other compilers?

A: The Instruction Frequency Analysis Pass is designed to work with the Clang compiler. However, you may be able to modify the analysis pass to work with other compilers by modifying the analysis pass to use the compiler's API.