[llvm-dev] [PM] I think that the new PM needs to learn about inter-analysis dependencies...

[Chandler Carruth via llvm-dev]

On Tue, Jul 12, 2016 at 11:34 PM Sean Silva <chisophugis at gmail.com> wrote:

> On Tue, Jul 12, 2016 at 11:32 PM, Xinliang David Li <davidxl at google.com>
> wrote:
>
>>
>>
>> On Tue, Jul 12, 2016 at 10:57 PM, Chandler Carruth <chandlerc at gmail.com>
>> wrote:
>>
>>> Yea, this is a nasty problem.
>>>
>>> One important thing to understand is that this is specific to analyses
>>> which hold references to other analyses. While this isn't unheard of, it
>>> isn't as common as it could be. Still, definitely something we need to
>>> address.
>>>
>>
>> We can call this type of dependencies (holding references)
>> hard-dependency. The soft dependency refers to the case where analysis 'A'
>> depends on 'B' during computation, but does not need 'B' once it is
>> computed.
>>
>> There are actually quite a few examples of hard-dependency case. For
>> instance LoopAccessInfo, LazyValueInfo etc which hold references to other
>> analyses.
>>
>> Problem involving hard-dependency is actually easier to detect, as it is
>> usually a compile time problem. Issues involving soft dependencies are more
>> subtle and can lead to wrong code gen.
>>
>
> Did you mean to say that soft-dependency problems are easier to detect? At
> least my intuition is that soft-dependency is easier because there is no
> risk of dangling pointers to other analyses.
>

The issue is that the fact that there is *any* dependency isn't clear.

However, I think the only real problem here are these "hard dependencies"
(I don't really like that term though). For others, only an analysis that
is *explicitly* preserved survives. So I'm not worried about the fact that
people have to remember this.

The question is how often there are cross-data-structure references. David
mentions a few examples, and I'm sure there are more, but it isn't clear to
me yet whether this is pervasive or occasional.

And even then it isn't clear how onerous explicitly managing this in
invalidate overrides will be.


>
> -- Sean Silva
>
>
>>
>> David
>>
>>
>>
>>>
>>> Some ideas about mitigating and fixing it below.
>>>
>>> On Tue, Jul 12, 2016 at 6:15 PM Sean Silva <chisophugis at gmail.com>
>>> wrote:
>>>
>>>> How should we solve this? I see two potential solutions:
>>>> 1. Analyses must somehow list the analyses they depend on (either by
>>>> overriding "invalidate" to make sure that they invalidate them, or
>>>> something "declarative" that would allow the AnalysisManager to walk the
>>>> transitive dependencies).
>>>>
>>>
>>> I think this is the right approach. I would personally start by
>>> overriding the invalidate callback everywhere that it is necessary, and see
>>> how bad that becomes.
>>>
>>> If it becomes common and burdensome, then we can change the way
>>> invalidation works such that the analysis manager is aware of the preserved
>>> analysis set in more detail, and have it build up the necessary data
>>> structures to know in-advance whether it must make an explicit invalidate
>>> call.
>>>
>>> However, I suspect this may not be *too* bad for two reasons:
>>>
>>> a) As I mentioned above, I'm hoping there aren't *too* many handles
>>> between different analyses. But I've not done a careful examination, so we
>>> can check this.
>>>
>>> b) For many analyses that might trigger this, I think we have a simpler
>>> option. If the analysis is *immutable* for any reason -- that is, it
>>> overrides its invalidate routine to always return "false" the way
>>> TargetLibraryInfo should (although I'm not sure it does currently), we
>>> shouldn't need to do this as it shouldn't be getting cleared out. Does this
>>> make sense? Do others see anything I'm missing with that approach?
>>>
>>> 2. The AnalysisManager must do a somewhat complicated dance to track
>>>> when analyses call back into it in order to get other analyses.
>>>>
>>>
>>> I would really rather avoid this, as currently the analysis manager's
>>> logic here is very simple, and in many cases we only need the analyses to
>>> *compute* our result, not to embed it. I'm tihnking of stuff like
>>> Dominators is used to build LoopInfo, but there isn't a stale handle there.
>>>
>>>
>>>
>>> There is another aspect of course in that if something is preserving
>>> LoopInfo, it really should be preserving Dominators too...
>>>
>>
>>
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