[PATCH] D66242: [POC][SCEV] Bounds for loop invariant exits

[Philip Reames via Phabricator via llvm-commits]

reames created this revision.
reames added reviewers: nikic, apilipenko, sanjoy.
Herald added subscribers: bollu, mcrosier.
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I noticed in one of the test updates for rL368898 <https://reviews.llvm.org/rL368898> that we hadn't been able to compute a backedge taken count for a loop with a constant exit.  This seems unfortunate, so I took a stab at a solution.

At the moment, this isn't ready to go in, I'm mostly looking for feedback on the idea and approach.  Is this a good idea?  Should I restrict it to only constants?  Should this be done somewhere else?

One point I'm a bit concerned about is whether setting the exact count to an arbitrary unreachable value breaks some assumption.  I can't think of any, but...

If this is the direction we want to take, I'll do the following:

- Autogen a couple of tests which need updated
- Submit an NFC which computes the max BE taken count in a pre-loop to the main one which tracks computable exits.
- Rebase this on top of both of the above for actual review.


Repository:
  rL LLVM

https://reviews.llvm.org/D66242

Files:
  lib/Analysis/ScalarEvolution.cpp
  test/Analysis/ScalarEvolution/finite-exit-count.ll

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