Re: CVS commit: src/sys/kern

[David Laight <david%l8s.co.uk@localhost>]

On Fri, Jan 16, 2009 at 10:58:41AM +0200, Alan Barrett wrote:
> On Fri, 16 Jan 2009, YAMAMOTO Takashi wrote:
> > cache_prune: use (a - b > 0) rather than (a > b) to compare ticks.
> 
> Why?  At first glance, both tests do the same thing.
> 
> Both the variables are signed int, so there could be cases where a-b is
> undefined while a>b is defined.  Is this likely to be a problem, or are
> the actual ranges of the variables such that overflow can't happen?

Probably because thay are actually modulo 2^32 values, so if the
'old' value is 0x7fffffff and the 'new' 0x80000001 you want the
difference of 'new - old' > 0 (2 > 0) being true not 'new > old'
which is false.

Of course you need to use unsigned ints for modulo arithmetic because
C allows i++ to saturate for integers.
(Then you need to cast the result of the subtract to signed ...)

        David

-- 
David Laight: david%l8s.co.uk@localhost