Re: [RESEND][PATCH] __isolate_lru_page:skip unneeded "not"

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 3 Apr 2010 06:25:08 +0800
Bob Liu <lliubbo@gmail.com> wrote:

> >> -	/*
> >> -	 * When checking the active state, we need to be sure we are
> >> -	 * dealing with comparible boolean values.  Take the logical not
> >> -	 * of each.
> >> -	 */
> >
> > You deleted a spelling mistake too!
> >
> >> -	if (mode != ISOLATE_BOTH && (!PageActive(page) != !mode))
> >> -		return ret;
> >> -
> >> -	if (mode != ISOLATE_BOTH && page_is_file_cache(page) != file)
> >> -		return ret;
> >> +	if (mode != ISOLATE_BOTH) {
> >> +		if ((PageActive(page) != mode) ||
> >> +			(page_is_file_cache(page) != file))
> >> +				return ret;
> >> +	}
> >
> > The compiler should be able to avoid testing for ISOLATE_BOTH twice,
> 
> Thanks for your kindly reply.
> then is the two "not" able to avoid by the compiler ?
> if yes, this patch is meanless and should be ignore.

I very much doubt if the compiler knows that these two variables can
only ever have values 0 or 1, so I expect that removing the !'s will
reduce text size.

That being said, it wouldn't be a good and maintainable change - 
one point in using enumerations such as ISOLATE_* is to hide their real
values.  Adding code which implicitly "knows" that a particular
enumerated identifier has a particular underlying value is rather
grubby and fragile.

But the code's already doing that.

It's also a bit fragile to assume that a true/false-returning C
function (PageActive) will always return 0 or 1.  It's a common C idiom
for such functions to return 0 or non-zero (not necessarily 1).


So a clean and maintainable implementation of

	if (mode != ISOLATE_BOTH && (!PageActive(page) != !mode))
		return ret;

would be

	if (mode != ISOLATE_BOTH &&
			((PageActive(page) && mode == ISOLATE_ACTIVE) ||
			 (!PageActive(page) && mode == ISOLATE_INACTIVE)))
		return ret;

which is just dying for an optimisation trick such as the one which is
already there ;)


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