[llvm-dev] RFC: Strong GC References in LLVM

[Andrew Trick via llvm-dev]

> On Jul 21, 2016, at 9:39 AM, Daniel Berlin <dberlin at dberlin.org> wrote:
> 
> 
> 
> On Thu, Jul 21, 2016 at 9:26 AM, Andrew Trick <atrick at apple.com <mailto:atrick at apple.com>> wrote:
> 
>> On Jul 21, 2016, at 7:45 AM, Philip Reames <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote:
>> 
>> Joining in very late, but the tangent here has been interesting (if rather OT for the original thread).
>> 
>> I agree with Danny that we might want to take a close look at how we model things like maythrow calls, no return, and other implicit control flow.  I'm not convinced that moving to a pure explicit model is the right idea because we get a lot of gain in practice from being able to reason about what are essentially a limited form of extended basic blocks.  I would welcome a design discussion about this, preferably in person, but also don't want to block any current (or future honestly) work on this until we have a reasonable firm plan of action.  
>> 
>> One idea would be to explicitly acknowledge that our "basic blocks" are actually "extended basic blocks" with internal exits due to exception propagation, noreturn, and (recently) guards.  I could see a couple of implementation strategies here:
>> 1) Extend BasicBlock to explicitly track potential early exiting instructions.  This would probably have to be conservative so that things like nothrow inference aren't required to update all callers in one go.
>> 2) Conservative assume that BasicBlock has an unknown number of early exiting instructions and write an analysis pass which answers questions about where those early exits are.  Any pass which does code motion would require this analysis.  (This is essentially the principled version of our current hacks.)
> 
> This analysis can be lazy/incremental. Most passes only do “safe” speculation and code sinking without side effects.
> 
> While I agree it can be lazy, and should be an analysis, i'm, again, really not sure which passes you are thinking about here that do code sinking/speculation that won't need it.
> 
> Here's the list definitely needing it right now:
> GVN
> GVNHoist
> LICM
> LoadCombine
> LoopReroll
> LoopUnswitch
> LoopVersioningLICM
> MemCpyOptimizer
> MergedLoadStoreMotion
> Sink
> 
> The list is almost certainly larger than this, this was a pretty trivial grep and examination.
> (and doesn't take into account bugs, etc)
> 
> 
> It would be nice to know which passes, specifically, you are thinking of when you say "most" :)

I don’t have a point to argue, just clarifying Philip's language. But if you want to know what I was thinking it’s that there are a couple of passes doing deliberate code motion: GVN and LICM (I didn’t think of the others) and a bunch of passes doing incidental code motion like InstCombine, CodeGenPrepare, a handful of passes calling SCEVExpander, etc.

I should have said “most passes at most do safe speculation…” so you don’t ask me to grep through the code looking for incidental code motion :)

-Andy

>  
> They don’t need to run the analysis.
> 
> 
> Thanks Philip. I forgot to mention that LLVM passes historically have taken advantage of implicit extended basic blocks (ISel) and were never improved to handle general EBBs. It took years for loop opts to finally handle early exits well.
> -Andy
> 

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