[LLVMdev] GC StackMaps (was Stackmap and Patchpoint Intrinsic Proposal)

[Andrew Trick]

I'm moving this to a different thread. I think the newly proposed
intrinsic definitions and their current implementation are valuable
regardless of how it gets tied into GC...

On Oct 22, 2013, at 6:24 PM, Philip R <listmail at philipreames.com> wrote:

> Adding Gael as someone who has previously discussed vmkit topics on the list.  Since I'm assuming this is where the GC support came from, I wanted to draw this conversation to the attention of someone more familiar with the LLVM implementation than myself.
> 
> On 10/22/13 4:18 PM, Andrew Trick wrote:
>> On Oct 22, 2013, at 3:08 PM, Filip Pizlo <fpizlo at apple.com> wrote:
>> 
>>> On Oct 22, 2013, at 1:48 PM, Philip R <listmail at philipreames.com> wrote:
>>> 
>>>> On 10/22/13 10:34 AM, Filip Pizlo wrote:
>>>>> On Oct 22, 2013, at 9:53 AM, Philip R <listmail at philipreames.com> wrote:
>>>>> 
>>>>>> On 10/17/13 10:39 PM, Andrew Trick wrote:
>>>>>>> This is a proposal for adding Stackmaps and Patchpoints to LLVM. The
>>>>>>> first client of these features is the JavaScript compiler within the
>>>>>>> open source WebKit project.
>>>>>>> 
>>>>>> I have a couple of comments on your proposal.  None of these are major enough to prevent submission.
>>>>>> 
>>>>>> - As others have said, I'd prefer an experimental namespace rather than a webkit namespace.  (minor)
>>>>>> - Unless I am misreading your proposal, your proposed StackMap intrinsic duplicates existing functionality already in llvm.  In particular, much of the StackMap construction seems similar to the Safepoint mechanism used by the in-tree GC support. (See CodeGen/GCStrategy.cpp and CodeGen/GCMetadata.cpp).  Have you examined these mechanisms to see if you can share implementations?
>>>>>> - To my knowledge, there is nothing that prevents an LLVM optimization pass from manufacturing new pointers which point inside an existing data structure.  (e.g. an interior pointer to an array when blocking a loop)  Does your StackMap mechanism need to be able to inspect/modify these manufactured temporaries?  If so, I don't see how you could generate an intrinsic which would include this manufactured pointer in the live variable list.  Is there something I'm missing here?
>>>>> These stackmaps have nothing to do with GC.  Interior pointers are a problem unique to precise copying collectors.
>>>> I would argue that while the use of the stack maps might be different, the mechanism is fairly similar.
>>> 
>>> It's not at all similar.  These stackmaps are only useful for deoptimization, since the only way to make use of the live state information is to patch the stackmap with a jump to a deoptimization off-ramp.  You won't use these for a GC.
>>> 
>>>> In general, if the expected semantics are the same, a shared implementation would be desirable.  This is more a suggestion for future refactoring than anything else.
>>> 
>>> I think that these stackmaps and GC stackmaps are fairly different beasts.  While it's possible to unify the two, this isn't the intent here.  In particular, you can use these stackmaps for deoptimization without having to unwind the stack.
>> 
>> I think Philip R is asking a good question. To paraphrase: If we introduce a generically named feature, shouldn’t it be generically useful? Stack maps are used in other ways, and there are other kinds of patching. I agree and I think these are intended to be generically useful features, but not necessarily sufficient for every use.
> Thank you for the restatement.  You summarized my view well.  
>> 
>> The proposed stack maps are very different from LLVM’s gcroot because gcroot does not provide stack maps! llvm.gcroot effectively designates a stack location for each root for the duration of the current function, and forces the root to be spilled to the stack at all call sites (the client needs to disable StackColoring). This is really the opposite of a stack map and I’m not aware of any functionality that can be shared. It also requires a C++ plugin to process the roots. llvm.stackmap generates data in a section that MCJIT clients can parse.
> Er, I think we're talking past each other again.  Let me lay out my current understanding of the terminology and existing infrastructure in LLVM.  Please correct me where I go wrong.
> 
> stack map - A mapping from "values" to storage locations.  Storage locations primarily take the form of register, or stack offsets, but could in principal refer to other well known locations (i.e. offsets into thread local state).  A stack map is specific to a particular PC and describes the state at that instruction only.  
> 
> In a precise garbage collector, stack maps are used to ensure that the stack can be understood by the collector.  When a stop-the-world safepoint is reached, the collector needs to be able to identify any pointers to heap objects which may exist on the stack.  This explicitly includes both the frame which actually contains the safepoint and any caller frames back to the root of thread.  To accomplish this, a stack map is generated at any call site and a stack map is generated for the safepoint itself.  
> 
> In LLVM currently, the GCStrategy records "safepoints" which are really points at which stack maps need to be remembered.  (i.e. calls and actual stop-the-world safepoints)  The GCMetadata mechanism gives a generic way to emit the binary encoding of a stack map in a collector specific way.  The current stack maps supported by this mechanism only allow abstract locations on the stack which force all registers to be spilled around "safepoints" (i.e. calls and stop-the-world safepoints).  Also, the set of roots (which are recorded in the stack map) must be provided separately using the gcroot intrinsic.  
> 
> In code:
> - GCPoint in llvm/include/llvm/CodeGen/GCMetadata.h describes a request for a location with a stack map.  The SafePoints structure in GCFunctionInfo contains a list of these locations.
> - The Ocaml GC is probably the best example of usage.  See llvm/lib/CodeGen/AsmPrinter/OcamlGCPrinter.cpp
> 
> Note: The summary of existing LLVM details above is based on reading the code.  I haven't actually implemented anything which used this mechanism yet.  As such, take it with a grain of salt.  

