[LLVMdev] GC questions.

[Chris Lattner]

On Mon, 19 Jul 2004, Tobias Nurmiranta wrote:

> Regarding llvm.gcroot, do I have to allocate stack-space for all
> pointers in a function? Right now I mostly use SSA-variables, and let
> llvm's register allocation allocate stack-space when needed.

Yes.  This reflects the fact that the GC can move objects (to compact the
heap) at unpredictable times.

> Also, what happens when I run the mem2reg pass, does it handle
> llvm.gcroot's that are moved from stack to registers?

Not currently.  In the future this will definitely be improved.  In
particular, when the code generators are enhanced to provide more accurate
mapping information for GC pointers (e.g. which registers contain
pointers), we can do this.  This is in the long term plans, but I suspect
that performance will be fine without it for many applications.

> I'm thinking along the lines, that should one not use llvm.gcroot on all
> SSA-variables that contains pointers to objects, and then depending on
> if the variables end up on the stack, or in registers, the compiler will
> use llvm.gcroot?

llvm.gcroot is an abstraction.  Your front-end is telling the code
generator and optimizer what locations can be modified unpredictably at
runtime by the GC system.   Right now we do not have many clients of the
gc interfaces, so they do not produce wonderful code, but this will change
in time.  :)

> All my objects are currently typetagged uint's. My llvm scheme code in
> the compiler explains this I think:

I think that this should be fine.  The GC interfaces are explicitly
designed to be able to support list/scheme style type tagging like this,
thought the current garbage collector is not fully implemented.

-Chris

>      (llvm-define (make-number x) (bit-shl x 2))
>      (llvm-define (raw-number x) (bit-shr x 2))
>      (llvm-define (clear-tag x) (bit-shl (bit-shr x 2) 2))
>      (llvm-define (get-tag x) (bit-and x 3))
>      (llvm-define (make-pointer x) (bit-or (clear-tag x) 1))
>      (llvm-define (make-function-pointer x) (bit-or (clear-tag x) 3))
>      (llvm-define (points-to x) (clear-tag x))
>      (llvm-define (number? x) (seteq (get-tag x) 0))
>      (llvm-define (vector? x) (seteq (get-tag x) 1))
>      (llvm-define (procedure? x) (seteq (get-tag x) 3))
>
>      (llvm-define (make-vector raw-size)
>                   (make-pointer
>                    (cast "uint*" (store raw-size (malloc (add raw-size 1))) "uint")))
>
> ,	Tobias
>
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-Chris

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