[llvm-dev] Optimizing diamond pattern in DAGCombine

[Amaury SECHET via llvm-dev]

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 ?
>>
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>> llvm-dev at lists.llvm.org
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>>
>>
>
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