[LLVMdev] Code for late safepoint placement available

Thu Jun 5 09:45:01 PDT 2014

Thanks for the comments and for taking a look.

On 06/05/2014 02:19 AM, David Chisnall wrote:
> Hi Philip,
>
> The first thing that I notice on looking at the code is the lack of comments.  For example, about the only comment that I see in include /llvm/IR/Statepoint.h is a note telling me that a class is only intended to be used on the stack.  Doxygen comments and, for a feature like this, some high-level overview of how it's intended to be used are essential.
Agreed.  This is mandatory before upstreaming.  We've gone through a 
couple of implementations and before sharing I tried to go through and 
rip out the actively misleading comments.  Unfortunately, that didn't 
leave much left.  :(
> We currently use address spaces on our architecture to differentiate between traditional pointers and fat pointers (both of which are supported on our system in hardware).  It would be nice, rather than hard-coding an address space, to have a property on DataLayout that would tell you if a specific address space contained GC pointers.  We could then designate an address space that was both GCable and contained fat pointers, and another for non-GCable fat pointers.
Making this configurable is definitely desirable.  I like your idea of 
including a list of address spaces in data layout which contain GC 
pointers and making that query-able.  That sounds quite reasonable.  
Unless anyone makes another proposal, I'll run with this.

>
> The tests don't look to be in a sensible format for LLVM - in particular, LLVM tests should not depend on clang.  It would be better, I think, to commit the IR generated from your Python script than the script itself (although putting it in utils might be helpful for people to regenerate them).
By the point we upstream this, those particular tests should probably 
just be removed.  When we were iterating on possible designs, it was 
useful not having the tests in IR form.  At this point, those tests have 
lost most of their value.  Creating a set of IR tests which use address 
spaces properly and cover all the same corner cases is probably the 
right approach.

Philip
>
> On 4 Jun 2014, at 17:35, Philip Reames <listmail at philipreames.com> wrote:
>
>> As I've mentioned on the mailing list a couple of times over the last few months, we've been working on an approach for supporting precise fully relocating garbage collection in LLVM.  I am happy to announce that we now have a version of the code available for public view and discussion.
>>
>> https://github.com/AzulSystems/llvm-late-safepoint-placement
>>
>> Our goal is to eventually see this merged into the LLVM tree.  There's a fair amount of cleanup that needs to happen before that point, but we are actively working towards that eventual goal.
>>
>> Please note that there are a couple of known issues with the current version (see the README).  This is best considered a proof of concept implementation and is not yet ready for production use.  We will be addressing the remaining issues over the next few weeks and will be sharing updates as they occur.
>>
>> In the meantime, I'd like to get the discussion started on how these changes will eventually land in tree.  Part of the reason for sharing the code in an early state is to be able to build a history of working in the open, and to to able to merge minor fixes into the main LLVM repository before trying to upstream the core changes.  We are aware this is a fairly major change set and are happy to work within the community process in that regard.
>>
>> I've included a list of specific questions I know we'd like to get feedback on, but general comments or questions are also very welcome.
>>
>> Open Topics:
>> 	• How should we factor the core GC support for review?  Our current intent is to separate logically distinct pieces, and share each layer one at a time.  (e.g. first infrastructure enhancements, then intrinsics and codegen support, then verifiers, then safepoint insertion passes)  Is this the right approach?
>> 	• How configurable does the GC support need to be for inclusion in LLVM?  Currently, we expect the frontend to mark GC pointers using address spaces.  Do we need to support alternate mechanisms?  If so, what interface should this take?
>> 	• How should we approach removing the existing partial support for garbage collection? (gcroot)  Do we want to support both going forward?  Do we need to provide a forward migration path in bitcode?  Given the usage is generally though MCJIT, we would prefer we simply deprecate the existing gcroot support and target it for complete removal a couple of releases down the road..
>> 	• What programmatic interface should we present at the IR level and where should it live?  We're moving towards a CallSite like interface for statepoints, gc_relocates, and gc_results call sites.  Is this the right approach?  If so, should it live in the IR subtree, or Support?  (Note: The current code is only about 40% migrated to the new interface.)
>> 	• To support invokable calls with safepoints, we need to make the statepoint intrinsic invokable.  This is new for intrinsics in LLVM.  Is there any reason that InvokeInst must be a subclass of CallInst? (rather than a view over either calls or invokes like CallSite)  Would changes to support invokable intrinsics be accepted upstream?  Alternate approaches are welcome.
>> 	• Is the concept of an abstract VM state something LLVM should know about?  If so, how should it be represented?  We're actively exploring this topic, but don't have strong opinions on the topic yet.
>> 	• Our statepoint shares a lot in the way of implementation and semantics with patchpoint and stackmap.  Is it better to submit new intrinsics, or try to identify a single intrinsic which could represent both?  Our current feeling is to keep them separate semantically, but share implementation where possible.
>>
>> Yours,
>> Philip (& team)
>>
>> p.s. Sanjoy, one of my co-workers,  will be helping to answer questions as they arise.
>>
>> p.p.s. For those wondering why the current gcroot mechanism isn't sufficient, I covered that in a previous blog post:
>> [1] http://www.philipreames.com/Blog/2014/02/21/why-not-use-gcroot/
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