[llvm] r222093 - ScalarEvolution: HowFarToZero was wrongly using signed division

[David Majnemer]

Alright, I'll fix this.  Sorry for the breakage!

On Sun, Nov 16, 2014 at 8:28 AM, Tobias Grosser <tobias at grosser.es> wrote:

> On 16.11.2014 17:02, Sebastian Pop wrote:
>
>> On Sun, Nov 16, 2014 at 4:19 AM, Tobias Grosser <tobias at grosser.es>
>> wrote:
>>
>>> On 16.11.2014 08:30, David Majnemer wrote:
>>>
>>>>
>>>> Author: majnemer
>>>> Date: Sun Nov 16 01:30:35 2014
>>>> New Revision: 222093
>>>>
>>>> URL: http://llvm.org/viewvc/llvm-project?rev=222093&view=rev
>>>> Log:
>>>> ScalarEvolution: HowFarToZero was wrongly using signed division
>>>>
>>>> HowFarToZero was supposed to use unsigned division in order to calculate
>>>> the backedge taken count.  However, SCEVDivision::divide performs signed
>>>> division.  Unless I am mistaken, no users of SCEVDivision actually want
>>>> signed arithmetic: switch to udiv and urem.
>>>>
>>>
>> findArrayDimensionsRec requires signed division in case Step is negative:
>>
>>      // Normalize the terms before the next call to
>> findArrayDimensionsRec.
>>      const SCEV *Q, *R;
>>      SCEVDivision::divide(SE, Term, Step, &Q, &R);
>>
>>
>>>> This fixes PR21578.
>>>>
>>>
>>>
>>> This commit breaks a delinearization test case in polly.
>>>
>>>
>> Tobi, could you say which testcase broke?
>>
> FAIL: Polly :: Isl/Ast/simple-run-time-condition.ll (160 of 391)
>
>
> ; CHECK: ({{(q == 100 && o <= 0|o <= 0 && q == 100)}})
>          ^
> <stdin>:8:5: note: scanning from here
> if ((o >= 1 && q == 0) || (o <= 0 && q == -2305843009213693852) ? 1 : 0)
>
>  I copied Sebastian who originally wrote the divide operation exactly as
>>> part
>>> of our delinearization analysis. He is possibly the best person to
>>> comment
>>> on this.
>>>
>>>
>> Most likely we will need to duplicate the code of SCEVDivision into a
>> SCEVUDivide.
>> The code that reused SCEVDivide contains this comment that expects
>> SCEVDivision
>> to perform an unsigned division:
>>
>
> This is my feeling as well.
>
> Cheers,
> Tobias
>
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