That's an excellent description of stack maps, GCStrategy, and
safepoints. Now let me explain how I see it.

GCStrategy provides layers of abstraction that allow plugins to
specialize GC metadata. Conceptually, a plugin can generate what looks
like stack map data to the collector. But there isn't any direct
support in LLVM IR for the kind of stack maps that we need.

When I talk about adding stack map support, I'm really talking about
support for mapping values to registers, where the set of values and
their locations are specific to the "safepoint".

We're adding an underlying implementation of per-safepoint live
values. There isn't a lot of abstraction built up around it. Just a
couple of intrinsics that directly expose the functionality.

We're also approaching the interface very differently. We're enabling
an MCJIT client. The interface to the client is the stack map format.


> In your change, you are adding a mechanism which is intended to enable runtime calls and inline cache patching.  (Right?)  Your stack maps seem to match the definition of a stack map I gave above and (I believe) the implementation currently in LLVM.  The only difference might be that your stack maps are partial (i.e. might not contain all "values" which are live at a particular PC) and your implementation includes Register locations which the current implementation in LLVM does not.  One other possible difference, are you intending to include "values" which aren't of pointer type?  

Yes, the values will be of various types (although only 32/64 bit
types are currently allowed because of DWARF register number
weirdness). More importantly, our stack maps record locations of a
specific set of values, which may be in registers, at a specific
location. In fact, that, along with reserving space for code patching,
is *all* we're doing. GCRoot doesn't do this at all. So there is
effectively no overlap in implementation.

> 
> Before moving on, am I interpreting your proposal and changes correctly?

Yes, except I don’t see a direct connection between the functionality we’re
adding and “the implementation currently in LLVM”.

> Assuming I'm still correct so far, how might we combine these implementations?  It looks like your implementation is much more mature than what exists in tree at the moment.  One possibility would be to express the needed GC stack maps in terms of your new infrastructure.  (i.e. convert a GCStrategy request for a safepoint into a StackMap (as you've implemented it) with the list of explicit GC roots as it's arguments).  What would you think of this?  

I can imagine someone wanting to leverage some of the new
implementation without using it end-to-end as-is. Although I'm not
entirely sure what the motivation would be. For example:

- A CodeGenPrepare pass could insert llvm.safepoint or llvm.patchpoint
  calls at custom safepoints after determining GC root liveness at
  those points.

- Something like a GCStrategy could intercept our implementation of
  stack map generation and emit a custom format. Keep in mind though
  that the format that LLVM emits does not need to be the format read
  by the collector. The JIT/runtime can parse LLVM's stack map data
  and encode it using it's own data structures. That way, the
  JIT/runtime can change without customizing LLVM.

As far as hooking the new stack map support into the GCMetaData
abstraction, I'm not sure how that would work. GCMachineCodeAnalysis
is currently a standalone MI pass. We can't generate our stack maps
here. Technically, a preEmitPass can come along later and reassign
registers invalidating the stack map. That's why we generate the maps
during MC lowering.

So, currently, the new intrinsics are serving a different purpose than
GCMetaData. I think someone working on GC support needs to be
convinced that they really need the new stack map features. Then we
can build something on top of the underlying functionality that works
for them.

-Andy
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