[PATCH] D23432: [AliasSetTracker] Degrade AliasSetTracker results when may-alias sets get too large.

[Xinliang David Li via llvm-commits]

On Fri, Aug 12, 2016 at 4:06 PM, Michael Kuperstein <mkuper at google.com> wrote:
> We could drop TBAA (I guess we can keep Scope and NoAlias AAMD), and have
> the degraded AST recompute the sets based on GetUnderlyingObject. That is,
> partition based on the UO if it isIdentifiedObject, and have a catch-all set
> for everything else. This will still catch noalias function parameters and a
> few other cases.
>
> But I'm also not sure it's worth the effort, given the eventual move to
> MemorySSA(?)
>

While this sounds very attractive, but devils are in details, so the
actual implementation effort may be larger than what is described --
so IMO it is probably not worth the effort.

David


> Hal, do you think we can live with full degradation?
>
>
> On Fri, Aug 12, 2016 at 3:50 PM, Daniel Berlin <dberlin at dberlin.org> wrote:
>>
>> I suggested this offline on the internal bug as an approach.
>> Unioning (literally generating a fake union that has all the original
>> members pointers) works sanely in all languages but is tricky and ugly in
>> practice (and llvm has no unions anymore).
>> You can't pick representatives sanely any other way if you keep tbaa
>> because it is not transitive. Same with other types of info other than uo.
>> if you keep those, discovering valid subsets requires querying every pair.
>>
>> If you drop tbaa, etc, you can partition.
>>
>> However consensus was long term to use memssa or something else, rather
>> than try to devise partitioning for a fundamentally n^2 representation
>>
>>
>> On Fri, Aug 12, 2016, 3:25 PM Hal Finkel <hfinkel at anl.gov> wrote:
>>>
>>> hfinkel added a subscriber: hfinkel.
>>> hfinkel added a comment.
>>>
>>> There might not be anything we can do in general that is better than
>>> this, but could we degrade to using summaries of some kind instead of N^2
>>> queries. For example, imagine that, in the degraded mode, we represented
>>> each set using the result of GetUnderlyingObject and the merged AA metadata.
>>> Then we queried using <Ptr, Size, AAMD> vs <UO, UnknownSize, MergedAAMD>,
>>> but only once per set, not once per pointer in each set? I'm trying to think
>>> of a way to keep getting the easy cases, such as noalias function
>>> parameters, even if we degrade on the hard things.
>>>
>>>
>>> https://reviews.llvm.org/D23432
>>>
>>>
>>>
>