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From: "arshamidi at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/59845] New: loop optimization problem when profiling
Date: Thu, 16 Jan 2014 18:12:00 -0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c++
X-Bugzilla-Version: 4.8.1
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59845

            Bug ID: 59845
           Summary: loop optimization problem when profiling
           Product: gcc
           Version: 4.8.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: arshamidi at gmail dot com

I'm using g++ with GCC 4.8.1; when I use fprofile-generate too optimize my
code, it actually slows down the process time: (constant folding problem within
loop?)

int main(){

    int n;
    int i;
    //int n2;
    printf("Enter a number: ");
    scanf("%d", &n);
    //n2 = n/2;
    printf("divisors are:\n\n");

    for (i=1; i <= /*n2*/ n/2; i++)
        if (n%i == 0)
            printf("\n%d", i);

    printf("\n\n end.\n");

    return 0;
}

Here I must use the constant 'n2' (commented out) too actually get the desired
speed -in combination with -fprofile-generate. 
I'm using the following switches:
-Ofast;-fomit-frame-pointer;-march=core2; -mfpmath=sse; -fprofile-generate;
-Wall

no problem without -fprofile-generate and -fprofile-use