[llvm-dev] RFC: Coroutine Optimization Passes

Philip Reames via llvm-dev llvm-dev at lists.llvm.org
Mon Jul 18 09:44:36 PDT 2016

I also haven't seen this discussion.  Can you provide a pointer to the 
thread where this was discussed?

p.s. If this discussion *hasn't* happened, that would definitely be a 
blocker for any of the specific work discussed below.


On 07/14/2016 10:48 PM, Vadim Chugunov via llvm-dev wrote:
> Hi!
> Sorry for jumping in late, but I have a general question (that 
> I perhaps should have asked during round 1):
> This proposal jumps straight into the thick of implementation, but I 
> don't think I've seen a motivation of why coroutines need to be 
> represented at the LLVM IR level.   Can't this transform be performed 
> entirely in the front-end?
> Vadim
> On Thu, Jul 14, 2016 at 9:28 PM, Gor Nishanov via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>     Hi David:
>     >> How do you deal with basic blocks which appear to be used by
>     multiple parts
>     >> of the coroutine? We handled this in WinEHPrepare by cloning
>     any BBs which
>     >> were shared.
>     I experimented with several approaches, but, cloning ended up
>     being the simplest
>     and most reliable. Suspend points express three different control
>     flows that
>     can happen at the suspend point: a suspension (default),
>     resumption (0) and
>     destruction (1).
>       %0 = call i8 @llvm.coro.suspend([..])
>       switch i8 %0, label %suspend [i8 0, label %resume,
>                                     i8 1, label %destroy]
>     I slap a switch that jumps to all suspend points in the function.
>     Then I clone
>     the function twice (one will become resume clone, and another
>     destroy clone).
>     This switch becomes the entry block in the clones. Then, I RAUW
>     coro.suspends
>     with -1, 0, or 1 (in original, resume and destroy clones
>     respectively) and let
>     SimplifyCFG do the rest. (This is slightly simplified explanation,
>     but it should
>      give the idea).
>     >> I would remove the attribute.  There are all sorts of tricks
>     you can do to
>     >> avoid scanning the function for calls to the intrinsic.  For
>     example, you
>     >> can see if a declaration of your intrinsic exists and, if so,
>     if it has an
>     >> users in the function in question (under the assumption that
>     there are few).
>     Aye-aye. Will remove the attribute.
>     With respect to lessening the impact of coroutine passes, one
>     approach I tried
>     was to look during doInitialize whether there are any uses of
>     coroutine
>     intrinsics and set a flag if there are any, or maybe build a set
>     of functions
>     with coroutines intrinsics in doInitialize, so that in
>     runOnFunction, I can just
>     check whether the function is in the set and skip if it is not.
>     Then, I scared myself silly that some optimization passes can
>     materialize
>     new functions or new function bodies and I will miss them. So I
>     stopped doing
>     that.
>     I think your approach takes care of my "materialization" concern.
>     (BTW, I don't
>     even know if that is a real concern). But may not be profitable if
>     there are a
>     lot of small functions that are faster to scan for coroutine
>     intrinsics then to
>     scan potentially longer list of coroutine intrinsics users.
>     BTW, Do you have a preference on how to restart CGSCC pipeline?
>     One of the four
>     options I listed (repeated in P.S), or even some better way I did
>     not think
>     about?
>     Thank you,
>     Gor
>     P.S.
>     Option 1: https://reviews.llvm.org/D21569 (no longer relevant,
>     since we are
>                                                removing
>     AttrKind::Coroutine)
>     Option 2: https://reviews.llvm.org/D21570 (bool& Devirt in runSCC)
>     Option 3: https://reviews.llvm.org/D21572 (virtual bool
>     restatedRequested())
>     Option 4: Fake devirtualized call in a function pass, so that
>     RefreshSCC will
>               detect devirtualization and restart the pipeline by itself.
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