Status of Garbage Collection with Statepoints in LLVM

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1

Hello LLVM community,

We have been experimenting with using LLVM IR as a target for a managed (dynamically typed) language via an AOT compiler (including a backend for ARM). One main challenge is getting the garbage collection right: We would like to be able to implement a moving collector. This requires us to a) find a precise set of root pointers and b) be able to rewrite those pointers after objects moved.

LLVM seems to provide two mechanisms for doing this: via “gcroot” and via “statepoints”. The [statepoints] documentation indicates the first option, namely “gcroot”, is only viable for conservative collectors and statepoints might eventually replace gcroot. So we wanted to try out the statepoints approach.

Though it turned out that:

  • the pass for inserting statepoints is hard-coded to only work with samples and CLR (see [placesafepoints])

  • the pass for rewriting statepoints is hard-coded to only work with samples and CLR (see [rewritestatepoints])

  • the only backend supporting statepoints right now seems to be 64-bit intel (see [backend-x64])

Since the ARM backend (and e.g. 32-bit intel) doesn’t seem to have support for lowering statepoints-using IR, we were rather disappointed.

We are now experimenting with keeping a shadow stack which contains all the managed pointers, but since the IR we emit contains reads/write to this shadow stack (and the shadow stack escapes on calls), the performance suffers significantly, since LLVM can’t perform a lot of the optimizations it could otherwise do (IIRC the statepoints approach doesn’t have this problem, since the statepoints can be inserted after optimization passes were run).

Is there any timeline for the statepoints support in LLVM?
Is there a list of things that currently work / don’t work with safepoints?
What is the recommended approach for moving GCs when using LLVM (what are others doing)?

Thanks in advance,
Martin

Sidenote: It would be beneficial for users if the [statepoints] documentation would highlight the current status and limitations.

[statepoints] http://llvm.org/docs/Statepoints.html
[placesafepoints] https://github.com/llvm-mirror/llvm/blob/a40ba754c3f765768d441b9b4b534da917f8ad3c/lib/Transforms/Scalar/PlaceSafepoints.cpp#L441

[rewritestatepoints] https://github.com/llvm-mirror/llvm/blob/a40ba754c3f765768d441b9b4b534da917f8ad3c/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp#L2286

[backend-x64] https://github.com/llvm-mirror/llvm/blob/a40ba754c3f765768d441b9b4b534da917f8ad3c/lib/Target/X86/X86MCInstLower.cpp#L841

2

Hello LLVM community,

We have been experimenting with using LLVM IR as a target for a managed (dynamically typed) language via an AOT compiler (including a backend for ARM). One main challenge is getting the garbage collection right: We would like to be able to implement a moving collector. This requires us to a) find a precise set of root pointers and b) be able to rewrite those pointers after objects moved.

Welcome to the community! Glad to have other interested users involved.

LLVM seems to provide two mechanisms for doing this: via "gcroot" and via "statepoints". The [statepoints] documentation indicates the first option, namely "gcroot", is only viable for conservative collectors and statepoints might eventually replace gcroot. So we wanted to try out the statepoints approach.

This is still true. I'd be very cautious about relying on the correctness of gcroot in it's current form. If you do decide you want to pursue that option, I'll have some suggestions on how to improve the situation in the backend, but I don't really recommend this.

Though it turned out that:
  * the pass for inserting statepoints is hard-coded to only work with samples and CLR (see [placesafepoints])
  * the pass for rewriting statepoints is hard-coded to only work with samples and CLR (see [rewritestatepoints])

In both cases, you're are going to need to introduce your own GCStrategy type. We don't have a good way to ask questions about the GCStrategy instance from IR transformation passes yet - it's on my long term todo, but got stalled due to some infrastructure issues - so we had to match names in a couple of places. I think you found both of them.

More generally, can I back up and ask an important question? Do you have to support deoptimization (i.e. osr side exits) in any form? If you do, you'll probably want to avoid the PlaceSafepoints utility pass. If you need to support this case, let me know and we can share some code which hasn't made it upstream yet.

  * the only backend supporting statepoints right now seems to be 64-bit intel (see [backend-x64])

Since the ARM backend (and e.g. 32-bit intel) doesn't seem to have support for lowering statepoints-using IR, we were rather disappointed.

