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Re: [PATCH, PR 40866] decrease the cost of an IV candidate by 1 when the IV is used in a test against zero


On Thu, Feb 11, 2010 at 05:35, Zdenek Dvorak <rakdver@kam.mff.cuni.cz> wrote:
> Hi,
>
>> The patch below fixes PR 40866 : [4.3/4.4/4.5 Regression] No counter
>> reversal for simple loops anymore.
>>
>> Problem:
>> The loop optimizer canonicalizes the loop to a downward counting loop
>> which may save one instruction if
>> the bound test is against zero. However, ivopts do not know that
>> comparing with zero is more efficient
>> than comparing with any other expressions and thus eliminate the
>> reversed IV based on the current cost
>> model.
>>
>> Solution:
>> In this patch, we changed the ivopts cost model to decrease the cost of
>> an IV candidate by 1 when the IV is
>> a test against zero. This idea was initially proposed by Zdenek Dvorak
>> as shown in the comment #3 of the bug
>> report.
>>
>> Bootstrapped and tested on x86-64-linux. OK for trunk?
>
> OK.

Committed to trunk.

Sebastian


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