[llvm-dev] Optimizing diamond pattern in DAGCombine

[Nirav Davé via llvm-dev]

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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