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

[Hal Finkel via llvm-commits]

----- Original Message -----
> From: "Daniel Berlin" <dberlin at dberlin.org>
> To: reviews+D23432+public+5838e63f935aeb78 at reviews.llvm.org, mkuper at google.com, davidxl at google.com
> Cc: hfinkel at anl.gov, llvm-commits at lists.llvm.org
> Sent: Friday, August 12, 2016 5:50:52 PM
> Subject: Re: [PATCH] D23432: [AliasSetTracker] Degrade AliasSetTracker results when may-alias sets get too large.
> 
> 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.
> 

Right. You'd need to partition based on UO and then merge the AA metadata (you might be able to partition based on the AA metadata too, but not using the pair-wise AA interface -- you'd need something that understood the metadata semantics and could partition in bulk).

> 
> However consensus was long term to use memssa or something else,
> rather than try to devise partitioning for a fundamentally n^2
> representation
> 

Okay. I can easily see how LICM would use MemorySSA. I suppose you can also partition in O(N) time using MemorySSA by walking the use/def graph to form equivalence classes (or maybe O(NlnN) in the presence of MemorySSA PHIs)?

 -Hal

> 
> 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
> 
> 
> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory