What exactly is LLVM

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I keep hearing about LLVM all the time  It s in Perl  then it s in Haskell  then someone uses it in some other language  What is it   What exactly distinguishes it from GCC  perspectives   safety etc

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LLVM is basically a library used to build compilers and or language oriented software  The basic gist is although you have gcc which is probably the most common suite of compilers  it is not built to be re-usable ie  it is difficult to take components from gcc and use it to build your own application  LLVM addresses this issue well by building a set of  quot modular and reusable compiler and toolchain technologies quot  which anyone could use to build compilers and language oriented software

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According to  Getting Started With LLVM Core Libraries  book  c    In fact  the name LLVM might refer to any of the following   The LLVM project infrastructure  This is an umbrella for several projects that  together  form a complete compiler  frontends  backends  optimizers  assemblers  linkers  libc    compiler-rt  and a JIT engine  The word  quot LLVM quot  has this meaning  for example  in the following sentence   quot LLVM is comprised of several projects quot    An LLVM-based compiler  This is a compiler built partially or completely with the LLVM infrastructure  For example  a compiler might use LLVM for the frontend and backend but use GCC and GNU system libraries to perform the final link  LLVM has this meaning in the following sentence  for example   quot I used LLVM to compile C programs to a MIPS platform quot    LLVM libraries  This is the reusable code portion of the LLVM infrastructure  For example  LLVM has this meaning in the sentence   quot My project uses LLVM to generate code through its Just-in-Time compilation framework quot    LLVM core  The optimizations that happen at the intermediate language level and the backend algorithms form the LLVM core where the project started  LLVM has this meaning in the following sentence   quot LLVM and Clang are two different projects quot    The LLVM IR  This is the LLVM compiler intermediate representation  LLVM has this meaning when used in sentences such as  quot I built a frontend that translates my own language to LLVM quot

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LLVM  used to mean  Low Level Virtual Machine  but not anymore  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 programming languages  Originally implemented for C C    the language-independent design  and the success  of LLVM has since spawned a wide variety of front-ends  including Objective C  Fortran  Ada  Haskell  Java bytecode  Python  Ruby  ActionScript  GLSL  and others   Read this for more explanation Also check out Unladen Swallow

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LLVM is a library that is used to construct  optimize and produce intermediate and or binary machine code   LLVM can be used as a compiler framework  where you provide the  front end   parser and lexer  and the  back end   code that converts LLVM s representation to actual machine code    LLVM can also act as a JIT compiler - it has support for x86 x86 64 and PPC PPC64 assembly generation with fast code optimizations aimed for compilation speed   Unfortunately disabled since 2013  there was the ability to play with LLVM s machine code generated from C or C   code at the demo page

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The LLVM Compiler Infrastructure is particularly useful for performing optimizations and transformations on code  It also consists of a number of tools serving distinct usages  llvm-prof is a profiling tool that allows you to do profiling of execution in order to identify program hotspots  Opt is an optimization tool that offers various optimization passes  dead code elimination for instance    Importantly LLVM provides you with the libraries  to write your own Passes  For instance if you require to add a range check on certain arguments that are passed into certain functions of a Program  writing a simple LLVM Pass would suffice    For more information on writing your own Pass  check this http   llvm org docs WritingAnLLVMPass html

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A good summary of LLVM is this     At the frontend you have Perl  and many other high level languages  At the backend  you have the natives code that run directly on the machine   At the centre is your intermediate code representation  If every high level language can be represented in this LLVM IR format  then analysis tools based on this IR can be easily reused - that is the basic rationale

[gcc] What exactly is LLVM?

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