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LI77357: INCORRECT LOOP COUNTER WITH -QIPA

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You can track all active APARs for this component.

APAR status

  • Closed as program error.

Error description

  • The counter in the for loop goes past its boundaries when
    compiling with -qipa and causes an incorrect output at runtime.
    
    The issue goes away if you remove -qipa from the compile line.
    
    
    ===== COMPILE COMMAND:
    xlC_r -o tc test.c -O -q64 -qipa
    
    ===== TESTCASE:
    $ cat test.c
    #include <stdio.h>
    
    int main()
    {
            unsigned int ctr=6;
            unsigned int stop =  (unsigned int)-1;
            unsigned int start =  5;
    
            printf("start=%u  ; stop=%u\n",start,stop);
    
            for(ctr = start ; ctr != stop ; --ctr)
            {
                    printf("ctr=%u\n",ctr);
    
                    if (ctr>start)
                    {
                            printf("Shouldn't get here\n");
                            exit(1);
                    }
            }
    }
    $
    
    ===== ACTUAL OUTPUT:
    $ ./tc
    start=5  ; stop=4294967295
    ctr=5
    ctr=4
    ctr=3
    ctr=2
    ctr=1
    ctr=0
    ctr=4294967295
    Shouldn't get here
    $
    
    ===== EXPECTED OUTPUT:
    $ ./tc
    start=5  ; stop=4294967295
    ctr=5
    ctr=4
    ctr=3
    ctr=2
    ctr=1
    ctr=0
    $
    

Local fix

  • Compile with -qstrict_induction. Although, you may need to
    monitor the runtime performance when using this option.
    

Problem summary

  • USERS AFFECTED:
    Users using -qhot=level=0 or above may be affected by this
    issue.
    
    PROBLEM DESCRIPTION:
    When -qhot=level=0 or higher is used, users may encounter
    incorrect output due to an internal induction variable
    overflow.
    

Problem conclusion

  • The fix consists of detecting a loop normalization opportunity
    where the induction variable overflows; so that users do not
    have to face a performance impact of using -qstrict_induction.
    

Temporary fix

Comments

APAR Information

  • APAR number

    LI77357

  • Reported component name

    XL C/C++ FOR LI

  • Reported component ID

    5725C7300

  • Reported release

    C10

  • Status

    CLOSED PER

  • PE

    NoPE

  • HIPER

    NoHIPER

  • Special Attention

    NoSpecatt

  • Submitted date

    2013-04-27

  • Closed date

    2013-04-27

  • Last modified date

    2013-04-27

  • APAR is sysrouted FROM one or more of the following:

    IV33243

  • APAR is sysrouted TO one or more of the following:

Fix information

  • Fixed component name

    XL C/C++ FOR LI

  • Fixed component ID

    5725C7300

Applicable component levels



Document information

More support for: XL C/C++ for Linux
Compiler

Software version: 12.1

Reference #: LI77357

Modified date: 27 April 2013