[LLVMdev] design question on inlining through statepoints and patchpoints

[Philip Reames]

The long term plan is a) evolving, and b) dependent on the specific use 
case.  :)

It would definitely be nice if we could support both early and late 
safepoint insertion.  I see no reason that LLVM as a project should pick 
one or the other since the infrastructure required is largely 
overlapping.  (Obviously, I'm going to be mostly working on the parts 
that I need, but others are always welcome to extend in other directions.)

One of the challenges we've run into is that supporting deoptimization 
points (which in practice are safepoints) require a lot of the same 
infrastructure as early safepoint insertion.  It's likely that we'll end 
with a scheme which inserts safepoint polls quite early (but with 
restricted semantics and optimization impact) and then converts them to 
explicit GC safepoints (with full invalidation semantics) quite late.  
We already have this distinction in tree in the form of PlaceSafepoints 
and RewriteStatepointsForGC.  I suspect we'll move further in this 
direction.

I suspect that for languages without deoptimization, you'll want to 
insert safepoint polls quite late.  Whether you do the same for 
safepoints-at-calls is debatable.  I used to think that you should do 
that quite late, but I'm no longer sure that's always the right answer.

Philip


On 06/17/2015 04:13 PM, Swaroop Sridhar wrote:
> With respect to phase ordering, is the long term plan to run the statepoint placement/transformation phases late (after all optimizations)?
> If so, will we need to support inlining post statepoint transformation?
>
> Thanks,
> Swaroop.
>
> -----Original Message-----
> From: Sanjoy Das [mailto:sanjoy at playingwithpointers.com]
> Sent: Tuesday, June 16, 2015 7:20 PM
> To: LLVM Developers Mailing List; Andrew Trick; Swaroop Sridhar; Chandler Carruth; Nick Lewycky
> Subject: design question on inlining through statepoints and patchpoints
>
> I've been looking at inlining invokes / calls done through statepoints and I want to have a design discussion before I sink too much time into something I'll have to throw away.  I'm not actively working on adding inlining support to patchpoints, but I suspect these issues are applicable towards teaching LLVM to inline through patchpoints as well.
>
>
> There are two distinct problems to solve before LLVM can inline through statepoints:
>
> # Managing data flow for the extra metadata args.
>
> LLVM needs some logic to "transfer" the extra live values attached to a statepoint/patchpoint into the body of the inlinee somehow.  How this is handled depends on the semantics of the live values (something the frontend knows).  There needs to be a clean way for the frontend to communicate this information to LLVM, or we need to devise a convention that is sensible for the kinds of frontends we wish to support.  Initially I plan to sidestep this problem by only inlining through statepoints have *no* extra live values / gc pointers.
>
>
> # Managing the call graph
>
> This is the problem we need to solve first.  Currently LLVM views the a statepoint or patchpoint call as
>
>    1. A call to an intrisic.  This does not add an edge to the call
>       graph (not even to the dedicated external node).
>
>    2. An escaping use of the callee.
>
> IIUC, (2) is (conservatively) imprecise and (1) is incorrect.  (1) makes LLVM believe that a function that calls @f via a statepoint does not call @f at all.  (2) makes LLVM believe that @f is visible externally, even if it has internal linkage.
>
> Given this starting point, I can think of three ways to model statepoint's (and patchpoint's) control flow semantics within a call
> graph:
>
>    1. Model calls to statepoint, patchpoint and stackmap intrinsics as
>       calling the external node.  Teach the inliner pass to
>       "devirtualize" calls through statepoints when posssible, except
>       that the "devirtualization" is only a facade (i.e. we don't
>       mutate the IR to change the statepoint to a direct call).  We add
>       some abstraction to the inlining utility functions to inline
>       through something more general than a CallSite.
>
>    2. Introduce a new abstraction InlineSite (bikeshedding on the name
>       is welcome).  InlineSite sits on top of a CallSite and knows how
>       to extract the semantic call out of a statepoint or a patchpoint
>       (similar to the llvm::Statepoint class).  The inliner and the
>       call graph analysis works on top of this InlineSite abstraction
>       instead of the CallSite abstraction.
>
>    3. Change all the places that matter (CallGraph, CallGraphSCCPass
>       etc.) from
>
>         if (CallSite CS = ...)
>
>       to
>
>         if (Statepoint SP = ...)
>            ...
>         else if (CallSite CS = ...)
>
>       or something equivalent to this.
>
> Personally, I'd prefer going with (1) if it is viable, and (2) if not.
>
> What do you think?
>
> -- Sanjoy
>
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