[LLVMdev] How to run two loop passes non-interleaved if they are registered one by one?

[Kevin Qin]

Hi Hal,

James told me that in PassManagerBuilder.cpp, BarrierNoopPass is already
used for this kind of purpose(though there's also a fixme saying it's
hacking). I think it's a good idea to use this pass here.

Thanks,
Kevin

2015-03-11 17:05 GMT+08:00 Hal Finkel <hfinkel at anl.gov>:

> ----- Original Message -----
> > From: "Kevin Qin" <kevinqindev at gmail.com>
> > To: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> > Sent: Wednesday, March 11, 2015 3:47:35 AM
> > Subject: [LLVMdev] How to run two loop passes non-interleaved if they
> are registered one by one?
> >
> >
> >
> > Hi LLVM developers,
> >
> >
> > I want to add LICM pass after loop unrolling pass in current
> > optimization pipeline. Because both of them are loop passes, so if I
> > registered them one by one, they will interleaved go through all
> > loops in bottom up way within same loop pass manager. Loop unroling
> > pass may create new inner loops from partial unrolling, and those
> > newly created loops can be visited only if the rest of loop passes
> > are finished on current loop. The problem is, this visit order may
> > be not in bottom up way, but that is order is required by LICM.
> >
> >
> > For example, the input has 2 nested loops(L1 and L2).
> >
> >
> > for () { //L1
> > for() { }//L2
> > }
> >
> >
> >
> > We registered LICM pass after loop unrolling pass, so these 2 passes
> > will share the same loop pass manager. The initialized work queue
> > will be (L1, L2). Loop pass manager always pops out the last object,
> > so loop unrolling pass will visit L2 first, and then LICM pass. When
> > L2 is visited by all passes, it will be removed from queue. At this
> > stage, everything is fine.
> >
> >
> > Next, L1 is on process, and it's partial unrolled from loop unrolling
> > pass, and finally L3 is created.
> >
> >
> >
> > for () { //L1
> > for() { }//L2
> > for() { }//L3
> >
> > }
> >
> >
> > Here's the problem. As LICM shared same loop pass manager will loop
> > unrolling pass, then L1 will be visited by LICM. But LICM extremely
> > requires a bottle up visit order, and at this moment, L1 is visited
> > earlier than L3, this will cause assertion failure.
> >
> >
> > The best solution for this is spiting two passes into different pass
> > manager, and let them run non-interleaved. But I don't know how to
> > implement this within legacy pass manager. Does anyone know how to
> > do that?
> >
>
> I believe that, generically, we've needed 'barrier' passes for this
> purpose. Creating a 'noop' function pass is an option. In this case,
> inserting a run of instcombine (or, if that's too expensive for some
> reason, instsimplify) could be reasonable. I can imagine unrolling creating
> things that turn out to be loop invariant only after simplification
> (especially considering that we rely on simplification to clean up the
> repeated induction variable increments, etc. after unrolling).
>
>  -Hal
>
> >
> > If you have any better solution, don't hesitate to share.
> >
> >
> >
> > --
> >
> >
> > Thanks a lot in advance,
> >
> >
> > Kevin Qin
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
>



-- 
Best Regards,

Kevin Qin
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