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For loop Error by Lenram


Status: 

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 as Fixed Help for as Fixed


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Type: Bug
ID: 729818
Opened: 3/8/2012 12:58:28 PM
Access Restriction: Public
Moderator Decision: Sent to Engineering Team for consideration
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Description

The following piece of "C" language code suppose to print K = 0 Ctr = 5
But its printing K = 0 C = 1. This is happening in "Release" configuration. Its printing correctly in Debug mode.

=====================
int k = 0;
int Ctr = 0;

            for (k = 0; k < 5; k++)
            {

                if(k == 0 )
                {
                    printf("K = 0");                    

                }
                Ctr++;                                
            }
                        printf(" Ctr = %d \n",iCtr );


==================

Note: - I tested in VS 2008. I hope it will happen in 2010 too.
Details
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Posted by Microsoft on 3/22/2012 at 2:36 PM
Hi, thanks for this report. This is a bug in the optimizer, and will be fixed in the release of VS11. If you are curious, here are the details.

There is an optimization called if-unswitching that turns:

for (i=L; i<U; i++) {
if (i == 3)
     // do something
}

Into this (removing the for-loop entirely):

if (L <= 3 < U) {
i=3;
// do something
}

The bug is that we were not considering uses of 'i' beyond the loop body when performing if-unswitching. This will be fixed in VS11.

Thanks!
Eric
Microsoft VC++ Team
Posted by MS-Moderator09 [Feedback Moderator] on 3/8/2012 at 10:01 PM
Thanks for your feedback. We are routing this issue to the appropriate group within the Visual Studio Product Team for triage and resolution. These specialized experts will follow-up with your issue.Thank you !
Posted by MS-Moderator01 on 3/8/2012 at 1:58 PM
Thank you for your feedback, we are currently reviewing the issue you have submitted. If this issue is urgent, please contact support directly(http://support.microsoft.com)
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