How to Create a Custom C++ Compiler Extension

Creating a custom C++ compiler extension involves understanding the underlying mechanisms of compilers, modifying or extending their functionality, and integrating these changes seamlessly into the existing compiler infrastructure. This tutorial will guide you through the process, from understanding the basics to implementing and testing your custom extension. The target audience for this tutorial is developers who are already familiar with C++ and have a basic understanding of compiler concepts.

Understanding Compilers

Before diving into the creation of a custom compiler extension, it’s crucial to have a clear understanding of what a compiler does and its various stages. A typical compiler performs the following tasks:

1. Lexical Analysis: This stage converts the source code into tokens. A token is a string with an assigned and thus identified meaning.
2. Syntax Analysis: Also known as parsing, this stage checks if the tokens form a valid sequence according to the language grammar. It produces a syntax tree (parse tree).
3. Semantic Analysis: This stage checks the syntax tree for semantic errors. It ensures that the parse tree follows the rules of the language, such as type checking.
4. Intermediate Code Generation: The compiler translates the parse tree into an intermediate representation (IR), which is easier to optimize and translate into machine code.
5. Optimization: The IR is optimized for performance improvements like reducing the number of instructions.
6. Code Generation: The optimized IR is translated into target machine code.
7. Code Linking: The generated machine code is linked with libraries and other modules to produce an executable.

Popular C++ Compilers

For this tutorial, we’ll focus on two popular open-source C++ compilers: GCC (GNU Compiler Collection) and Clang (part of the LLVM project).

GCC (GNU Compiler Collection)

GCC is a compiler system produced by the GNU Project supporting various programming languages. It is a standard compiler for many Unix-like operating systems, including Linux. GCC has a modular architecture that allows for extensions and modifications.

Clang/LLVM

Clang is a compiler front end for the C, C++, and Objective-C programming languages. It uses LLVM as its back end. LLVM (Low-Level Virtual Machine) is a collection of modular and reusable compiler and toolchain technologies. Clang aims to provide a lightweight and modular compiler that can be used to build larger systems.

Setting Up the Development Environment

Before you start developing your custom compiler extension, you need to set up your development environment.

Installing GCC

On a Linux system, you can install GCC using the package manager. For example, on Ubuntu:

sudo apt-get update
sudo apt-get install build-essentialCode language: Bash (bash)

This command installs GCC along with other essential build tools.

Installing Clang/LLVM

Similarly, you can install Clang and LLVM on Ubuntu:

sudo apt-get install clang llvmCode language: Bash (bash)

For other operating systems, refer to the respective documentation for installation instructions.

Setting Up the Project

For this tutorial, we’ll set up a project directory where we’ll keep all our code and related files. Create a new directory for your project:

mkdir CustomCompilerExtension
cd CustomCompilerExtensionCode language: Bash (bash)

Extending the GCC Compiler

We’ll start with extending the GCC compiler. Suppose we want to add a custom attribute to functions that will trigger specific behavior during compilation. This could be useful for various purposes, such as custom optimizations or code generation tweaks.

Understanding GCC Plugins

GCC supports plugins, which are dynamic shared objects loaded at runtime. Plugins can extend GCC by adding new optimization passes, custom attributes, or even new language features.

Writing a GCC Plugin

Let’s write a simple GCC plugin that introduces a new attribute called custom_attr.

1. Create the Plugin Source File Create a file named custom_plugin.c in your project directory:

   #include <gcc-plugin.h>
   #include <tree.h>
   #include <plugin-version.h>
   #include <cp/cp-tree.h>

   int plugin_is_GPL_compatible;

   static void handle_custom_attr(tree *node, tree name, tree args, int flags, bool *no_add_attrs) {
       if (TREE_CODE(*node) == FUNCTION_DECL) {
           fprintf(stderr, "Function %s has custom_attr attribute\n", IDENTIFIER_POINTER(DECL_NAME(*node)));
       }
   }

   static struct attribute_spec custom_attr = {
       "custom_attr", 0, 0, false, false, false, handle_custom_attr, false
   };

   static void register_attributes(void *event_data, void *data) {
       register_scoped_attributes(&custom_attr, 1);
   }

   int plugin_init(struct plugin_name_args *plugin_info, struct plugin_gcc_version *version) {
       if (!plugin_default_version_check(version, &gcc_version)) {
           fprintf(stderr, "This GCC plugin is for version %s\n", gcc_version.basever);
           return 1;
       }

       register_callback(plugin_info->base_name, PLUGIN_ATTRIBUTES, register_attributes, NULL);
       return 0;
   }Code language: C++ (cpp)

This plugin defines a new attribute custom_attr and a handler function handle_custom_attr. When a function with this attribute is encountered, the handler prints a message to stderr.

