[LLVMdev] Proposal for improving llvm.gcroot (summarized)

[Talin]

On Fri, Apr 1, 2011 at 1:58 AM, Jay Foad <jay.foad at gmail.com> wrote:

> On 30 March 2011 19:08, Talin <viridia at gmail.com> wrote:
> > llvm.gc.declare(alloca, meta). This intrinsic marks an alloca as a
> garbage
> > collection root. It can occur anywhere within a function, and lasts
> either
> > until the end of the function, or a until matching call to
> > llvm.gc.undeclare().
> > llvm.gc.undeclare(alloca). This intrinsic unmarks and alloca, so that it
> is
> > no longer considered a root from that point onward.
>
> Hi Talin,
>
> What changes to code generation would be necessary to support this?
>
> I can only describe this in abstract terms, since I know very little about
LLVM's code generation. (I am primarily a user of LLVM, not a developer of
it, although I have made a few minor contributions.)


> Is there any intention of supporting a collector that has static stack
> maps for each function, i.e. a table telling you, for each point in
> the code, where all the roots are on the stack (similar to unwind info
> for exception handling)? If so, I think it's a bit dodgy to use
>

That is already supported in the current LLVM. The changes I am proposing
are merely an extension of what we have now, allowing frontends to emit more
efficient code.


> intrinsic function calls to mark the start/end of the lifetime of a GC
> root, because function calls are inherently dynamic. For example, you
> can't represent this code with a static stack map:
>
> if (cond) {
>  llvm.gc.declare(foo, bar);
> }
> ...
> // foo may or may not be a root here
> ...
> if (cond) { // same condition as above
>  llvm.gc.undeclare(foo);
> }
>
> You would need to do the same as what is done today: Move the declare
outside of the condition, and initialize the variable to a null state, such
that the garbage collector will know to ignore the variable. In the
if-block, you then overwrite the variable with actual data.

The difference is that in the today's LLVM, the variable declaration has to
be in the first block, and lasts for the entire function - so you have to
initialize all of your stack variables to a null state in the first block.
By extending the notation to allow stack roots to have a limited lifetime,
we can avoid having to initialize the stack root unless we actually enter
the block where it is defined.

I should mention that the declare/undeclare intrinsics are the least
important part of this proposal. The important part is the ability to
declare SSA values as roots - that is what will make a world of difference
to folks like myself that are writing frontends that use garbage collection.


> Even if you're careful not to generate code like this in your front
> end, how can you guarantee that LLVM optimisation passes won't
> introduce it?
>
> The old llvm.dbg.region.start and llvm.dbg.region.end had the same
> kind of problem.
>
> Thanks,
> Jay.
>



-- 
-- Talin
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