Re: LLVM

[David Chisnall]

This post is from Chris Lattner, LLVM lead developer and now head of  
Apple's LLVM team.  The compiler he is talking about is gcc-llvm,  
which uses the scanner and parser from GCC and the code generation  
from LLVM.

For various reasons (licensing, ability to use the code in other  
projects, compilation speed, NIH), Apple have since started the clang  
project, which aims to produce a C/C++/Objective-C scanner and parser.  
This is then transformed into an AST which is transformed into LLVM  
IR, which can then be compiled to native code (or interpreted).  The  
current status of clang is that C parsing and code generation is  
pretty much finished, as is Objective-C parsing.  I have started  
working on code generation for Objective-C, and there are other people  
working on C++.

For me, the big advantage of LLVM is made obvious by the way in which  
the GSLS implementation on OS X works.  To implement a Smalltalk  
compiler on LLVM and an Objective-C runtime you would need a Smalltalk  
parser (trivial), something that generates LLVM IR message sends (I've  
already written this for the GNU runtime) and an Objective-C  
implementation of a few core classes, like BlockClosure.  You then get  
something like StepTalk but compiled to native speed with runtime  
optimisations in significantly less code (and can add languages like  
Self or Io fairly easily).  Note that these would be first-class  
GNUstep development languages, not simply scripting languages.  For  
this reason, I am keeping the runtime-specific bits of the code  
generation free of clang-specific code.  They only use pure LLVM  
constructs and so can be easily reused elsewhere.

David

On 1 Mar 2008, at 08:03, hns@computer.org wrote:

> As usual, world isn't white or black:
>
> There was/is a proposal to integrate LLVM into gcc and this message
> also addresses some of the licence issues.
>
> http://gcc.gnu.org/ml/gcc/2005-11/msg00888.html
>
> He reports a LLVM based compiler that is command-line compatible to
> gcc. I don't know anything about the status though.
>
> Nikolaus
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