C17 compiler implementation from scratch

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29 days ago

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29 days ago

#Kefir C compiler

This repository contains an implementation of C17 language compiler from scratch. No existing open source compiler infrastructure is being reused. The main priority is self-sufficiency of the project, compatibility with platform ABI and compliance with C17 language standard. Any omissions or incompatibilities between the language standard and Kefir behavior which are not explicitly documented (see Implementation & Usage quirks section below) shall be considered bugs.

Kefir supports modern x86-64 Linux, FreeBSD, OpenBSD and NetBSD environments (see Supported environments section below). Compiler is also able to produce JSON streams containing program representation on various stages of compilation (tokens, AST, IR), as well as printing source code in preprocessed form. By default, the compiler outputs GNU As-compatible assembly (Intel syntax with/without prefixes and ATT syntax are supported). Position-independent code generation is supported. Kefir features cc-compatible command line interface.

Kefir website also provides some additional information.

Note to the users of Kefir: if you encounter any behavior that does not comply with C language standard or significantly diverges from other compilers, please do no hestitate to reach me out via email directly or through the mailing list. Code snippets for easier reproduction are especially welcome.

#Project name

Kefir compiler is named after fermented milk drink, no other connotations are meant or intended.

#Supported environments

Kefir targets x86-64 ISA and System-V ABI. Supported systems include modern Linux, FreeBSD, OpenBSD and NetBSD operating systems (full test suite is executed regularly in these environments). A platform is considered supported if full automated test suite (see Test suite section below) sucessfully executes there -- no other guarantees and claims are made. On Linux, glibc and musl standard libraries are supported; musl might be preferable because it's header files are more compilant with standard C language without extensions, however as of now Kefir supports enough GCC extensions to reasonably use include files from glibc, on BSDs system libc can be used (additional macro definitions, such as __GNUC__, __GNUC_MINOR__, could be necessary depending on used system libc features). Kefir supports selection of target platform via --target command line option.

For each respective target, compiler expects a set of environment variables (e.g. KEFIR_GNU_INCLUDE, KEFIR_GNU_LIB, KEFIR_GNU_DYNAMIC_LINKER) to be present in order to correctly configure system include and library paths -- the default values for these variables are detected upon Kefir build, however in the event of any changes to system toolchain (e.g. after upgrades) configuration needs to be re-generated.

In addition, see Implementation & Usage quirks section below for some other specifics of Kefir.

#Motivation & goals

The main motivation of the project is deeper understanding of C programming language, as well as practical experience in the broader scope of compiler implementation aspects. Based on this, following goals were set for the project:

  • Self-sufficiency - project shall use minimal number of external dependencies. Runtime dependencies should include only C standard library and operating system APIs.
  • Compliance with C17 standard - resulting product should be reasonably compliant with language standard. All intentional deviations and exceptions shall be described and justified.
  • Compatibility with platform ABI - produced code should adhere ABI of target platform. It should be possible to transparently link it with code produced by commonly used compilers of the target platform.
  • Manageable scope of the project - full-fledged implementation of C17 compiler is demanding task. Project scope shall be managed so that implementation as a single-person pet-project is feasible. For instance, standard library implementation is currently out-of-scope. Instead, compiler supports some of widespread C extensions in order to re-use existing libc implementations.
  • Portability - compiler code itself should be easily portable across different environments. Currently, the development is concentrated on a single target platform, however it might be extended in the future.

Following things are NON-goals:

  • Performance - trying to outcompete well-established compiler backends, such as GCC backend or LLVM, is not reasonable, thus performance was never considered a goal, even though some improvements can be occasionally made. In fact, performance is deliberately sacrificed to facilitate implementation of other goals.
  • Compatibility with other compiler extensions - C compilers are known to include different extensions that are not described by language standard. Considerable number of those are implemented, however it is not project goal per se, thus there are no guarantees of extension compatibility.

Note on the language standard support: initially the compiler development was focused on C11 language standard support. The migration to C17 happened when the original plan was mostly finished. During the migration applicable DRs from those included in C17 were inspected and code was updated accordingly. The compiler itself is still written in compliance with C11 language standard.

