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InterlockedExchangeAdd equivalent

Doug Price
Note: This was originally posted on 11th February 2013 at http://forums.arm.com

I'm porting a Windows application to an ARM® Cortex-A9 based system running Ubuntu 12.04 and I need something equivalent the Windows function InterlockedExchangeAdd().  Is there a function like this available for ARM?  If not, can anyone provide some equivalent assembly code?
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  • Note: This was originally posted on 11th February 2013 at http://forums.arm.com

    Thanks so much for your help.  For anyone else that may be interested, here's what I came up with.

    long myExchangeAdd( volatile long *pTarget, long nValue )
    {
    #if defined( _WIN32 )
    return InterlockedExchangeAdd( pTarget, nValue );
    #else
    int nResult, nModified;

    asm volatile
    (
         "1:"                               "\t@ loop label"   "\n\t"
         "LDREX %0,[%2]"        "\t@ load value"  "\n\t"
         "ADD   %0,%0,%3"   "\t@ increment it"   "\n\t"
         "STREX %1,%0,[%2]" "\t@ attempt store" "\n\t"
         "CMP   %1,#0"           "\t@ if not atomic"  "\n\t"
         "BNE   1b"                   "\t@ then repeat"   "\n\t"
         : "=&r" (nResult), "=&r" (nModified)
         : "r" (pTarget), "r" (nValue)
         : "cc", "memory"
    );

    return nResult;
    #endif
    }
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  • Note: This was originally posted on 11th February 2013 at http://forums.arm.com

    Thanks so much for your help.  For anyone else that may be interested, here's what I came up with.

    long myExchangeAdd( volatile long *pTarget, long nValue )
    {
    #if defined( _WIN32 )
    return InterlockedExchangeAdd( pTarget, nValue );
    #else
    int nResult, nModified;

    asm volatile
    (
         "1:"                               "\t@ loop label"   "\n\t"
         "LDREX %0,[%2]"        "\t@ load value"  "\n\t"
         "ADD   %0,%0,%3"   "\t@ increment it"   "\n\t"
         "STREX %1,%0,[%2]" "\t@ attempt store" "\n\t"
         "CMP   %1,#0"           "\t@ if not atomic"  "\n\t"
         "BNE   1b"                   "\t@ then repeat"   "\n\t"
         : "=&r" (nResult), "=&r" (nModified)
         : "r" (pTarget), "r" (nValue)
         : "cc", "memory"
    );

    return nResult;
    #endif
    }
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