[LLVMdev] Re: LLVM and PyPy

[Armin Rigo]

Hello Chris,

On Fri, Oct 31, 2003 at 10:58:45AM -0600, Chris Lattner wrote:
> These are definitely features that we plan to add, but just haven't gotten
> to yet.  In particular, Alkis is working on a Java front-end, which will
> require similar features.  In the beginning, we will probably just use a
> conservative collector, eventually adding support for precise GC.

Great!

> We already have the capability of doing function-at-a-time code
> generation: what is basic-block at a time generation used for?  How do you
> do global optimizations like register allocation?

It is central to Psyco, the Python just-in-time specializer
(http://psyco.sourceforge.net) whose techniques we plan to integrate with
PyPy.  Unlike other environments like Self, which collects execution profiles
during interpretation and use them to recompile whole functions, Psyco has no
interpretation stage: it directly emits a basic block and run it; the values
found at run-time trigger the compilation of more basic blocks, which are run,
and so on.  So each function's machine code is a dynamic network of basic
blocks which are various specialized versions of a bit of the original
function.  This network is not statically known, in particular because basic
blocks often have a "switch" exit based on some value or type collected at
run-time.  Every new value encountered at this point triggers the compilation
of a new switch case jumping to a new basic block.

We will also certainly consider Self-style recompilations, as they allow more
agressive optimizations.  (Register allocation in Psyco is done using a simple
round-robin scheme; code generation is very fast.)

> That would be great!  We've tossed around the idea of creating C bindings
> for LLVM, which would make interfacing from other languages easier than
> going directly to the C++ API, but we just haven't had a chance to yet.
> Maybe you guys would be interested in helping with that project?

Well, as the C++ API is nice and clean it is probably simpler to bind it
directly to Python.  We would probably go for Boost-Python, which makes C++
objects directly accessible to Python.  But nothing is sure about this; maybe
driving LLVM from LLVM code is closer to our needs.  Is there a specific
interface to do that?  Is it possible to extract from LLVM the required code
only, and link it with the final executable?  In my experience, there are a
few limitations of C that require explicit assembly code, like building calls
dynamically (i.e. the caller's equivalent of varargs).


A bientot,

Armin.