[LLVMdev] LLVM for dynamic languages

[Chris Lattner]

On Thu, 11 Sep 2003, Rayiner Hashem wrote:

> How suitable do the developers think that LLVM would be as a code-generator
> for a dynamically typed langage? Would the lack of static type information
> make the traditional code optimizations performed by LLVM relatively
> ineffective?

LLVM is designed to be able to support "any" language, efficiently,
without restriction.  However, the mapping for some languages may not be a
clean as for others (lots of casting might be involved for example).  That
said, many dynamic languages should be cleanly mappable to the LLVM layer.
What are you thinking about in particular?

As far as the optimizations already in LLVM: LLVM provides an excellent
infrastructure for interprocedural optimizations.  As such, I would
recommend writing some language tuned optimizations (as necessary) that
would lower the dynamic objects into the primitives the optimizers
already understand.

For example, think of a language like smalltalk (the most dynamic language
that I know of).  Say you have a program that looks like this:

(8*(4/2)) print

The LLVM constant folding optimizations will not be able to fold these
constants, because they are actually instances of the SmallInteger class,
which have dynamically dispatched messages to perform the operations.

Now, however, if you preceed the standard LLVM optimizations with some
simple partial evaluation/resolution transformations, you can show that *
and / will resolve to the implementations in the SmallInteger class.  As
such, you can convert it to a direct call, have it inlined, and the
objects themselves would be turned into primitives.

If you can express the method dispatch in a way that is useful to the
preexisting optimizations, of course, you wouldn't have to do anything
special to support this (C++/Java v-tables fit into the catagory).

Basically the idea is that some optimizations are much more important to
certain languages than others.  In this case, it's method resolution, for
functional languages, it's tail call elimination, for C++ it's scalar
replacement of aggregates, etc.

Since LLVM provides the facilities needed to efficiently implement these
types of optimizations, it is a pretty good infrastructure for doing them
in, even though it currently does not support "all possible" optimizations
out-of-the-box (yet :)

Please let me know if I have confused the issue or missed your question
completely.  :)

-Chris

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