#Current status


The initial implementation has been finished: at the momement the compiler features all necessary components, and with some known minor idiosyncrasies supports C17 language standard. Kefir is able to re-use standard library provided by target systems. Further effort is concentrated on improving and extending the compiler, including:

  • Implementing optimizing code generator -- initial basic implementation has been finished. Pending subtasks are:
    • Implementing actual optimization passes -- several basic optimizations have been implemented.
    • Implementing atomics natively instead of relying on software library.
    • Producing debug information during code generation.
    • Position-independent code generation, building position-independent executables and shared libraries is supported.
  • Adding support for upcoming C23 standard.
  • Improving compatibility with mainstream compiler by implementing additional extensions and built-ins.
  • Bugfixes, improvements in error reporting.
  • Extending the number of supported platforms.
  • Reimplementing parser, lexer and register allocator for better performance.
  • Refactoring and cleaning up analysis and translation stage implementation.

#Implementation quirks

Some implementation details that user needs to take into account:

  • Attention: code produced by Kefir shall be linked with a runtime library libkefirrt.a. The library is linked automatically if environment is configured correctly. Kefir can also link built-in versions of runtime, however make sure that correct --target is specified during link phase. Kefir might provide own versions of some header files as well -- if environment is configured correctly, they are also added to include path automatically.
  • Atomic implementation fully relies on software atomic library (libatomic for GCC, libcompiler_rt for Clang), thus any program that employs atomic operations need to link a libatomic-compatible library. It happens by default for Glibc and *BSD targets. Furthermore, if <stdatomic.h> header from Clang includes is used (the default on FreeBSD and OpenBSD), -D__GNUC__=4 -D__GNUC_MINOR__=20 command line arguments shall be added to Kefir invocation.
  • Atomic operations on long double variables (both scalar and complex) might result in undefined behavior due to uninitialized padding contained at the last 48 bits of long double storage unit. Kefir implements padding zeroing upon long double storage to mitigate this issue, however linking object files produced by Kefir and other compilers might provoke the undefined behavior and needs to be done with care.
  • Unicode and wide strings are supported under the assumption that source and target character sets are the same. No re-encoding is performed.
  • No STDC pragmas are implemented in preprocessor. Kefir does not perform respective optimizations and implements conservative behavior, thus these pragmas would be no-op.

#Standard library

Kefir can be used along with musl libc standard library. Kefir also supports glibc, as well as libc implementations provided with FreeBSD, OpenBSD and NetBSD, however header files from these libraries tend to include non-standard compiler features, and thus support might vary for different library versions. In practice, Kefir implements enough compiler extensions to make use of all target system standard libraries. However, additional macro definitions (such as __GNUC__ and __GNUC_MINOR__ on BSD systems) might be needed for successful compilation.

#Language extensions

Several C language extensions are implemented for better compatibility with GCC. All of them are enabled by default in the driver, but disabled if the compiler is invoked directly (consult the manual for details). No specific compability guarantees are provided. Among the implemented extensions (non-exaustive list):

  • Implicit function declarations -- if no function declaration is present at call-site, int function_name() is automatically defined. The feature was part of previous C standards, however it's absent from C11 onwards.
  • int as implicit function return type -- function definition may omit return type, int will be used instead.
  • Designated initializers in fieldname: form -- old, deprecated form which is still supported by GCC.
  • Labels-as-values -- labels can be addressed with && operator, gotos support arbitratry addresses in goto * form.
  • Automatic type inference -- __typeof__, __typeof_unqual__, __auto_type type specifiers.
  • Vararg builtins.
  • Some of __atomic* and __sync* builtins.
  • Full list of built-ins can be obtained here.

Kefir also defines a few non-standard macros by default, such as macros indicating data model (__LP64__), endianess (__BYTE_ORDER__ and __ORDER_LITTLE_ENDIAN__), as well as __KEFIRCC__ which can be used to identify the compiler.