This is explicitly documented: Garbage Collection Safepoints in LLVM — LLVM 18.0.0git documentation

Adding support for ARM (32?, 64?) shouldn't be too complicated. If you search for STATEPOINT in lib/Target/X86, you'll see there are only a small handful of places which need architectural support. Most of the complexity is in the generic CodeGen parts.

Adding support for 32 bit x86 should be even easier. If I'm reading the code correctly, it looks like the only issue is in generating the right call sequence.

We are now experimenting with keeping a shadow stack which contains all the managed pointers, but since the IR we emit contains reads/write to this shadow stack (and the shadow stack escapes on calls), the performance suffers significantly, since LLVM can't perform a lot of the optimizations it could otherwise do (IIRC the statepoints approach doesn't have this problem, since the statepoints can be inserted *after* optimization passes were run).

That's pretty much the entire idea behind the late rewriting model. I'll comment just for clarity that statepoints *could* be inserted early as well, but I don't recommend it.

One thing I want to ask: have you implemented inlining? One thing we found was that the relative importance of how we represented safepoints dropped substantially once we got aggressive inlining in place. Essentially, all of our hot safepoints disappeared or became inliner bugs. :slight_smile:

Is there any timeline for the statepoints support in LLVM?

Not explicitly. This is directly driven by those of us using and contributing to them. As we find problems in our use cases, we fix them.

Just to give some context, we (Azul) have reached what we believe to be a stable state and are mostly focused on (non-gc related) performance issues. Not all of our changes have made it upstream - specifically, the gc-pointer distinction and exception handling mentioned in the list above - but most of them have. On the platform we care about (x86-64) and the configurations we use (early poll insert, late rewriting), things appear stable.

Is there a list of things that currently work / don't work with safepoints?

There wasn't a public list. Rather than replying with one here, I've added to the statepoint documentation with the start of such a list.

http://llvm.org/docs/Statepoints.html#problem-areas-and-active-work

If you have questions on any of these, please ask.

What is the recommended approach for moving GCs when using LLVM (what are others doing)?

Currently, we (Azul) and the CoreCLR llilc team are the only folks I know of using LLVM with a relocating GC. Both of us are using statepoints.

Thanks in advance,
Martin

Sidenote: It would be beneficial for users if the [statepoints] documentation would highlight the current status and limitations.

If you have suggestions for documentation fixes, please let me know. I'm happy to either review changes or make the changes myself if you point out problems.

[statepoints] Garbage Collection Safepoints in LLVM — LLVM 18.0.0git documentation
[placesafepoints] https://github.com/llvm-mirror/llvm/blob/a40ba754c3f765768d441b9b4b534da917f8ad3c/lib/Transforms/Scalar/PlaceSafepoints.cpp#L441
[rewritestatepoints] https://github.com/llvm-mirror/llvm/blob/a40ba754c3f765768d441b9b4b534da917f8ad3c/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp#L2286
[backend-x64] https://github.com/llvm-mirror/llvm/blob/a40ba754c3f765768d441b9b4b534da917f8ad3c/lib/Target/X86/X86MCInstLower.cpp#L841

Philip

p.s. We're happy to talk on the phone or in person about these topics as well. Having a higher bandwidth conversation can be quite helpful. Let me know if you're interested in arranging such a meeting. Or, if you're local to the bay area, consider coming to one of the socials. Sanjoy and I both generally attend and either of us can answer further questions you might have.

3

Hi Martin,

Philip covered all of it very well, I'll just add one minor comment:

More generally, can I back up and ask an important question? Do you have to
support deoptimization (i.e. osr side exits) in any form? If you do, you'll
probably want to avoid the PlaceSafepoints utility pass. If you need to

PlaceSafepoints is inadequate only if you have asynchronous
invalidation -- i.e. thread X is spinning in a long running loop while
thread Y loaded a class that makes the code running in thread X
invalid, so thread X needs to be polling for deopt safepoints. If all
your invalidation events are synchronous then deopt bundles +
PlaceSafepoints should be enough for both deoptimization and precise
relocating GC.

If you don't need to *poll* for safepoints at all (perhaps the entire
VM is fully single threaded, so you safepoint only on allocation),
then PlaceSafepoints is not needed at all, and you can directly run
RewriteStatepointsForGC.

-- Sanjoy

4

Presumably this also depends on the memory model that your language provides. If it’s something like Go, where no happens-before relationships are established between threads by any mechanism other than explicit synchronisation (atomics or message passing), then it’s perfectly fine for a thread in a long-running loop that doesn’t contain any safepoints to see a stale copy for a very long time.

