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

[Daniel Berlin via llvm-commits]

On Thu, Aug 11, 2016 at 3:44 PM, Michael Kuperstein <mkuper at google.com>
wrote:

> mkuper created this revision.
> mkuper added reviewers: davidxl, dberlin.
> mkuper added a subscriber: llvm-commits.
>
> The cause of PR28832 is the quadratic behavior of repeated inserts into
> AliasSetTracker - inserting a pointer into AST is linear, since it requires
> walking over all "may" alias sets, and running an alias check vs. each
> pointer in such a set.
> This patch tracks (more or less) the total number of pointers in all "may"
> sets, and when that number exceeds a threshold, declares the tracker
> "saturated" - and lumps all pointers into a single "may" set.
>
> For the example in PR28832, without this patch, I have:
>    ---User Time---   --System Time--   --User+System--   ---Wall Time---
> --- Name ---
>   43.4251 ( 47.4%)   0.0440 ( 33.2%)  43.4691 ( 47.4%)  43.4471 ( 47.4%)
> Loop Invariant Code Motion
>   19.6565 ( 21.5%)   0.0040 (  3.0%)  19.6605 ( 21.5%)  19.6512 ( 21.5%)
> Loop Invariant Code Motion
>   17.5580 ( 19.2%)   0.0040 (  3.0%)  17.5620 ( 19.2%)  17.5534 ( 19.2%)
> Loop Invariant Code Motion
>    3.0498 (  3.3%)   0.0080 (  6.0%)   3.0578 (  3.3%)   3.0563 (  3.3%)
> Value Propagation
>
> And with this patch, with a threshold of 250:
>    ---User Time---   --System Time--   --User+System--   ---Wall Time---
> --- Name ---
>    5.6446 ( 29.1%)   0.0160 (  5.6%)   5.6606 ( 28.8%)   5.6570 ( 28.8%)
> Value Propagation
>    2.2423 ( 11.6%)   0.0040 (  1.4%)   2.2463 ( 11.4%)   2.2448 ( 11.4%)
> Loop Invariant Code Motion
>    1.6510 (  8.5%)   0.0039 (  1.4%)   1.6549 (  8.4%)   1.6526 (  8.4%)
> Loop Invariant Code Motion
>    1.5398 (  8.0%)   0.0040 (  1.4%)   1.5438 (  7.9%)   1.5429 (  7.9%)
> Loop Invariant Code Motion
>
> A threshold of 1000 produces a significant slowdown:
>    ---User Time---   --System Time--   --User+System--   ---Wall Time---
> --- Name ---
>   10.0644 ( 25.7%)   0.0080 (  2.4%)  10.0724 ( 25.5%)  10.0689 ( 25.5%)
> Loop Invariant Code Motion
>    7.5912 ( 19.4%)   0.0159 (  4.8%)   7.6071 ( 19.3%)   7.6063 ( 19.3%)
> Loop Invariant Code Motion
>    7.4471 ( 19.0%)   0.0120 (  3.6%)   7.4591 ( 18.9%)   7.4571 ( 18.9%)
> Loop Invariant Code Motion
>    5.5252 ( 14.1%)   0.0200 (  6.0%)   5.5452 ( 14.1%)   5.5454 ( 14.1%)
> Value Propagation
>
> This isn't in a committable state - it needs some cleanup, more testing,
> and some substantive fixes (per the TODOs).
> So no point in reviewing it in detail yet, but I'd like to get a direction
> check.
>
> Danny, w.r.t to our earlier discussion - I'm not a fan of the idea of
> having a saturated AST be unsafe to query, so this is required anyway.


Yup.

IMHO, you should always just return "may-alias" to any query once saturated
:P.


> We can add isSaturated() checks to LICM to speed it up further, as a
> separate patch.
>
>
> https://reviews.llvm.org/D23432
>
> Files:
>   include/llvm/Analysis/AliasSetTracker.h
>   lib/Analysis/AliasSetTracker.cpp
>   test/Analysis/AliasSet/saturation.ll
>
>
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