[LLVMdev] Interfacing llvm with a precise, relocating GC
Michael Lewis
don.apoch at gmail.com
Fri Oct 25 21:37:15 PDT 2013
I'm also highly interested in relocating-GC support from LLVM. Up until now
my GC implementation has been non-relocating which is obviously kind of a
bummer given that it inhibits certain classes of memory
allocation/deallocation tricks.
I wrote up a bunch of my findings on the implementation of my GC here:
https://code.google.com/p/epoch-language/wiki/GarbageCollectionScheme
Frankly I haven't had time to get much further than idle pondering of how
I'd go about implementing relocation, but it seems to me like the existing
read/write barrier intrinsics might be sufficient if abused properly by the
front-end and lowered carefully. My operating hypothesis has been to stash
barriers at key points - identified by the front-end itself - and possibly
elide them during lowering if it's safe to do so. If my understanding is
correct, it should be possible to lower the barriers into exactly the kind
of boxing/unboxing procedure described in this thread.
Based on my experimentation so far, which is admittedly minimal, I think
the overhead of updating relocated pointers is actually not as bad as it
sounds. It isn't strictly necessary to store both a boxed *and* unboxed
pointer in every frame. In fact, in the current incarnation of gcroot,
marking an alloca as a gcroot effectively forces a stack spill for that
alloca; this means that the relocating collector just updates the single
existing pointer on the stack when it does its magic, and you're done. With
proper barriers in place, and/or careful location of safepoints by the
front-end, it's not that hard to make sure that a relocated pointer gets
updated.
The real trick, as near as I can tell, would be updating registers that
have live roots during a collection. But again based on my past
investigations this should just be a matter of ensuring that
post-collection you have a barrier that is lowered into a register refresh
based on the on-stack relocated pointer value.
One thing I've been meaning to try and do is use this barrier-abuse scheme
to work around the existing lack of support for tracing roots in machine
registers, by effectively setting up an artificial stack spill just prior
to a collection, and otherwise operating on registers as much as possible
instead of gcroot'ed allocas. Again, I've only considered this as a thought
exercise until now, so I apologize if there's some obvious flaw I'm not
aware of in my reasoning. I haven't actually done any of this stuff yet so
it's more speculative than anything - just trying to creatively engineer
around the existing limitations :-)
- Mike
On Fri, Oct 25, 2013 at 5:35 PM, Philip Reames <listmail at philipreames.com>wrote:
> On 10/25/13 1:10 PM, Ben Karel wrote:
>
>
>
>
> On Thu, Oct 24, 2013 at 6:42 PM, Sanjoy Das <sanjoy at azulsystems.com>wrote:
>
>> Hi Rafael, Andrew,
>>
>> Thank you for the prompt reply.
>>
>> One approach we've been considering involves representing the
>> constraint "pointers to heap objects are invalidated at every
>> safepoint" somehow in the IR itself. So, if %a and %b are values the
>> GC is interested in, the safepoint at the back-edge of a loop might
>> look like:
>>
>> ; <label>: body
>> %a = phi i32 [ %a.relocated, %body ] [ %a.initial_value, %pred ]
>> %b = phi i32 [ %b.relocated, %body ] [ %b.initial_value, %pred ]
>> ;; Use %a and %b
>>
>> ;; The safepoint starts here
>> %a.relocated = @llvm.gc_relocate(%a)
>> %b.relocated = @llvm.gc_relocate(%b)
>> br %body
>>
>> This allows us to not bother with relocating derived pointers pointing
>> inside %a and %b, since it is semantically incorrect for llvm to reuse
>> them in the next iteration of the loop.
>
>
> This is the right general idea, but you can already express this
> constraint in LLVM as it exists today, by using llvm.gcroot(). As you
> noted, this also solves the interior-pointer problem by making it the front
> end's job to convey to LLVM when it would/would not be safe to cache
> interior pointers across loop iterations. The invariant that a front-end
> must maintain is that any pointer which is live across a given
> potential-GC-point must be reloaded from its root after a (relocating) GC
> might have occurred. This falls naturally out of the viewpoint that %a is
> not an object pointer, it's the name of an object pointer's value at a
> given point in time. So, of course, whenever that object pointer's value
> might change, there must be a new name.
>
> To rephrase, you're saying that the current gcroot mechanism is analogous
> to a boxed object pointer. We could model a safepoint by storing the
> unboxed value into the box, applying an opaque operation to the box, and
> reloading the raw object pointer from the box. (The opaque operator is key
> to prevent the store/load pair from being removed. It also implements the
> actual safepoint operation.) Is this a correct restatement?
>
> Here's an example:
> gcroot a = ...; b = ...;
> object* ax = *a, bx = *b;
> repeat 500000 iterations {
> spin for 50 instructions
> *a = ax;
> *b = bx;
> safepoint(a, b)
> ax = *a;
> bx = *b;
> }
>
> If so, I would agree with you that this is a correct encoding of object
> relocation. I think what got me confused was using the in-tree examples to
> reverse engineer the semantics. Both the Erlang and Ocaml examples insert
> safepoints after all the LLVM IR passes are run and derived pointers were
> inserted. This was the part that got me thinking that the gcroot semantics
> were unsuitable for a precise relocating collector. If you completely
> ignored these implementations and inserted the full box/safepoint
> call/unbox implementation at each safepoint before invoking any
> optimizations, I think this would be correct.
>
> One problem with this encoding is that there does not currently exist an
> obvious mechanism to describe a safepoint in the IR itself. (i.e. there is
> no llvm.gcsafepoint) You could model this with a "well known" function
> which a custom GCStrategy recorded a stackmap on every call. It would be
> good to extend the existing intrinsics with such a mechanism.
>
> At least if I'm reading things right, the current *implementation* is not
> a correct implementation of the intended semantics. In particular, the
> safepoint operation which provides the opaque manipulation of the boxed
> gcroots is not preserved into the SelectionDAG. As a result, optimizations
> on the SelectionDAG could eliminate the now adjacent box/unbox pairs. This
> would break a relocating collector.
>
> Leaving this aside, there are also a number of performance problems with
> the current implementation. I'll summarize them here for now, but will
> expand into approaches for resolving each if this looks like the most
> likely implementation strategy.
> 1) Explicitly adding box/safepoint/unbox prevents creation of derived
> pointers. This prevents optimizations such as loop-blocking. Ideally,
> we'd rather let the derived pointers be created and capture them for
> updating at the safepoint.
> 2) The redundant loads and stores required for box/unbox. (These might be
> partially eliminated by other optimization passes.)
> 3) The inability to allocate GC roots to registers.
> 4) Frame sizes are implicitly doubled since both an unboxed and boxed
> representation of every object pointer is required.
>
> Frankly, I think that (3) is likely solvable, but that (1) and (4) could
> be fatal for a high performance implementation. I don't have hard numbers
> on that; I'm simply stating my intuition.
>
>
> The fact that the mutable memory associated with a gcroot() is allocated
> on the stack (rather than, say, a machine register) is an implementation
> detail; fixing it doesn't require altering the (conceptual) interface for
> LLVM's existing GC support, AFAICT.
>
> While in principal, I agree with you here, in practice the difference is
> actually quite significant. I am not convinced that the implementation
> would be straight forward. Let's set this aside for the moment and focus
> on the more fundamental questions.
>
>
>
>> We lower gc_relocate to a
>> pseudo opcode which lowered into nothing after register allocation.
>>
>> The problem is, of course, the performance penalty. Does it make
>> sense to get the register allocator "see" the gc_relocate instruction
>> as a copy so that they get the same register / slot? Will that
>> violate the intended semantics of gc_relocate (for example, after
>> assigning the same register / slot to %a and %a.relocated, are there
>> passes that will try to cache derived pointers across loop
>> iterations)?
>>
>> Thanks,
>> -- Sanjoy
>>
>>
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