[llvm] r258184 - [SCEV] Fix PR26207

[Hans Wennborg via llvm-commits]

On Tue, Jan 19, 2016 at 12:53 PM, Sanjoy Das via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: sanjoy
> Date: Tue Jan 19 14:53:51 2016
> New Revision: 258184
>
> URL: http://llvm.org/viewvc/llvm-project?rev=258184&view=rev
> Log:
> [SCEV] Fix PR26207
>
> In some cases, the max backedge taken count can be more conservative
> than the exact backedge taken count (for instance, because
> ScalarEvolution::getRange is not control-flow sensitive whereas
> computeExitLimitFromICmp can be).  In these cases,
> computeExitLimitFromCond (specifically the bit that deals with `and` and
> `or` instructions) can create an ExitLimit instance with a
> `SCEVCouldNotCompute` max backedge count expression, but a computable
> exact backedge count expression.  This violates an implicit SCEV
> assumption: a computable exact BE count should imply a computable max BE
> count.
>
> This change
>
>  - Makes the above implicit invariant explicit by adding an assert to
>    ExitLimit's constructor
>
>  - Changes `computeExitLimitFromCond` to be more robust around
>    conservative max backedge counts

Is this something that should be merged to 3.8?