David

5

Thank you very much for the quick answers :slight_smile:

Though it turned out that:

  • the pass for inserting statepoints is hard-coded to only work with samples and CLR (see [placesafepoints])
  • the pass for rewriting statepoints is hard-coded to only work with samples and CLR (see [rewritestatepoints])

In both cases, you’re are going to need to introduce your own GCStrategy type. We don’t have a good way to ask questions about the GCStrategy instance from IR transformation passes yet - it’s on my long term todo, but got stalled due to > some infrastructure issues - so we had to match names in a couple of places. I think you found both of them.

I did have my own GCStrategy type, but these two passes are currently silently ignoring any strategies which don’t have the right name. It would be great if an error could be reported instead, but I saw the todo’s to address this.

More generally, can I back up and ask an important question? Do you have to support deoptimization (i.e. osr side exits) in any form? If you do, you’ll probably want to avoid the PlaceSafepoints utility pass. If you need to support this case,
let me know and we can share some code which hasn’t made it upstream yet.

We don’t have any plans for deoptimizations right now.

  • the only backend supporting statepoints right now seems to be 64-bit intel (see [backend-x64])

Since the ARM backend (and e.g. 32-bit intel) doesn’t seem to have support for lowering statepoints-using IR, we were rather disappointed.

This is explicitly documented: http://llvm.org/docs/Statepoints.html#supported-architectures

Adding support for ARM (32?, 64?) shouldn’t be too complicated. If you search for STATEPOINT in lib/Target/X86, you’ll see there are only a small handful of places which need architectural support.
Most of the complexity is in the generic CodeGen parts.
Adding support for 32 bit x86 should be even easier. If I’m reading the code correctly, it looks like the only issue is in generating the right call sequence.

That’s good news :slight_smile:

One thing I want to ask: have you implemented inlining? One thing we found was that the relative importance of how we represented safepoints dropped substantially once
we got aggressive inlining in place. Essentially, all of our hot safepoints disappeared or became inliner bugs. :slight_smile:

We haven’t implemented inlining. Currently we do a feasibility experiment to find out if LLVM would be a good option for our problem (and GC is one big hurdle to pass, the other one is probably exceptions which we haven’t looked at yet – code size also matters to us).
Since we are in control of lowering source language to IR, inlining could happen before we generate IR and/or on the IR-level via LLVM inlining, but since we would use it in an AOT rather than a JIT setting for a dynamic language, there are many calls where the receiver is not known at compile time (maybe profile-based guarded optimization would be an option).

Is there any timeline for the statepoints support in LLVM?

Not explicitly. This is directly driven by those of us using and contributing to them. As we find problems in our use cases, we fix them.

Just to give some context, we (Azul) have reached what we believe to be a stable state and are mostly focused on (non-gc related) performance issues.
Not all of our changes have made it upstream - specifically, the gc-pointer distinction and exception handling mentioned in the list above - but most of
them have. On the platform we care about (x86-64) and the configurations we use (early poll insert, late rewriting), things appear stable.

Are the changes you have on top of LLVM ToT available somewhere (some of the issues on the list seem to be relevant to us as well)?
May I ask: Do you use it in a production system already?

Is there a list of things that currently work / don’t work with safepoints?

There wasn’t a public list. Rather than replying with one here, I’ve added to the statepoint documentation with the start of such a list.
http://llvm.org/docs/Statepoints.html#problem-areas-and-active-work
If you have questions on any of these, please ask.

Thanks a lot for adding this.

Sidenote: It would be beneficial for users if the [statepoints] documentation would highlight the current status and limitations.

If you have suggestions for documentation fixes, please let me know. I’m happy to either review changes or make the changes myself if you point out problems.

One thing I ran into is the silent ignoring of GCStrategy’s with wrong name, but that seems to be already on the list.

p.s. We’re happy to talk on the phone or in person about these topics as well. Having a higher bandwidth conversation can be quite helpful.
Let me know if you’re interested in arranging such a meeting. Or, if you’re local to the bay area, consider coming to one of the socials.
Sanjoy and I both generally attend and either of us can answer further questions you might have.

Thanks for the offer :slight_smile:
We’ll spend a bit more time experimenting with the 64-bit intel implementation for now and see how far we get with it. We might reach out if we have more questions on the way.