[PATCH] Loop Rerolling Pass

[Nick Lewycky]

Andrew Trick wrote:
>
> On Oct 16, 2013, at 6:25 PM, Andrew Trick <atrick at apple.com
> <mailto:atrick at apple.com>> wrote:
>
>>> But it seems, that being the case, we could still return 'n/5' for
>>> loops with only one exiting block? I thought that is what
>>> SE->hasLoopInvariantBackedgeTakenCount(L) was for. It would only say
>>> that there was a backedge-taken count if the loop structure was
>>> simple enough that there was one unambiguous answer. Is this just an
>>> implementation oversight, or are there additional complications?
>>>
>>>>
>>>> So you could get a minimum taken count for a particular loop back
>>>> edge in this case if we adapt the SCEV API to communicate properly.
>>>
>>> I recall discussing this before, and so I apologize, but can you
>>> elaborate on what 'communicate properly' will entail?
>>
>> I’m open to anything. For each branch exit we could distinguish
>> between a min vs. exact backedge taken count.
>
> Minimum taken count is the wrong term. We have either an
> “exact-may-exit” count or an “exact-must-exit” count. The implication of
> exact-may-exit is that either:
> - the loop exits at this iteration
> - the loop exits at a later iteration via another exit
> - the loop terminates at a later iteration
> - the loop exhibits undefined behavior (at a later iteration)

One more case:

  - the loop either exits here, or does not terminate

Nick

>
> Of course, in either case, the loop may exit earlier via another exit or
> terminate early.
>
> -Andy
>
>
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