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

Gerald S. Williams gsw@agere.com
Thu, 17 Oct 2002 18:03:39 -0400

I wrote:

But I said I was done tweaking for now. :-)

Well, I was done tweaking, but then I decided to try out one more performance tweak. This is in desperate need of commenting, but it's finally faster than checking via division on my machine:

#define FULL_BITS (sizeof(size_t) * 8U) #define TOP_BIT (((size_t)1) << ((FULL_BITS)-1)) #define HALF_BITS (sizeof(size_t) * 4U) #define MID_BIT (((size_t)1) << ((HALF_BITS)-1)) #define LO_MASK ((((size_t)1) << (HALF_BITS))-1) #define HI_MASK (~(LO_MASK))

#define Q_BITS (sizeof(size_t) * 2U) #define TQ_BITS (sizeof(size_t) * 6U) #define TQ_BIT (((size_t)1) << ((TQ_BITS)-1)) #define LTQ_MASK ((((size_t)1) << (TQ_BITS))-1) #define TQ_MASK (~(LTQ_MASK))

#define SAFE_MULTIPLY(dest,src1,src2,on_overflow)
{
size_t _x = src1;
size_t _y = src2;
size_t _dest = _x * _y;

dest = _dest;

if ((_x | _y) & HI_MASK)
{
if (_safe_multiply_check_for_overflow(_x,_y,_dest))
{
on_overflow;
}
}
}

int _safe_multiply_check_for_overflow( size_t h, size_t l, size_t dest) { if (l > h) { size_t temp = l; l = h; h = temp; }

if ((h & HI_MASK) && (l))
{
    size_t lgt;
    size_t leq;

    if (l & HI_MASK)
    {
        lgt = (size_t)(-1);
        leq = 0;
    }
    else
    {
        size_t hbit;
        size_t mask;

        if (h & TQ_MASK)
        {
            for ((mask=TOP_BIT),(hbit=0);!(mask&h);mask>>=1)
            {
                ++hbit;
            }
        }
        else
        {
            for ((mask=TQ_BIT),(hbit=Q_BITS);!(mask&h);mask>>=1)
            {
                ++hbit;
            }
        }
        leq = 1 << hbit;
        mask = leq - 1;
        lgt = ~mask ^ leq;
    }
    if ((l & lgt) || ((l & leq) && !(dest & TOP_BIT)))
    {
        return 1;
    }
}

return 0;

}

-Jerry