# Hacking on Clang

This document provides some hints for how to get started hacking on Clang for developers who are new to the Clang and/or LLVM codebases.

- Developer Documentation
- Debugging
- Testing
- LLVM IR Generation

## Developer Documentation

Both Clang and LLVM use doxygen to provide API documentation. Their respective web pages (generated nightly) are here:

- Clang
- LLVM

For work on the LLVM IR generation, the LLVM assembly language reference manual is also useful.

## Debugging

Inspecting data structures in a debugger:

- Many LLVM and Clang data structures provide a `dump()` method which will print a description of the data structure to `stderr`.
- The `QualType` structure is used pervasively. This is a simple value class for wrapping types with qualifiers; you can use the `isConstQualified()`, for example, to get one of the qualifiers, and the `getTypePtr()` method to get the wrapped `Type*` which you can then dump.

## Testing

Clang includes a basic regression suite in the tree which can be run with `make test` from the top-level clang directory, or just `make` in the test sub-directory. `make report` can be used after running the tests to summarize the results, and `make VERBOSE=1` can be used to show more detail about what is being run.

For more intensive changes, running the LLVM Test Suite with clang is recommended. Currently the best way to override LLVMGCC, as in: `make LLVMGCC="ccc -std=gnu89" TEST=nightly report` (make sure ccc is in your PATH or use the full path).

## LLVM IR Generation

The LLVM IR generation part of clang handles conversion of the AST nodes output by the Sema module to the LLVM Intermediate Representation (IR). Historically, this was referred to as "codegen", and the Clang code for this lives in `lib/CodeGen`.

The output is most easily inspected using the `-emit-llvm` option to clang (possibly in conjunction with `-o -`). You can also use `-emit-llvm-bc` to write an LLVM bitcode file which can be processed by the suite of LLVM tools like `llvm-dis`, `llvm-nm`, etc. See the LLVM Command Guide for more information.