Kefir has support of asm directive, both in file and function scope. Implementation supports output and input parameters, parameter constraints (immediate, register, memory), clobbers and jump labels, however there is no compatibility with respective GCC/Clang functionality (implemented bits behave similarly, though, thus basic use cases shall be compatible). Additionally, asm-labels are supported for non-static non-local variables.

Kefir supports __attribute__(...) syntax on parser level, however attributes are ignored in most cases except aligned/__aligned__ and __gnu_inline__ attributes. Presence of attribute in source code can be turned into a syntax error by CLI option.

#Build & Usage

Disclaimer: Use at your own risk. This is experimental project which is not meant for production purposes. No guarantees are being made for correctness, completeness, stability and fitness for any particular purpose.

Kefir depends on a C11 compiler (tested with gcc and clang), GNU As assembler, GNU Makefile as well as basic UNIX utilities for build. Development and test dependencies include valgrind (for test execution) as well. After installing all dependencies, kefir can be built with a single command: make all EXTRA_CFLAGS="-march=native" -j$(nproc). By default, kefir builds a shared library and links executables to it. Static linkage can be used by specifying USE_SHARED=no in make command line arguments. Sample PKGBUILD is provided in dist/kefir directory.

It is also advised to run basic test suite:

LC_ALL=C.UTF-8 make test all   # Linux
gmake test all CC=clang        # FreeBSD
gmake test all CC=clang AS=gas # OpenBSD
gmake test all CC=gcc AS=gas   # NetBSD

Optionally, Kefir can be installed via: make install DESTDIR=.... Short reference on compiler options can be obtained by running kefir --help, as well as in the manual which is supplied in the compiler distribution.

At the moment, Kefir is automatically tested in Ubuntu 22.04, FreeBSD 13.2 and OpenBSD 7.3 and NetBSD 9.3 environments. Arch Linux is used as a primary development environment.

#Web playground

Kefir supports compilation with Emscripten into a WebAssembly library, which can be invoked from client-side JavaScript in Web-applications. Kefir functionality in that mode in limited due to absence of normal POSIX environment, linker and assembler utilities: only text output (assembly code, preprocessed code, tokens, ASTs, IR) can be produced from a single input file. Furthermore, all include files need to be explicitly supplied from JavaScript side in order to be available during compilation. Note that this does not imply support for WebAssembly as a compilation target: it only serves as a host environment. To build kefir.js and kefir.wasm in bin/web directory, use:

make web -j$(nproc) # Requires Emscripten installed

A simple playground Web application is also available. It bundles Kefir web build with Musl include files and provides a simple Godbolt-like interface. Build as follows:

make webapp -j$(nproc)

The Web application is static and requires no server-side logic. An example of simple server command-line:

python -m  http.server 8000 -d bin/webapp

A hosted version of the Web application is available at Kefir playground (please note that the Web page uses JavaScript and WebAssembly).


Kefir is capable of bootstraping itself (that is, compiling it's own source code). It can be performed as follows:

make bootstrap -j$(nproc)

Alternatively, bootstrap can be performed manually:

# Stage 0: Build & Test initial Kefir version with system compiler.
#          Produces dynamically-linked binary in bin/kefir and
#          shared library bin/libs/libkefir.so
make test all -j$(nproc)
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:$(pwd)/bin/libs
# Stage 1: Use previously built Kefir to compile itself.
#          Replace $MUSL with actual path to musl installation.
#          Produces statically-linked binary bin/bootstrap1/kefir
make -f bootstrap.mk bootstrap SOURCE=$(pwd)/source HEADERS=$(pwd)/headers BOOTSTRAP=$(pwd)/bootstrap/stage1 KEFIRCC=./bin/kefir \
rm -rf bin # Remove kefir version produced by stage 0
# Stage 2: Use bootstrapped Kefir version to compile itself once again.
#          Replace $MUSL with actual path to musl installation.
#          Produces statically-linked binary bin/bootstrap2/kefir
make -f bootstrap.mk bootstrap SOURCE=$(pwd)/source HEADERS=$(pwd)/headers BOOTSTRAP=$(pwd)/bootstrap/stage2 KEFIRCC=./bootstrap/stage1/kefir \
# Stage 3: Diff assembly files generated by Stage 1 and Stage 2.
#          They shall be identical
./scripts/bootstrap_compare.sh bootstrap/stage1 bootstrap/stage2

