[llvm] r258184 - [SCEV] Fix PR26207

[Andrew Trick via llvm-commits]

This fix looks safe to me too.

Thanks Sanjoy!

-Andy

> On Jan 19, 2016, at 3:26 PM, Sanjoy Das <sanjoy at playingwithpointers.com> wrote:
> 
> +CC Andy
> 
> I'd wait for Andy to chime in, I think this is okay to merge into 3.8
> provided no problems are seen over the next 1-2 days.
> 
> 
> 
> On Tue, Jan 19, 2016 at 2:34 PM, Hans Wennborg <hans at chromium.org <mailto:hans at chromium.org>> wrote:
>> 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?
> 
> 
> 
> -- 
> Sanjoy Das
> http://playingwithpointers.com <http://playingwithpointers.com/>
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