On Thu, 30 Aug 2012 17:10:47 -0700 Guenter Roeck <li...@roeck-us.net> wrote:
> DIV_ROUND_CLOSEST returns a bad result for dividends with different sign:
> DIV_ROUND_CLOSEST(-2, 2) = 0
>
> Most of the time this does not matter. However, in the hardware monitoring
> subsystem, DIV_ROUND_CLOSEST is sometimes used on integers which can be
> negative (such as temperatures).
>
> ...
>
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -84,8 +84,11 @@
> )
> #define DIV_ROUND_CLOSEST(x, divisor)( \
> { \
> - typeof(divisor) __divisor = divisor; \
> - (((x) + ((__divisor) / 2)) / (__divisor)); \
> + typeof(x) __x = x; \
> + typeof(divisor) __d = divisor; \
> + ((__x) < 0) == ((__d) < 0) ? \
> + (((__x) + ((__d) / 2)) / (__d)) : \
> + (((__x) - ((__d) / 2)) / (__d)); \
> } \
> )
Your v2 had that sneaky little "(typeof(x))-1 >= 0" trick in it, so
half the code gets elided at compile time if `x' (why isn't this called
"dividend") has an unsigned type.
Would retaining that be of any benefit? We do want to avoid doing the
compare-and-branch in as many cases as possible.
Also, this would be a great opportunity to document the macro's beahviour
(I do go on). That would be a useful thing to do, given that we're now
handling the four +/+, +/-, -/+, -/- cases and the behaviour for each
case isn't terribly obvious.
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