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LI76635: INEFFICIENT REGISTER MOVE USING XVCPSGND

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APAR status

  • Closed as program error.

Error description

  • When analyzing listing files that contain "xvcpsgnd"
    instructions, the compiler should be generating more efficient
    code for the that instruction.
    
    
    Compiler uses register moves xvcpsgndp. But instead, it can
    make use of the xxlor and xxpermdi instructions.
    xxlor only takes 2c (can run only on pipe 0).
    xxpermdi takes 3c (can run only on pipe 1).
    xvcpsgnd always takes 7c and can run on both pipes.
    ==> A register move with with xxlor is much faster.
    ==> 2 register moves can be implemented with xxlor + xxpermdi
    
    Moving a register pair can be required for duplicating
    _Decimal128 values.
    
    So, the following instructions:
    1435| 000044 xvcpsgnd F0021780   1     LRVS      vs0=vs2
    1435| 000048 xvcpsgnd F0231F80   1     LRVS      vs1=vs3
    
    can be replaced with:
    
    xxlor vs0,vs2,vs2
    xxpermdi vs1,vs3,vs3,0
    

Local fix

  • n/a
    

Problem summary

  • Users Affected:
    Users who use _Decimal128 values which internally makes use of
    xvcpsgnd can be potentially affected by this issue.
    
    Problem Description:
    The compiler inefficiently creates xvcpsgnd instructions which
    can potentially be replaced with xxlor and xxpermdi
    instructions. However, this is not always the case, as it
    depends on which architecture the final exe is being executed
    on.
    

Problem conclusion

  • Replacing xvcpsgnd with xxlor and xxpermdi is beneficial on
    PWR7. As a result, the compiler will now recognize such
    circumstances under the -qtune=pwr7 option and will replace
    xvcpsgnd with xxlor and xxpermdi.
    

Temporary fix

Comments

APAR Information

  • APAR number

    LI76635

  • Reported component name

    XL C/C++ FOR LI

  • Reported component ID

    5724X1400

  • Reported release

    B10

  • Status

    CLOSED PER

  • PE

    NoPE

  • HIPER

    NoHIPER

  • Special Attention

    NoSpecatt / Xsystem

  • Submitted date

    2012-02-06

  • Closed date

    2012-02-06

  • Last modified date

    2012-02-06

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

    IV12845

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

Fix information

  • Fixed component name

    XL C/C++ FOR LI

  • Fixed component ID

    5724X1400

Applicable component levels



Document information

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

Software version: B10

Reference #: LI76635

Modified date: 06 February 2012