[llvm-dev] RFC: Strong GC References in LLVM

Chandler Carruth via llvm-dev llvm-dev at lists.llvm.org
Mon Jul 18 16:46:55 PDT 2016

Sorry, I missed this at first but I have one issue here:

On Mon, Jul 18, 2016 at 11:55 AM Sanjoy Das via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi all,
> I think it is time to start getting more concrete here.  As a starting
> point, I want to send out for review (roughly) the following changes:
>   - Add a "gc" address space to the datalayout string

I don't really understand the need for this yet, because the following

>   - Start implementing the non-controversial rules (i.e. everything
>     except the bits that initiated the "nospeculate" attribute
>     discussion):

I think everything here should apply to *all* non-zero address spaces. I
think the thing we would want is for a tagged address space to opt *out* of
this conservative behavior, not the other way around.

So I don't think you need a tagged address space to implement everything
here, and I'd like to avoid tagging the address space until the last
possible second to make sure that this is implemented as generically as
possible. I'm actually hopeful that the tagging isn't necessary at all.


>      - No pointer <-> integer casts for GC address spaces to begin with
>      - Add an intrinsic (with control dependence) to
>        convert GCrefs -> integers (we need this for GC load/store
>        barriers)
>      - Disable some of the problematic "cast by round tripping through
>        memory" type optimizations for loads and stores that are of GC
>        ref type
> The things above are things we know we need, and even if all we do is
> implement those, we will be in a better position overall.
> One thing I want a design opinion on (already discussed on IRC): I'm
> planning to phrase RewriteStatepointsForGC (a ModulePass) that
> "implements" GC references "in terms of" normal pointers.  One way to
> do this is to rewrite each def and user of GC refs to use a normal
> pointer, but that's unnecessary data structure churn, so I was
> wondering if instead we can flip the meaning of what a GC ref is by
> modifying the datalayout instead?  RewriteStatepointsForGC can then be
> seen as changing IR that can be lowered to run on only a "machine"
> that directly supports GC pointers to IR that can be lowered to run on
> machines that don't.  That is RewriteStatepointsForGC will change IR
> from
> "No explicit relocations, addrspace(k) is marked as 'gc' in the
> datalayout" to "All relocations explicit, addrspace(k) is not marked
> specially in the datalayout"
> However, Chandler had some (strong?) reservations on IRC about
> modifying datalayout in an optimization, in the face of which I have a
> couple of alternatives:
>   - Have RewriteStatepointsForGC rewrite defs and users of GC
>     references to use a "normal" pointer type.  I'm a little hesitant
>     to to do this since it seems wasteful (no evidence yet that it will
>     matter), and may complicate keeping side data structures correct in
>     the face of mass invalidations.
>   - Represent the gc address space in something other than the
>     datalayout that we all can agree is fair game to be modified by a
>     ModulePass.  Not a great option since datalayout seems the most
>     natural place to put the "gc-ref-addrspace" information.
>   - Don't do anything, i.e. RewriteStatepointsForGC does what it does
>     today: it rewrites pointers of addrspace(1) (or addrspace(k) for
>     some k) to be explicit but does not change the meaning of
>     "addrspace(k)".  I'm hesitant to do this because then I can't
>     concisely answer "what does RewriteStatepointsForGC do?".
> I want to see what others think about this, but in the absence of any
> specific opinion here I'll go with the first option (and consider
> using mutateType if things turn out to be too slow).
> In parallel with all this, I'll try to come up with a concrete notion
> of how the nospeculate attributes on loads and function calls will
> look like, how it would interact with optimizations like mem2reg etc.
> I'll consider potential interactions with
> https://reviews.llvm.org/D20116 "Add speculatable function attribute"
> and generally just kick it around to see if the idea holds up and
> gives us all of the constraints we need.
> Sounds good?
> -- Sanjoy
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