[llvm-dev] Optimizing diamond pattern in DAGCombine

[Amaury SECHET via llvm-dev]

You can see an instance of that problem in https://reviews.llvm.org/D32756

Depending on which order the combiner pass on the nodes, the optimization
kicks in or not. I have other diamond pattern that I want to add and they
all suffer in the same way.

2017-05-22 12:19 GMT-07:00 Nirav Davé <niravd at google.com>:

> I suspect the most direct resolution here is to see the problem directly.
> Can you post a patch?
>
> -Nirav
>
> On Mon, May 22, 2017 at 3:01 PM, Amaury SECHET <deadalnix at gmail.com>
> wrote:
>
>> Explicitly re-adding a node to be processed doesn't work, because the
>> processing order is canonical.
>>
>> 2017-05-22 11:39 GMT-07:00 Nirav Davé <niravd at google.com>:
>>
>>> You can always explicitly add D to the worklist when you make the
>>> transformation with AddToWorklist. Presuambly this was the cause for your
>>> infinite loop.
>>>
>>> -Nirav
>>>
>>>
>>> On Mon, May 22, 2017 at 2:07 PM, Amaury SECHET <deadalnix at gmail.com>
>>> wrote:
>>>
>>>> The root problem is that, when A gets modified, D doesn't get added
>>>> back to the worklist. I could match the pattern on A, but the problem
>>>> remains: when D gets modified, A do not get added back tot he worklist.
>>>>
>>>> I also considered ding several round of DAGCombine, but it is very easy
>>>> to run into infinite loops, even with a fair amount of sanity checks.
>>>>
>>>> 2017-05-22 7:30 GMT-07:00 Nirav Davé <niravd at google.com>:
>>>>
>>>>> This is a little hard to diagnose in the abstract, but it sounds like
>>>>> you're having a loop along the lines of visit A -> visit B/C -> visit D ->
>>>>> visit A and that at each step you're making a real reasonable change to the
>>>>> DAG and returning to the original DAG.
>>>>>
>>>>> One possible solution is to to rework the optimization to occur when
>>>>> visiting A not D. so the last edge in the visit loop no longer happens.
>>>>>
>>>>> If that's not feasible, I don't think there's much you can do that is
>>>>> not effectively preventing one of those transitions from occurring when it
>>>>> would cause the infinite visit loop.
>>>>>
>>>>> HTH,
>>>>>
>>>>> -Nirav
>>>>>
>>>>>
>>>>> On Mon, May 22, 2017 at 2:21 AM, Amaury SECHET via llvm-dev <
>>>>> llvm-dev at lists.llvm.org> wrote:
>>>>>
>>>>>> I'm trying to optimize a pattern that goes roughly as:
>>>>>>     A
>>>>>>    / \
>>>>>>   B   C
>>>>>>    \ /
>>>>>>     D
>>>>>>
>>>>>> Problem is, when A gets modified, B and C get added back to the
>>>>>> worklist, but D doesn't. Readding D to the worklist just create an infinite
>>>>>> loop where one process D again and again.
>>>>>>
>>>>>> Is there a proper way to make this work ?
>>>>>>
>>>>>> _______________________________________________
>>>>>> LLVM Developers mailing list
>>>>>> llvm-dev at lists.llvm.org
>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170522/edf28a11/attachment.html>