Jeff Epler wrote:
> Is this correct for all operands? Consider 4-bit types, with valid
> values 0..15.
> 3*9 == 27, but 27%16 == 11 and 11>max(3, 9)
You're right. It's a case of "fingers quicker than the
brain". There may be something worth investigating in
that type of relationship, but that isn't the answer.
I came up with it as I was responding with a somewhat
more working answer.
What I was originally going to post before my mental
lapse was this:
One solution to what Guido asked for is as follows. It
avoids the division, but I don't think it's worth it
(it doesn't make much difference on my platform):
#define FULL_BITS (sizeof(size_t) * 8U)
#define TOP_BIT (((size_t)1) << ((FULL_BITS)-1))
#define HALF_BITS (sizeof(size_t) * 4U)
#define LO_MASK ((((size_t)1) << (HALF_BITS))-1)
#define HI_MASK (~(LO_MASK))
#define SAFE_MULTIPLY(dest,src1,src2,on_error)\
{\
size_t _x = src1;\
size_t _y = src2;\
\
dest = _x * _y;\
\
if ((_x|_y) & HI_MASK)\
{\
size_t h;\
size_t l;\
size_t mask;\
size_t low_bit;\
size_t low_bit_mask;\
\
if (_x >= _y)\
{\
h = _x;\
l = _y;\
}\
else\
{\
h = _y;\
l = _x;\
}\
\
for (low_bit=0,mask=TOP_BIT; mask & HI_MASK; ++low_bit,(mask>>=1))\
{\
if (h & mask)\
{\
break;\
}\
}\
\
low_bit_mask = (size_t)1 << low_bit;\
\
do\
{\
if (l & mask)\
{\
on_error;\
}\
mask >>= 1;\
}\
while (!(mask & low_bit_mask));\
}\
}
Well, at least it demonstrates how to avoid multiply-invoking
its parameters. :-)
-Jerry
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