Re: [PATCH 00/18] lib: bitmap: Various improvements

[Joe Perches]

On Mon, 2014-07-07 at 16:20 -0700, Andrew Morton wrote:
> On Fri,  4 Jul 2014 00:42:46 +0200 Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx> wrote:
> 
> > Many functions in lib/bitmap.c start with an expression such as lim =
> > bits/BITS_PER_LONG. Since bits has type (signed) int, and since gcc
> > cannot know that it is in fact non-negative, it generates worse code
> > than it could. These patches, mostly consisting of changing various
> > parameters to unsigned, gives a slight overall code reduction:
> 
> Yes, we have a bad habit of using signed types for things where
> negative values are absurd.

Linus at one time wrote:
http://thread.gmane.org/gmane.linux.kernel/1526174

---------------------------------------------------------
The fact is, the x86 bitop instructions act on a
signed index. Making the index be "unsigned long" would violate the
actual *behavior* of the function, so it would be singularly stupid.
---------------------------------------------------------

> The patches look OK to me.

Me too.

But this does have the unfortunate effect of allowing
bitmap tests on bitmaps that are unable to be set/cleared
when the bitmap is larger than INT_MAX size.

There could be some check to make sure size <= INT_MAX.

The docs could be clearer here too about the actual
maximum size of a bitmap.


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