[LLVMdev] llvm.gcroot suggestion

[Talin]

On Mon, Mar 7, 2011 at 2:05 PM, Joshua Warner <joshuawarner32 at gmail.com>wrote:

>
>
> On Mon, Mar 7, 2011 at 12:48 PM, Talin <viridia at gmail.com> wrote:
>
>> On Mon, Mar 7, 2011 at 10:58 AM, Joshua Warner <joshuawarner32 at gmail.com>wrote:
>>
>>> Hi Talin,
>>>
>>> Sorry to interject -
>>>
>>>
>>>>  For example, suppose I have a type "String or (float, float, float)" -
>>>> that is, a union of a string and a 3-tuple of floats. Most of the time what
>>>> LLVM will see is { i1; { float; float; float; } } because that's bigger than
>>>> { i1; String* }. LLVM won't even know there's a pointer in there, except
>>>> during those brief times when I'm accessing the pointer field. So tagging
>>>> the pointer in a different address space won't help at all here.
>>>>
>>>>
>>> I think this is a fairly uncommon use case that will be tricky to deal
>>> with no matter what method is used to track GC roots.  That said, why not do
>>> something like make the pointer representation (the {i1, String*}) the
>>> long-term storage format, and only bitcast *just* before loading the
>>> floats?  You could even use another address space to indicate that something
>>> is *sometimes* a pointer, dependent upon some other value (the i1, perhaps
>>> indicated with metadata).
>>>
>>
>> I don't know if it's an uncommon use case or not, but it is something that
>> I handle already in my frontend. (I suppose it's uncommon in the sense that
>> almost no one uses the garbage collection features of LLVM, but part of the
>> goal of this discussion is to change that.)
>>
>
> I actually meant uncommon in the sense of having stack-allocated unions
> that participate in garbage collection.  Off the top of my head, I could
> only name one language (ML) that might use a feature like that.  Even then,
> I suspect most ML implementations would actually push that stuff onto the
> heap.
>
>
>> The problem with making { i1, String* } the long-term storage format is
>> that it isn't large enough in the example I gave, so you'll overwrite other
>> fields if you try to store the three floats. The more general issue is that
>> the concepts we're talking about simply aren't expressible in IR as it
>> exists today.
>>
>
> Good catch - what I actually intended to indicate was the String "half" of
> the union, properly padded - so something more like {i1, String*, float}
> (for 64-bit pointers).
>
>
>>
>>> My vote (not that it really counts for much) would be the address-space
>>> method.  It seems much more elegant.
>>>
>>
>> I agree that the current solution isn't the best. The problem I have is
>> that the solutions that are being suggested are going to break my code
>> badly, and with no way to fix it.
>>
>> The *real* solution is to make root-ness a function of type. In other
>> words, you can mark any type as being a root, which exposes the base address
>> of all objects of that type to the garbage collector. This is essentially
>> the same as the pointer-address-space suggestion, except that it's not
>> limited to pointers. (In practice, it would only ever apply to pointers and
>> structs.)
>>
>> (Heck, I'd even be willing to go with a solution where only structs and
>> not pointers could be roots - it means I'd have to wrap every pointer in a
>> struct, which would be a royal pain, but it would at least work.)
>>
>
> Hmm... do you mean something like a "marked" bit (or maybe a vector of
> mark_ids) in every type, where you could query a function for values of
> "marked" types at particular safe points?  This sounds like something that
> might solve the problem described below with compressed pointers (not that I
> am actually encountering this problem) - but in the near-term, it seems to
> me that everything that you could conceivably mark as a GC root would
> somehow contain a pointer value.  In this case, union support in LLVM would
> make the generated IR cleaner, but not necessarily any more correct.
>
> Being able to make a "marked" version of every type seems unnecessary, and
> in some cases, somewhat non-intuitive.  Take for instance, making a "marked"
> float type - which I can't think of any good use for.  I like the idea of
> using address spaces because it keeps the concepts in IR largely orthogonal,
> rather than having features that overlap in purpose in many cases.  That,
> and IMO it just makes sense for pointers into the (or, in general, a) heap
> be considered in a different address space from "normal" pointers.  This
> could extend well to tracking pointers onto the stack (as seen in C#
> out/ref) for the purpose of generating closures (in .NET - which doesn't
> currently have this feature).
>
> Let me ask a question before we go too much further. Currently the argument
to llvm.gcroot must be an alloca instruction. You cannot GEP an internal
field within the alloca and pass it to the gcroot intrinsic. So the entire
alloca is considered a root, even if it has non-pointer fields. My question
is, in this new address-space proposal, are we talking about changing this
so that the garbage collector only "sees" the internal pointer fields within
the alloca, or will it still be able to "see" the entire alloca? This is the
crucial point for me - I've written my GC strategy to deal with complex
allocas, and there are several data types - such as unions - which depend on
this.

I can probably work around the union issue using methods like you suggest -
that is building some "dummy" type containing a pointer as the long-term
storage format - as long as the GC can still see the entire struct. It's
ugly because it means that my frontend has to know about padding and
alignment and such, issues which I prefer to leave to LLVM to figure out.

But if we change it so that the GC only sees pointers, then I'm dead in the
water.

As far as my suggestion of marking types go, you are right, it doesn't make
sense for most types. It really only matters for structs and pointers.
Imagine if structs had an "isRoot" flag that lived next to "isPacked", which
makes the struct a distinct type. This would be written in IR as "gcroot {
i1, float }" or something like that. The presence of this flag has the same
effect as marking a pointer in the GC address space. Combine that with the
ability to mark SSA values as roots, and my life would get vastly simpler
and my generated code would get about 20% faster :)

>
>>> The only thing that I think would be unusually difficult for the
>>> address-space method to handle would be alternative pointer representations,
>>> such as those used in the latest version of Hotspot (see
>>> http://wikis.sun.com/display/HotSpotInternals/CompressedOops).
>>> Essentially, a 64-bit pointer is packed into 32-bits by assuming 8-byte
>>> alignment and restricting the heap size to 32GB.  I've seen similar
>>> object-reference bitfields used in game engines.  In this case, there is no
>>> "pointer" to attach the address space to.
>>>
>>> (Yes, I know that Hotspot currently uses CompressedOops ONLY in the heap,
>>> decompressing them when stored in locals, but it is not inconceivable to
>>> avoid decompressing them if the code is just moving them around, as an
>>> optimization.)
>>>
>>> Just my few thoughts.
>>>
>>> -Joshua
>>>
>>
>>
>>
>> --
>> -- Talin
>>
>
> -Joshua
>
>


-- 
-- Talin
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