Clang Build Extension

Overview

clang-build-ext is a setuptools plugin that builds Python C/C++ extensions using the LLVM/Clang compiler toolchain instead of the system default compiler. Either a system-installed LLVM/Clang or the karellen-llvm-clang package can be used.

The plugin provides drop-in replacements for setuptools' build_ext and build_clib commands, along with features such as glob pattern expansion in source lists, Drakon IR bytecode embedding, and thin static library support.

Basic Usage

Add the following to your setup.py:

from setuptools import setup, Extension
from karellen.clang_build_ext import ClangBuildExt, ClangBuildClib

setup(
    ...,
    ext_modules=[Extension("myext", ["src/*.c"])],
    cmdclass={
        "build_ext": ClangBuildExt,
        "build_clib": ClangBuildClib,
    },
)

LLVM Toolchain

The compiler uses the full LLVM toolchain:

Features

Glob Pattern Expansion

Source file lists in both extensions and libraries support shell glob patterns. Patterns are expanded at build time, so you don't need to enumerate every source file in setup.py:

setup(
    ...,
    ext_modules=[
        Extension("myext", ["src/module/*.c", "src/module/**/*.c"]),
    ],
    libraries=[
        ("mylib", {"sources": ["src/lib/*.c", "src/lib/**/*.c"]}),
    ],
    cmdclass={
        "build_ext": ClangBuildExt,
        "build_clib": ClangBuildClib,
    },
)

Drakon Enhancements

Drakon mode embeds LLVM intermediate representation (IR) bytecode into compiled binaries as custom ELF sections. This enables post-compilation IR analysis and transformation of the final binary.

When enabled, the build:

1. Compiles with --save-temps=obj -fno-discard-value-names to produce .bc (bitcode) files alongside object files
2. Includes .bc files in static libraries created via build_clib
3. After linking, extracts .bc files from all linked objects and static libraries and embeds them into the output binary as .drakon.<name> ELF sections (marked noload,readonly)

Enable via command line or environment variable:

# Command line
python setup.py build_ext --drakon
python setup.py build_ext -d

# Environment variable
DRAKON=1 python setup.py build_ext

Thin Static Libraries

Thin static libraries store references to object files rather than copies, reducing build artifact size during development.

Enable via command line or environment variable:

# Command line
python setup.py build_ext --thin
python setup.py build_ext -T

# Environment variable
THIN=1 python setup.py build_ext

Both options can be combined:

python setup.py build_ext --drakon --thin

The build_clib command inherits drakon and thin settings from build_ext automatically.

Setuptools Compatibility

clang-build-ext maintains compatibility across setuptools versions, including the API changes in setuptools 75+ (new C++ compiler executables) and 82+ (removal of the dry_run parameter).