backward threading heuristics tweek

[Jeff Law]

On 08/15/2016 02:06 PM, Jan Hubicka wrote:
>>> So the threaded path lives fully inside loop1: 6->8->9->3->4->6 propagating
>>> that phi_inserted is 0 after the first iteration of the loop.  This looks like
>>> useful loop peeling oppurtunity which does not garble loop structure. So
>>> perhaps threading paths starting and passing loop latch (i.e. peeling) is
>>> sane? Perhaps all paths fully captured in the loop in question are?
>> Peeling like this has long been a point of contention -- it totally
>> mucks things up like vectorizing.
>>
>> The general issue that the threader knows nothing about the
>> characteristics of the loop -- thus peeling is at this point is
>> premature and just as likely to hinder performance as improve it.
>>
>> I'm never been happy with how this aspect of threading vs loop opts
>> turned out and we have open BZs related to this rats nest of issues.
>
> Ok, then we perhaps just want to silence the testcase?
We might.  I'll have to take a closer look though.  Which means I have 
to stop losing time to other things every day ;(

jeff