The LLVM Project is a collection of modular and reusable compiler and
toolchain technologies. Despite its name, LLVM has little to do with
traditional virtual machines. The name
"LLVM" itself is not an acronym; it is the full name of the project.
LLVM began as a research
project at
the University of Illinois, with
the goal of providing a modern, SSA-based compilation strategy capable
of supporting both static and dynamic compilation of arbitrary
programming languages. Since then, LLVM has
grown to be an umbrella project consisting of a number of
subprojects, many of which are being used in production by a wide variety of
commercial and open source projects
as well as being widely used in academic research. Code
in the LLVM project is licensed under the
"Apache 2.0 License with LLVM exceptions"
The primary sub-projects of LLVM are:
The LLVM Core libraries provide a modern source- and
target-independent optimizer, along with
code generation support for many
popular CPUs (as well as some less common ones!) These libraries are built
around a well specified code representation
known as the LLVM intermediate representation ("LLVM IR"). The LLVM Core
libraries are well documented, and it is particularly
easy to invent your own language (or port an existing compiler) to use
LLVM as an optimizer and code generator.
Clang is an "LLVM native"
C/C++/Objective-C compiler, which aims to deliver amazingly fast compiles,
extremely useful error
and warning messages and to provide a platform for building great
source level tools.
The Clang Static Analyzer and
clang-tidy are
tools that automatically find bugs in your code, and are great examples of the
sort of tools that can be built using the Clang frontend as a library to
parse C/C++ code.
The LLDB project builds on
libraries provided by LLVM and Clang to provide a great native debugger.
It uses the Clang ASTs and expression parser, LLVM JIT, LLVM disassembler,
etc so that it provides an experience that "just works". It is also
blazing fast and much more memory efficient than GDB at loading symbols.
The libc++ and
libc++ ABI projects provide
a standard conformant and high-performance implementation of the C++
Standard Library, including full support for C++11 and C++14.
The compiler-rt project
provides highly tuned implementations of the low-level code generator
support routines like "__fixunsdfdi" and other calls generated when
a target doesn't have a short sequence of native instructions to implement
a core IR operation. It also provides implementations of run-time libraries
for dynamic testing tools such as
AddressSanitizer,
ThreadSanitizer,
MemorySanitizer,
and
DataFlowSanitizer.
The MLIR subproject is a novel
approach to building reusable and extensible compiler infrastructure. MLIR
aims to address software fragmentation, improve compilation for heterogeneous
hardware, significantly reduce the cost of building domain specific compilers,
and aid in connecting existing compilers together.
The OpenMP subproject
provides an OpenMP runtime for use with the
OpenMP implementation in Clang.
The polly project implements
a suite of cache-locality optimizations as well as auto-parallelism and
vectorization using a polyhedral model.
The libclc project aims to
implement the OpenCL standard library.
The klee project implements a
"symbolic virtual machine" which uses a theorem prover to try to evaluate
all dynamic paths through a program in an effort to find bugs and to prove
properties of functions. A major feature of klee is that it can produce a
testcase in the event that it detects a bug.
The LLD project is a new
linker. That is a drop-in replacement for system linkers
and runs much faster.
The
BOLT
project is a post-link optimizer. It achieves the improvements by optimizing
application's code layout based on execution profile gathered by sampling
profiler.
In addition to official subprojects of LLVM, there are a broad variety of
other projects that use components
of LLVM for various tasks. Through these external projects you can use
LLVM to compile Ruby, Python, Haskell, Rust, D, PHP, Pure, Lua, Julia, and a number of
other languages. A major strength of LLVM is its versatility, flexibility, and
reusability, which is why it is being used for such a wide variety of different
tasks: everything from doing light-weight JIT compiles of embedded languages
like Lua to compiling Fortran code for massive super computers.
As much as everything else, LLVM has a broad and friendly community of people
who are interested in building great low-level tools. If you are interested in
getting involved, a
good first place is to skim the LLVM Blog and join LLVM Discourse. For information on how to send in a patch, get commit access, and
copyright and license topics, please see the
LLVM Developer Policy.