2. Compile the Plugin To compile the plugin, use the following command:

   gcc -fPIC -shared -o custom_plugin.so custom_plugin.c -I$(gcc --print-file-name=plugin)Code language: Bash (bash)

This command generates a shared object file custom_plugin.so.

3. Using the Plugin To use the plugin, you need to pass the -fplugin option to GCC along with the path to the shared object file:

   gcc -fplugin=./custom_plugin.so -c your_source_file.cCode language: Bash (bash)

If your_source_file.c contains a function with the custom_attr attribute, you should see the corresponding message printed to stderr.

Example Usage

Consider the following C++ source file example.cpp:

void __attribute__((custom_attr)) my_function() {
    // Function implementation
}

int main() {
    my_function();
    return 0;
}Code language: C++ (cpp)

Compile it using the custom plugin:

gcc -fplugin=./custom_plugin.so -o example example.cppCode language: Bash (bash)

You should see the message “Function my_function has custom_attr attribute” printed to stderr.

Extending the Clang Compiler

Next, we’ll extend the Clang compiler. Suppose we want to add a custom diagnostic that warns whenever a function has more than a specified number of parameters.

Understanding Clang Plugins

Clang supports plugins, which allow you to extend its capabilities. Plugins can add new diagnostics, AST (Abstract Syntax Tree) visitors, or even custom code transformations.

Writing a Clang Plugin

Let’s write a Clang plugin that introduces a custom diagnostic for functions with too many parameters.

1. Create the Plugin Source File Create a file named TooManyParams.cpp in your project directory:

   #include "clang/AST/AST.h"
   #include "clang/Frontend/FrontendPluginRegistry.h"
   #include "clang/Frontend/CompilerInstance.h"
   #include "clang/AST/RecursiveASTVisitor.h"
   #include "clang/Basic/Diagnostic.h"

   using namespace clang;

   namespace {

   class TooManyParamsVisitor : public RecursiveASTVisitor<TooManyParamsVisitor> {
   public:
       explicit TooManyParamsVisitor(ASTContext *Context)
           : Context(Context) {}

       bool VisitFunctionDecl(FunctionDecl *D) {
           if (D->param_size() > 3) {
               DiagnosticsEngine &Diag = Context->getDiagnostics();
               unsigned DiagID = Diag.getCustomDiagID(DiagnosticsEngine::Warning, "Function has too many parameters");
               Diag.Report(D->getLocation(), DiagID);
           }
           return true;
       }

   private:
       ASTContext *Context;
   };

   class TooManyParamsConsumer : public ASTConsumer {
   public:
       explicit TooManyParamsConsumer(ASTContext *Context)
           : Visitor(Context) {}

       void HandleTranslationUnit(ASTContext &Context) override {
           Visitor.TraverseDecl(Context.getTranslationUnitDecl());
       }

   private:
       TooManyParamsVisitor Visitor;
   };

   class TooManyParamsAction : public PluginASTAction {
   protected:
       std::unique_ptr<ASTConsumer> CreateASTConsumer(CompilerInstance &CI, llvm::StringRef) override {
           return std::make_unique<TooManyParamsConsumer>(&CI.getASTContext());
       }

       bool ParseArgs(const CompilerInstance &CI, const std::vector<std::string> &args) override {
           return true;
       }
   };

   }

   static FrontendPluginRegistry::Add<TooManyParamsAction>
   X("too-many-params", "warn about functions with too many parameters");Code language: C++ (cpp)

This plugin defines a custom AST visitor that checks the number of parameters for each function declaration. If a function has more than three parameters, it emits a warning.

2. Compile the Plugin To compile the plugin, use the following command:

   clang++ -fPIC -shared -o TooManyParams.so TooManyParams.cpp `llvm-config --cxxflags --ldflags --system-libs --libs all`Code language: Bash (bash)

This command generates a shared object file TooManyParams.so.

3. Using the Plugin To use the plugin, you need to pass the -Xclang -load -Xclang options to Clang along with the path to the shared object file:

   clang++ -Xclang -load -Xclang ./TooManyParams.so -c your_source_file.cppCode language: Bash (bash)

If your_source_file.cpp contains a function with more than three parameters, you should see the corresponding warning.

Example Usage

Consider the following C++ source file example.cpp:

void my_function(int a, int b, int c, int d) {
    // Function implementation
}

int main() {
    my_function(1, 2, 3, 4);
    return 0;
}Code language: C++ (cpp)

Compile it using the custom plugin:

clang++ -Xclang -load -Xclang ./TooManyParams.so -o example example.cppCode language: Bash (bash)

You should see the warning “Function has too many parameters” emitted by the Clang compiler.

