Low Level Virtual Machine

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LLVM
Developed by	LLVM Developer Group & Apple, Inc
Initial release	2000
Latest release	2.3 / 9 June 2008
OS	Cross-platform
Type	Compiler
License	University of Illinois/NCSA Open Source License
Website	http://llvm.org/

The Low Level Virtual Machine, generally known as LLVM, is a compiler infrastructure, written in C++, which is designed for compile-time, link-time, run-time, and "idle-time" optimization of programs written in arbitrary imperative programming languages. The LLVM project started in 2000 at the University of Illinois at Urbana-Champaign.

LLVM currently supports the compilation of C, C++, Objective C, Ada, and Fortran programs, using front-ends derived from version 4.0.1 and 4.2 of the GNU Compiler Collection (GCC). An alternate front-end for C-like languages is being developed under the name clang.

Using LLVM, one can create a virtual machine for languages, like Java, that require them, a code generator for a specific machine architecture, and optimizers independent from particular platforms or languages. The LLVM intermediate representation (IR) is language and architecture independent; it lies between a language-specific module and a code generator for a specific machine. LLVM includes aggressive inter-procedural optimization support, static and JIT compilers.

It has many components in various stages of development; including, but not limited to, a Java bytecode frontend, a CIL frontend, a Python frontend, a new graph coloring register allocator. The LLVM JIT compiler is capable of optimizing unnecessary static branches out of a program at runtime, and is therefore useful for partial evaluation in cases where a program has many options—most of which can easily be determined unnecessary in a specific environment. Because of this, it is used in the OpenGL pipeline of Mac OS X 10.5 (Leopard) to provide support for missing hardware features.

1 Code representation
2 Licensing
3 References
4 See also
5 External links

Code representation

LLVM supports a language-independent instruction set and type system. Most of the instructions have a form similar to three address code. Each instruction is in Static single assignment form (SSA), meaning that each variable (called a typed register) is assigned once and is frozen. This helps simplify the analysis of dependencies among variables.

Any form of type conversion, from coercion to the downcasting of an object, is performed explicitly using the cast instruction. LLVM has basic types, like integers of fixed sizes, and exactly five derived types: pointers, arrays, vectors, structures, and functions. A type construct in a concrete language can be represented by combining these basic types in LLVM. For example, a class in C++ can be represented by a combination of structures, functions and arrays of function pointers.

Licensing

LLVM and clang are publicly available under the University of Illinois/NCSA Open Source License [1], a GPL-compatible,^[1] OSI-approved license that is very similar to the BSD license.