[Python-Dev] Memory size overflows

2002年10月16日 12:00:06 -0400

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