[llvm] r258184 - [SCEV] Fix PR26207

[Hans Wennborg via llvm-commits]

Thanks! The commit looks good:
http://llvm.org/viewvc/llvm-project?view=revision&revision=258869
And I verified that the lit tests pass on the branch.

Cheers,
Hans

On Tue, Jan 26, 2016 at 3:21 PM, Sanjoy Das
<sanjoy at playingwithpointers.com> wrote:
> Merged to 3.8 as rL258869.
>
> It would be great if you could please verify that it indeed went through.
>
> -- Sanjoy
>
>
> Hans Wennborg wrote:
>>
>> This has been in a few days now. Sanjoy, do you want to merge this to
>> 3.8 using the script in utils/release/merge.sh (or let me know if
>> you'd prefer me to do it).
>>
>> Thanks,
>> Hans
>>
>> On Tue, Jan 19, 2016 at 3:38 PM, Andrew Trick<atrick at apple.com>  wrote:
>>>
>>> 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>  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
>>>
>>>
>