V3 PATCH: numeric_limits<> support, fix PR/3865

[Nathan Sidwell]

Richard Henderson wrote:

>   #define digits10_for_int_type(TYPE)                           \
>     (sizeof (TYPE) * __CHAR_BIT__ == 8 ? 2                      \
>      : sizeof (TYPE) * __CHAR_BIT__ == 16 ? 4                   \
>      : sizeof (TYPE) * __CHAR_BIT__ == 32 ? 9                   \
>      : sizeof (TYPE) * __CHAR_BIT__ == 64 ? 18 + ((TYPE)-1 > 0) \
>      : /* error */ 0)
> 
>   static const int digits10 = digits10_for_int_type (wchar_t);
surely something based on the following, culled from elsewhere
would be more concise?
	// ln (2)/ln (10) * 2^12 == 1233.0188
	#define DECIMALFORBITS(n) (((n) * 1233 + 4095) >> 12)
	#define DECIMALINBITS(n) (((n) * 1233) >> 12)
leads to
#define digits10_for_int_type(TYPE) \
	(((sizeof (TYPE) * __CHAR_BIT__ - ((TYPE)-1 < 0)) * 1233) >> 12)

nathan
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