[PATCH] Remove dead code from LoopUnswitch

[Hal Finkel]

----- Original Message -----
> 
> On Oct 26, 2013, at 11:09 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> 
> > Chris, et al.,
> > 
> > LoopUnswitch's code simplification routine has code to convert
> > conditional to unconditional branches, after unswitching makes the
> > condition constant, and then remove any blocks that renders dead.
> > Unfortunately, this code is dead, currently broken, and
> > furthermore, has never been alive (at least as far back at 2006).
> > I propose that, at least for now, we nuke it.
> > 
> > Regarding the general functionality, I see (at least) two possible
> > options:
> > 
> > 1. Fix/rewrite this functionality within LoopUnswitch
> > 
> > 2. Run SimplifyCFG after running LoopUnswitch: we currently run
> > InstCombine after running LoopUnswitch, and so the loop pass
> > manager is already done after running LoopUnswitch, and SE is
> > already being invalidated by InstCombine, so it does not look
> > like, if we added SimplifyCFG after InstCombine, we'd really lose
> > much.
> > 
> > Thoughts?
> 
> Hi Hal,

Andy,

In retrospect, my phrasing of this question was bad. What I should have said is, "If we still want this functionality at this point in the pipeline..." -- I have no particular evidence that it is needed (although it looks like Chris thought that it was needed at some point in the "distant" past).

It sounds like you don't think it is needed, and we can just remove the current dead code.

> 
> SimplifyCFG does run after Unswitch... way after. But at least it
> will be rerun before any other loop opts, like Vectorizer. Actually
> I think SCCP would catch this first.

Looks like jump threading catches some of them too.

> The problem with running SimplifyCFG in between loop pass managers is
> that it invalidates loop info.

Good point.

> 
> Which pass are you concerned will see the dead blocks? GVN?

Idiom recognition?

> 
> Long term I envision the pass order looking like: GVN -> Unswitch ->
> SCCP -> … -> LoopVectorize.

Sounds good.

Thanks again,
Hal

> 
> -Andy
> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory