[polly] r238905 - Translate power-of-two floor-division into ashr

[David Majnemer]

On Wed, Jun 3, 2015 at 7:56 AM, Tobias Grosser <tobias at grosser.es> wrote:

> On 06/03/2015 09:58 AM, David Majnemer wrote:
>
>>
>>
>> On Tue, Jun 2, 2015 at 11:31 PM, Tobias Grosser <tobias at grosser.es
>> <mailto:tobias at grosser.es>> wrote:
>>
>>     Author: grosser
>>     Date: Wed Jun  3 01:31:30 2015
>>     New Revision: 238905
>>
>>     URL: http://llvm.org/viewvc/llvm-project?rev=238905&view=rev
>>     Log:
>>     Translate power-of-two floor-division into ashr
>>
>>     Power-of-two floor divisions can be translated into an arithmetic
>> shift
>>     operation. This allows us to replace a complex lowering that
>>     requires division
>>     operations:
>>
>>
>> This only holds if the numerator is non-negative.  -1/INT_MIN should
>> give 0 but calculating it with a right-shift would give -1.
>>
>
> Your example -1/INT_MIN is uses negative operands in both the numerator
> and the denominator. This it indeed failed with my code. However, the case
> of negative numerators with positive denominators should be fine, as we are
> translating here a division with rounding towards -inf, not rounding
> towards zero as the sdiv instruction in LLVM uses. Or did I miss something
> here.
>

Here is an example using a negative numerator and a positive denominator:
%r = sdiv i32 2550202368, 1073741824 => -1

Here is what shifting produces:
%r = ashr i32 2550202368, 30 => -2


>
> I know exclude negative values 'b' from this optimization and also fixed
> the dyn_cast issue in 238927.
>

Thanks!


>
> Best,
> Tobias
>
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