Re: [PATCH] Avoid invalid loop transformations in jump threading registry.

[Jeff Law <jeffreyalaw@gmail.com>]

On 9/24/2021 5:34 AM, Aldy Hernandez wrote:
>
>
> On 9/23/21 6:10 PM, Jeff Law wrote:
>>
>>
>> On 9/23/2021 5:15 AM, Aldy Hernandez wrote:
>>> My upcoming improvements to the forward jump threader make it thread
>>> more aggressively.  In investigating some "regressions", I noticed
>>> that it has always allowed threading through empty latches and across
>>> loop boundaries.  As we have discussed recently, this should be avoided
>>> until after loop optimizations have run their course.
>>>
>>> Note that this wasn't much of a problem before because DOM/VRP
>>> couldn't find these opportunities, but with a smarter solver, we trip
>>> over them more easily.
>> We used to be much more aggressive in this space -- but we removed 
>> the equivalency tracking on backedges in the main part of DOM which 
>> had the side effect to reducing the number of threads related to back 
>> edges in loops.
>
> I thought we couldn't thread through back edges at all in the old 
> threader, or are we talking about the same thing?  We have a hard fail 
> on backedge thread attempts for anything but the backward threader and 
> its custom copier.
We used to have it in the distant past IIRC.

Jeff