[LLVMdev] Criticism of garbage collection support in LLVM

[Lennart Augustsson]

I just want to say that the "functional language people don't like
LLVM" saga is not complete.
I think the LLVM is very cool, and although I've not looked into
supporting GC yet, I'm sure it's possible.
I'd rather try that than duplicating the LLVM effort (I've written
enough code generators).

  -- Lennart

On Sat, Jan 17, 2009 at 7:02 PM, Chris Lattner <clattner at apple.com> wrote:
>
> On Jan 17, 2009, at 10:18 AM, Mikhail Glushenkov wrote:
>
>> This may be of interest:
>>
>> http://lhc-compiler.blogspot.com/2009/01/case-against-cllvm.html
>>
>> People implementing a new Haskell compiler explain why LLVM is an
>> unsuitable target for them.
>
> I find the article, and particularly the preceding one (http://lhc-compiler.blogspot.com/2009/01/what-is-lhc.html
> ) to be quite amusing.  First, this article should be titled "the case
> against C / LLVM 2.5", not against LLVM in general.  There is nothing
> in the design that prevents holding GC roots in registers, it is just
> that noone has implemented support for it yet.  It would actually be a
> pretty simple hack to move the current GC implementation to use the
> address space support, and say that all pointers into address space 42
> are GC'd pointers (for example).  This would work fine for languages
> like haskell and java where the GC doesn't need static metadata
> associated with the pointers.
>
> I assert that the time and cost to implement an entirely new x86
> backend (to start with?) is far more than would be required to add
> this feature to LLVM (also note that their non-existent code generator
> doesn't support this feature either).  Further, the skill set required
> to implement a very performant low-level code generator is very
> different than that required to produce a performant implementation of
> a high level language like Haskell.
>
> In the end, I consider this to be a yet-another chapter in the
> "functional language people don't like LLVM" saga.  Whether it be
> small improvements in LLVM tail calls or the fact that LLVM currently
> pins GC roots to the stack, there is always a "good reason" to justify
> writing something new, rather than learning and extend an existing
> system.  I'm sure that LLVM being written in C++ (as opposed to, say,
> ocaml) is part of this as well.
>
> In the end, I think that diversity is good in the compiler world, and
> wish them the best.  It's just not the approach I would take in
> 2009. ;-)
>
> -Chris
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