[Python-Dev] Re: Memory size overflows (original) (raw)

Gerald S. Williams gsw@agere.com
Tue, 22 Oct 2002 15:48:48 -0400

I didn't find any way to improve the actual overflow check, although if you entirely replace the "fast path" check with checks involving unsigned masking, you get some performance improvement. For a wide variety of input patterns, I get about an 18% speedup versus the core long multiply code, when modified as shown below:

#define UL_LO_HI_BIT (((unsigned long)1) << (sizeof(unsigned long) * 4U)) #define UL_LO_MASK ((UL_LO_HI_BIT) - 1) #define UL_HI_MASK ((UL_LO_MASK)) #define UL_HI_LO_BIT (((unsigned long)1) << ((sizeof(unsigned long) * 4U)-1)) #define UL_OVERFLOW_IMPOSSIBLE_MASK ((UL_HI_LO_BIT) - 1) #define UL_OVERFLOW_POSSIBLE_MASK ((UL_OVERFLOW_IMPOSSIBLE_MASK))

long core_int_mul(long a, long b) { long longprod = a * b; unsigned long ma = a & UL_HI_MASK;

if (ma == ((a < 0) ? UL_HI_MASK : 0))
{
    unsigned long mb = b & UL_OVERFLOW_POSSIBLE_MASK;

    if (mb == ((b < 0) ? UL_OVERFLOW_POSSIBLE_MASK : 0))
    {
        return longprod;
    }
}
{
    double doubleprod = (double)a * (double)b;
    double doubled_longprod = (double)longprod;
    double diff = doubled_longprod - doubleprod;
    double absdiff = (diff >= 0.0) ? diff : -diff;
    double absprod = (doubleprod >= 0.0) ? doubleprod : -doubleprod;

    /* absdiff/absprod <= 1/32 iff
       32 * absdiff <= absprod -- 5 good bits is "close enough" */
    if (32.0 * absdiff <= absprod)
    {
        return longprod;
    }
    else
    {
        SIGNAL_AN_ERROR;
    }
}

}

This version suffers no apparent degradation versus the existing implementation when fed sets of multiplicands evenly distributed over range(-sys.maxint, sys.maxint), and almost always shows an improvement.

Shall I submit a patch?

-Jerry