[llvm-dev] RFC: alloca -- specify address space for allocation

Swaroop Sridhar via llvm-dev llvm-dev at lists.llvm.org
Thu Aug 27 17:43:35 PDT 2015


> -----Original Message-----
> From: Philip Reames [mailto:listmail at philipreames.com]
> Sent: Thursday, August 27, 2015 2:50 PM
> To: Swaroop Sridhar <Swaroop.Sridhar at microsoft.com>; llvm-dev <llvm-
> dev at lists.llvm.org>; Sanjoy Das <sanjoy at playingwithpointers.com>
> Cc: Joseph Tremoulet <jotrem at microsoft.com>; Andy Ayers
> <andya at microsoft.com>; Russell Hadley <rhadley at microsoft.com>
> Subject: Re: RFC: alloca -- specify address space for allocation
> On 08/27/2015 02:40 PM, Swaroop Sridhar wrote:
> > Philip: I think there are two separate issues:
> > 1) Handling GC pointers stored in stack locations -- tracking their liveness,
> reporting them to the GC. etc.
> > 2) Treating pointers to stack locations as GC-pointers.
> >
> > I think the two options that you presented here are for solving (1).
> Your right.  I was focused on (1), mostly because I don't understand the
> challenge with (2).
> > I'm trying to solve issue (2) using alloca in addrspace(1).
> >
> > Basically, we want to create a pointer to a stack location (the location itself
> may not have any gc-pointers) and give it a type such that it can be used
> interchangeably with GC-pointers.
> Can you explain what you mean here?  Is this an issue with being able to pass
> the pointer to a function which expects gc references without having to
> bitcast?  Or is there a lowering challenge I'm not seeing?

Yes this is a matter of interoperability of the stack and heap pointers, when passed as managed pointers.
I don't think there is a problem with lowering.

> > Today, we can achieve this by using an alloca followed by an addrspacecast.
> It could be done in one instruction if the alloca can specify the addrspace of
> the pointer created.
> >
> > The advantage of doing this in one step is, for example, that we can check
> that there are no (other) addrspace(0) -> addrspace(1) casts.
> > (introduced inadvertently by user code or compiler transformations).
> Is there a reason that your custom verifier can't use metadata or pattern
> matching to accept these special addrspace casts of allocas?  We have a
> similar custom verifier which uses a metadata based exception mechanism
> which has been pretty successful for us. We've had to occasionally fix a
> dropped metadata issue, but I think we've hit one or two of these ever.

OK, let me think about it. 
It is possible to add specific pattern matching cases (to handle specific addrspacecasts) to analysis phases including RewriteStatepointsForGC.
Apart from alloca, there are some other cases in MSIL (like pinning of GC pointers) where we'll need to perform the addrspacecast to fo unmanaged <-> managed pointer conversion. These will also need special handling.

I'm not sure if the analysis will be cumbersome after optimization transforms -- but likely OK for the case of addrspacecasts fed by alloca. 


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