tree 233344fd2ee7c641151183498d5731c1c617a15d
parent b93a52f5fec53c51628fa923f8a573f9f6ce2c98
author Philip Reames <listmail@philipreames.com> 1571614682 +0000
committer Philip Reames <listmail@philipreames.com> 1571614682 +0000

[IndVars] Eliminate loop exits with equivalent exit counts

We can end up with two loop exits whose exit counts are equivalent, but whose textual representation is different and non-obvious. For the sub-case where we have a series of exits which dominate one another (common), eliminate any exits which would iterate *after* a previous exit on the exiting iteration.

As noted in the TODO being removed, I'd always thought this was a good idea, but I've now seen this in a real workload as well.

Interestingly, in review, Nikita pointed out there's let another oppurtunity to leverage SCEV's reasoning.  If we kept track of the min of dominanting exits so far, we could discharge exits with EC >= MDE.  This is less powerful than the existing transform (since later exits aren't considered), but potentially more powerful for any case where SCEV can prove a >= b, but neither a == b or a > b.  I don't have an example to illustrate that oppurtunity, but won't be suprised if we find one and return to handle that case as well.  

Differential Revision: https://reviews.llvm.org/D69009


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375379 91177308-0d34-0410-b5e6-96231b3b80d8