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Google summer of Code proposal
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Date: 2009-03-23 (14:19)
From: Xavier Leroy <Xavier.Leroy@i...>
Subject: Re: [Caml-list] Google summer of Code proposal
> I would like to develop LLVM frontend to Ocaml language. LLVM  does 
> participate in GSoC. LLVM do not mind to developed a ocaml frontend as LLVM 
> GSoC project. I want to discuss details with you before I will make an 
> official proposal to LLVM. [...]
> Do authors of ocaml has something to say about the idea?

Da. A number of things, actually.

1- I know of at least 3, maybe 4 other projects that want to do
something with OCaml and LLVM.  Clearly, some coordination between
these efforts is needed.

2- If you're applying for funding through a summer of code project,
you need to articulate good reasons why you want to combine OCaml and
LLVM.  "Ocaml is cool, LLVM is cool, so OCaml+LLVM would be extra
cool" is not enough.  "It will generate PIC-16 code" isn't either.
Run-time code generation could be a good enough reason, but you still
need to weigh the development cost of the LLVM approach against, for
example, hacking the current OCaml code generator so that it emits
machine code in memory instead of assembly code.

3- A language implementation like OCaml breaks down in four big parts:
        1- Front-end compiler
        2- Back-end compiler and code emitter
        3- Run-time system
        4- OS interface
Of these four, the back-end is not the biggest part nor the most
complicated part.  LLVM gives you part 2, but you still need to
consider the other 3 parts.  Saying "I'll do 1, 3 and 4 from scratch",
Harrop-style, means a 5-year project.  To get somewhere within a
reasonable amount of time, you really need to reuse large parts of the
existing OCaml code base.

4- From a quick look at LLVM specs, the two aspects that appear most
problematic w.r.t. Caml are exception handling and GC interface.
LLVM seems to implement C++/Java "zero-cost" exceptions, which are
known to be quite costly for Caml.  (Been there, done that, in the
early 1990s.)  I regret that LLVM does not provide support for
alternate implementations of exceptions, like the C-- intermediate
language of Ramsey et al does:

The big issue, however, is GC interface.  There are GC techniques that
need no support from the back-end: stack maps and conservative
collection.  Stack maps are very costly in execution time.
Conservative GC like the Boehm-Weiser GC is already much better.  But
for full efficiency, back-end support is required.  LLVM documents a
minimal interface to produce stack maps and make them available to the
GC at run-time:

The first thing you want to investigate is whether this interface is
enough to support an exact, relocating collector such as
OCaml's. According to
Gordon Henriksen did succeed in interfacing OCaml's GC with LLVM.
Maybe there is still some more work to do here, in which case I
recommend you look into this first.

6- Here is a schematic of the Caml compiler.  (Use a fixed-width font.)

             | parsing and preprocessing
          Parsetree (untyped AST)
             | type inference and checking
          Typedtree (type-annotated AST)
             | pattern-matching compilation, elimination of modules, classes
           /  \
          /    \ closure conversion, inlining, uncurrying,
         v      \  data representation strategy
      Bytecode   \
                  | code generation
               Assembly code

In my opinion, the simplest approach is to start at Cmm and generate
LLVM code from there.  Starting at one of the higher-level
intermediate forms would entail reimplementing large chunks of the
OCaml compiler.  Note that Cmm is quite close to the C-- intermediate
language mentioned earlier, so there is a lot to learn from Fermin
Reig's experience with an OCaml/C-- back-end.

7- To finish, I'll preventively deflect some likely reactions by Jon

"But you'll be tied to OCaml's data representation strategy!"
   Right, but 1- implementing you own data representation strategy is
   a lot of work, especially if it is type-based, and 2- OCaml's
   strategy is close to optimal for symbolic computing.

"But LLVM assembly is typed, so you must have types!"
   Just use int32 or int64 as your universal type and cast to
   appropriate pointer types in loads or stores.

"But your code will be tainted by OCaml's evil license!"
   It is trivial to make ocamlopt dump Cmm code in a file or pipe.
   (The -dcmm debug option already does this.)  Then, you can have a
   separate, untainted program that reads the Cmm code and transforms it.

"But shadow stacks are the only way to go for GC interface!"
   No, it's probably the worst approach performance-wise; even a
   conservative GC should work better.

Hope this helps,

- Xavier Leroy