[llvm-dev] Optimizing diamond pattern in DAGCombine

[Amaury SECHET via llvm-dev]

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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