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LI77159: PERFORMANCE DEGRADATION WITH POPCNT ON POWER7

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You can track all active APARs for this component.

APAR status

  • Closed as program error.

Error description

  • A performance degradation is seen with the __popcnt4 and
    __popcnt8 population count functions when POWER7 hardware is
    targeted with the -qarch/-qtune options, and -qhot/IPA
    optimization is used.  Slower code is generated using
    the popcntb instruction instead of the popcntw/popcntd
    instructions.
    

Local fix

  • - Do not use higher -qhot levels, use -qhot=level=0
    

Problem summary

  • PROBLEM DESCRIPTION:
    Performance degradation with popcnt optimization.
    
    USERS AFFECTED:
    Customers who use popcnt instructions on POWER7.
    

Problem conclusion

  • The popcnt optimization on POWER7 has been disabled, since it
    is not profitable on POWER7.
    

Temporary fix

Comments

APAR Information

  • APAR number

    LI77159

  • 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

    2012-12-17

  • Closed date

    2012-12-17

  • Last modified date

    2012-12-17

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

    IV29519

  • 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 #: LI77159

Modified date: 17 December 2012