[llvm-dev] Optimizing diamond pattern in DAGCombine

[Nirav Davé via llvm-dev]

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
>>>>>
>>>>>
>>>>
>>>
>>
>
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