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LI76945: -O CAUSING INCORRECT OUTPUT

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You can track all active APARs for this component.

APAR status

  • Closed as program error.

Error description

  • When compiling the test case below at -O, the resulting binary
    produces incorrect output at runtime.
    
    ===== COMPILE COMMAND:
    xlc -O -q64 -o di6 test.c
    
    
    ===== TESTCASE:
    $cat test.c
    #include <stdio.h>
    
    static void quorem(unsigned int *ax, unsigned int *bx)
    {
            unsigned int *bxe;
            unsigned long long borrow, carry, ys, y;
    
            borrow = 0;
            carry = 0;
    
            bxe = bx + 2;
            while (bx < bxe)
            {
                    ys = *bx++ * 2ULL + carry;
                    carry = ys >> 32;
                    y = *ax - (ys & 0xFFFFFFFFU) - borrow;
                    borrow = (y >> 32) & 1;
                    *ax++ = y;
            }
    }
    
    int main()
    {
            unsigned int a[] = { 0x75600000U, 0x146D9833U };
            unsigned int b[] = { 0xF0400000U, 0x08E1BC9BU };
            quorem(a, b);
            printf("%08X\n", a[1]); /* should print 02AA1EFB */
            return 0;
    }
    $
    
    
    ===== ACTUAL OUTPUT:
    $xlc -O -q64 -o di6 test.c && ./di6
    02AA1EFC
    $
    
    
    
    ===== EXPECTED OUTPUT:
    $xlc -O -q64 -o di6 test.c && ./di6
    02AA1EFB
    $
    

Local fix

  • n/a
    

Problem summary

  • USERS AFFECTED:
    Users that compile with optimization in 64-bit mode and that
    have the problem pattern may by affected by this issue.
    
    PROBLEM DESCRIPTION:
    The issue is that the compiler incorrectly removes the
    truncation to 32-bit before 64-bit rotate.
    

Problem conclusion

  • This issue has been fixed.
    

Temporary fix

Comments

APAR Information

  • APAR number

    LI76945

  • 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

    2012-08-28

  • Closed date

    2012-08-28

  • Last modified date

    2012-08-28

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

    IV20936

  • 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 #: LI76945

Modified date: 28 August 2012