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

[David Majnemer]

This should be fixed in r222104.

On Sun, Nov 16, 2014 at 11:51 AM, David Majnemer <david.majnemer at gmail.com>
wrote:

> 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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