Integrating and Testing Custom Extensions

Once you’ve created your custom compiler extensions, it’s essential to integrate and test them thoroughly to ensure they work as expected.

Automated Testing

Automated tests help ensure that your custom compiler extensions function correctly and consistently. You can use testing frameworks or write custom scripts to automate the testing process.

Using a Testing Framework

For C++ projects, you can use frameworks like Google Test or Catch2 to write and run automated tests.

1. Installing Google Test On Ubuntu, you can install Google Test using the package manager:

   sudo apt-get install libgtest-devCode language: Bash (bash)

Then, compile the Google Test library:

   cd /usr/src/gtest
   sudo cmake CMakeLists.txt
   sudo make
   sudo cp *.a /usr/libCode language: Bash (bash)

2. Writing Tests Create a test file named test_example.cpp in your project directory:

   #include <gtest/gtest.h>

   extern void my_function(int, int, int, int);

   TEST(MyFunctionTest, TooManyParams) {
       EXPECT_NO_FATAL_FAILURE(my_function(1, 2, 3, 4));
   }

   int main(int argc, char **argv) {
       ::testing::InitGoogleTest(&argc, argv);
       return RUN_ALL_TESTS();
   }Code language: C++ (cpp)

3. Compiling and Running Tests Compile the test file along with your source file using Google Test and your custom plugin:

   clang++ -Xclang -load -Xclang ./TooManyParams.so -o test_example test_example.cpp example.cpp -lgtest -lgtest_main -pthread
   ./test_exampleCode language: Bash (bash)

This command compiles and runs the test, and you should see the Google Test output indicating whether the test passed or failed.

Manual Testing

In addition to automated tests, you can perform manual tests to verify the behavior of your custom compiler extensions. Create various test cases with different scenarios to ensure comprehensive coverage.

Example Manual Test Cases

1. Function with Less Than or Equal to Three Parameters

   void my_function(int a, int b, int c) {
       // Function implementation
   }Code language: C++ (cpp)

Expected Result: No warning emitted.

2. Function with More Than Three Parameters

   void my_function(int a, int b, int c, int d) {
       // Function implementation
   }Code language: C++ (cpp)

Expected Result: Warning “Function has too many parameters” emitted.

3. Functions with Different Signatures

   void my_function(int a) {
       // Function implementation
   }

   void another_function(double a, double b, double c, double d, double e) {
       // Function implementation
   }Code language: C++ (cpp)

Expected Result: Warning emitted only for another_function.

Continuous Integration

Integrating your custom compiler extensions into a continuous integration (CI) pipeline ensures that they are tested automatically with every code change. You can use CI services like GitHub Actions, Travis CI, or Jenkins to set up automated testing and deployment.

Example GitHub Actions Workflow

Create a .github/workflows/ci.yml file in your repository:

name: CI

on: [push, pull_request]

jobs:
  build:
    runs-on: ubuntu-latest

    steps:
    - name: Checkout code
      uses: actions/checkout@v2

    - name: Install dependencies
      run: sudo apt-get install -y clang llvm libgtest-dev cmake

    - name: Compile Google Test
      run: |
        cd /usr/src/gtest
        sudo cmake CMakeLists.txt
        sudo make
        sudo cp *.a /usr/lib

    - name: Build custom plugin
      run: clang++ -fPIC -shared -o TooManyParams.so TooManyParams.cpp `llvm-config --cxxflags --ldflags --system-libs --libs all`

    - name: Run tests
      run: |
        clang++ -Xclang -load -Xclang ./TooManyParams.so -o test_example test_example.cpp example.cpp -lgtest -lgtest_main -pthread
        ./test_exampleCode language: YAML (yaml)

This workflow checks out your code, installs the necessary dependencies, compiles the Google Test library and your custom plugin, and runs the tests.

Conclusion

Creating a custom C++ compiler extension can significantly enhance your development workflow by adding new features, diagnostics, or optimizations tailored to your needs. This tutorial covered the basics of extending GCC and Clang compilers, from writing simple plugins to integrating and testing them. By following these steps, you can create powerful and flexible compiler extensions to suit your specific requirements.

Further Reading

• GCC Internals Documentation
• Clang Plugins Documentation
• LLVM Programmer’s Manual

References

• GCC Plugins
• Clang Documentation
• Google Test Documentation

By understanding the internals of compilers and experimenting with custom extensions, you can unlock new possibilities for optimizing and analyzing your C++ code. Happy coding!