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
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