Low Level Virtual Machine (LLVM) is:

1. A compilation strategy designed to enable effective program optimization across the entire lifetime of a program. LLVM supports effective optimization at compile time, link-time (particularly interprocedural), run-time and offline (i.e., after software is installed), while remaining transparent to developers and maintaining compatibility with existing build scripts.

2. A virtual instruction set - LLVM is a low-level object code representation that uses simple RISC-like instructions, but provides rich, language-independent, type information and dataflow (SSA) information about operands. This combination enables sophisticated transformations on object code, while remaining light-weight enough to be attached to the executable. This combination is key to allowing link-time, run-time, and offline transformations.

3. A compiler infrastructure - LLVM is also a collection of source code that implements the language and compilation strategy. The primary components of the LLVM infrastructure are a GCC-based C & C++ front-end, a link-time optimization framework with a growing set of global and interprocedural analyses and transformations, static back-ends for the X86, PowerPC, IA-64, Alpha, & SPARC V9 architectures, a back-end which emits portable C code, and a Just-In-Time compiler for X86, PowerPC, and SPARC V9 processors.

4. LLVM does not imply things that you would expect from a high-level virtual machine. It does not require garbage collection or run-time code generation (In fact, LLVM makes a great static compiler!). Note that optional LLVM components can be used to build high-level virtual machines and other systems that need these services.

LLVM is a robust system, particularly well suited for developing new mid-level language-independent analyses and optimizations of all sorts, including those that require extensive interprocedural analysis. LLVM is also a great target for front-end development for conventional or research programming languages, including those which require compile-time, link-time, or run-time optimization for effective implementation, proper tail calls or garbage collection. We have an incomplete list of projects which have used LLVM for various purposes, showing that you can get up-and-running quickly with LLVM, giving time to do interesting things, even if you only have a semester in a University course. We also have a list of ideas for projects in LLVM.

LLVM was started by the Lifelong Code Optimization Project, led by Vikram Adve at the University of Illinois, Urbana-Champaign. Since the first public release, LLVM has grown to include contributions from several other people! We welcome external contributions, so please send e-mail to llvmdev@cs.uiuc.edu if you are interested in contributing code to the LLVM infrastructure.

Nov 08, 2005: LLVM 1.6 is now available for download! LLVM is publicly available under an open source License. Also, you might want to check out the new features in CVS that will appear in the next LLVM release. If you want them early, download LLVM through anonymous CVS.

If you'd like to experiment with LLVM, but don't want to download it and compile it, we've got just the thing for you. You can now compile C and C++ in your browser, to see what the LLVM representation looks like, to see what various C/C++ constructs map to in LLVM, and try out some of the optimizers.