[PATCH] PR rtl-optimization/64081: Enable RTL loop unrolling for duplicated exit blocks and back edges.

[Jeff Law]

On 02/06/2016 12:08 PM, David Edelsohn wrote:

>> Normally I'd say that if it was approved before, then it's still good to go
>> since there haven't been major conceptual changes in this code since the
>> patch was originally written and now.
>>
>> However, in this instance the patch had been reported to cause problems on
>> AIX, problems that we can't reproduce now -- which makes me want to be more
>> cautious.  Was it a problem with the patch, or some other latent issue -- we
>> don't know at this point.
>>
>> So I think the way to go is to apply this patch on top of r219827 where it
>> caused the AIX failure.  Then bootstrap on aix and determine the root cause
>> of of the AIX bootstrap failure.  If it's this patch, then update the patch
>> as needed.  If the patch is just exposing a latent bug elsewhere, we should
>> evaluate whether or not that latent but has been fixed or not before
>> applying this fix to the trunk.
>>
>> It's considerably more work, but ISTM it's the right thing to do.
>
> I'm on the fence about this patch.  I definitely don't think that it
> should be merged for GCC 6.
>
> If the patch were to be proposed during Stage 1 for GCC 7 and had not
> caused bootstrap problems for AIX, no one would have any question.
>
> The problem is we don't know if the patch exposed a latent bug that
> independently was fixed after the patch was reverted or if the patch
> still contains a bug that has been rendered latent by another change.
>
> Another approach to track down the cause would be to bisect which
> patch fixed the bootstrap failure if the patch had not been reverted.
Yes, that would be a good approach as well.  The concern here would be 
that without doing the root cause analysis, bisection may just find a 
patch which made the issue go latent.  To be sure we still have to do 
some root cause analysis.

Given this fixes a regression, I'm still open to incorporating the 
patch, but we've got to know what went wrong when the patch was 
previously applied and that whatever that problem was got fixed.
Jeff