Furthermore, kefir can also be bootstrapped using normal build process:

make all CC=$PATH_TO_KEFIR -j$(nproc)

#Test suite

Kefir relies on following tests, most of which are executed as part of CI:

  • Own (base) test suite that includes:
    • Unit tests
    • Integration tests -- each test is a self-contained program that executes some part of compilation process, produces a text output which is then compared to the expected.
    • System tests -- each test is a self-contained program that performs compilation process, starting from some stage (e.g. compiling a program defined as AST structure or IR bytecode) and produces an assembly output, which is then combined with the remaining part of test case containing asserts (compiled with system compiler) and executed.
    • End-to-end tests -- each test consists of multiple *.c files which are compiled either using system compiler or kefir depending on file extension. Everything is then linked together and executed. The test suite is executed on Linux with gcc and clang compilers, on FreeBSD with clang and on OpenBSD with clang. In Linux and FreeBSD environments valgrind is used to control test suite correctness at runtime.
  • Bootstrapping test -- kefir is used to compile itself using 2-stage bootstrap technique as described above.
  • GCC Torture Suite -- compile & execute parts of GCC torture test suite are executed with kefir compiler, with some permissive options enabled. At the moment, out of 3445 tests, 537 fail and 29 are skipped due to being irrelevant (e.g. SIMD or profiling test cases; there is no exhaustive skip list yet). All failures happen on compilation stage, no abortions occur at runtime. The work with torture test suite will be continued in order to reduce the number of failures. The torture tests are included into CI pipeline with some basic test result checks.
  • Miscallenous tests:
    • Lua test -- kefir is used to build Lua 5.4 interpreter and then Lua basic test suite is executed on the resulting executable
    • Test suite which is a fork of c-testsuite is executed. Currently, the test suite reports 3 failures that happen due to C language extensions used in the tests. Failing test cases are skipped.

Furthermore, Kefir also provides an external test suite comprised of open source software that is known to work with Kefir. This suite are not included in the CI, however, it is regularly executed manually. Currently, the external test suite includes: bash 5.2.21, git 2.44.0, libsir 2.2.4, nano 7.2, oksh 7.5, sqlite 3.45.3, tcc 0.9.27, tcl 8.6.14, tin 2.6.3, zlib 1.3.1. The external test suite is used to verify Kefir compatbility with real world software.

Own test suite is deterministic (that is, tests do not fail spuriously), however there might arise problems when executed in unusual environments (e.g. with non-Unicode locale). For instance, some tests contain unicode characters and require the environment to have appropriate locale set. Also, issues with local standard library version might cause test failures.

Currently, extension of the test suite is a major goal. It helps significantly in eliminating bugs, bringing kefir closer to C language standard support, improving compiler UX in general.

#Design notes

In order to simplify translation and facilitate portability, intermediate representation (IR) layer was introduced. It defines architecture-agnostic 64-bit stack machine bytecode, providing generic calling convention and abstracting out type layout information. Compiler is structured into separate modules with respect to IR: code generation, AST analysis and translation. The IR code is then converted into optimizer SSA-like representation. IR layer provides several interfaces for AST analyzer to retrieve necessary target type layout information (for instance, for constant expression analysis). AST analysis and translation are separate stages to improve code structure and reusability. Parser uses recursive descent approach with unlitmited back-tracking. Lexer was implemented before preprocessor and can be used independently of it (preprocessing stage can be completely omitted), thus both lexer and preprocessor modules share the same lexing facilities. Driver links kefir as a library and uses fork syscalls in order to isolate each file processing.

#Source code hosting

The primary code repository is hosted at Sourcehut, with secondary mirrors at Codeberg and author's personal website.

#Author and license

Author: Jevgenijs Protopopovs

The code base also includes patches from:


  • Main body of the compiler - GNU GPLv3
  • Runtime library and includes - BSD 3-clause