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
<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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David Chisnall
2014-Jun-05 09:19 UTC
[LLVMdev] Code for late safepoint placement available
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. 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. 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). David 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/ > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
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/ >> _______________________________________________ >> LLVM Developers mailing list >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
On Wed, Jun 4, 2014 at 9:35 AM, 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.) > > Chris and I had a discussion about 3 years ago where we talked aboutkeeping both, but it really depends on how difficult it is. Although the existing intrinsics have many different kinds of horribleness, the one advantage that they have is that roots don't have to be pointers - they can be structs containing pointers, such as tagged unions or Go-style interface values, which have fields that may contain either a pointer or some other data type depending on the value of some other field. I know we talked in email about ways to work around this limitation, but those workarounds have some complex edge cases which it would be nice to avoid - like for example passing a tagged union as a parameter. That being said, I'm probably the only person who cares about this particular issue :) And while removing support for non-pointer roots will make my life harder in some ways, the new system will make it easier in many other ways.> > - 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/ > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev > >-- -- Talin -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140610/3dbfed8b/attachment.html>
On 06/10/2014 11:23 AM, Talin wrote:> On Wed, Jun 4, 2014 at 9:35 AM, Philip Reames > <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote: > > > 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 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.. > > Chris and I had a discussion about 3 years ago where we talked about > keeping both, but it really depends on how difficult it is.First, did I trim the response correctly? It sounds like you're responding to the question of how to integrate late safepoint placement with gcroot, but your actual response was inlined after another unrelated question. Honestly, other than the maintenance headache, the two code paths basically don't touch. Keeping both is entirely viable, though I'd have to ask: why? What benefit does gcroot have? (You started addressing this already, just making the question explicit.)> Although the existing intrinsics have many different kinds of > horribleness, the one advantage that they have is that roots don't > have to be pointers - they can be structs containing pointers, such as > tagged unions or Go-style interface values, which have fields that may > contain either a pointer or some other data type depending on the > value of some other field. I know we talked in email about ways to > work around this limitation, but those workarounds have some complex > edge cases which it would be nice to avoid - like for example passing > a tagged union as a parameter. > > That being said, I'm probably the only person who cares about this > particular issue :) And while removing support for non-pointer roots > will make my life harder in some ways, the new system will make it > easier in many other ways.For those who weren't part of the previous discussion, I summarized the subset of IR which our safepoint placement scheme handles in this blog post: http://www.philipreames.com/Blog/2014/06/12/ir-restrictions-for-late-safepoint-placement/ Short version as it applies to Talin's point: We do not currently support pointers to garbage collected objects in aggregate types. This should be fairly straight forward to add. Patches are welcome and we should support them at some point. We have no plans to support tagged unions as roots. If you're willing to bake in knowledge of the tag functions, doing so wouldn't be particularly hard, but I don't know of any way to do so in a general way. One easy to implement support for aggregate types would be to explode and reconstruct the aggregate over the safepoint. A more efficient, but also more complicated, scheme would be to encode the interior reference to the aggregate type into the arguments of the statepoint intrinsic. Both schemes suffer from the fact they have to rely on type information (of the aggregate itself) being accurate. This is a stronger assumption than we currently need for non-aggregates and we would need to verify that the optimizer actually upheld it. Talin, hopefully my restatement above helps clarify. I'll also note that tagged unions as parameters are fine if the values are extracted immediately on entry before any safepoint happens. The safepoint mechanism only considers values which are live (have potential uses reachable from) the safepoint itself. As a result, dead tagged unions are fine. Admittedly, that's a bit of a hack, but you might find it a useful hack. :) Philip -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20140613/887385c8/attachment.html>