Re: linux/kernel.h (&_x == &_y); why

[Jan Hudec <bulb@ucw.cz>]

On Tue, Dec 02, 2003 at 15:12:52 -0800, John Tyner wrote:
> I believe that it is so that gcc will throw a warning if the two types 
> are not compatible.

Actualy, it should throw a warning if they are not exactly the same,
because if they are not, the comparsion breaks aliasing rules.
Especially it should throw a warning if one is signed and the other is
not.

> John
> 
> On Tue, Dec 02, 2003 at 03:32:57PM -0700, Pat LaVarre wrote:
> > Anybody here understand the '"unnecessary" pointer comparison' of the
> > min/max of linux/kernel.h?
> > 
> > Are we trusting gcc to disregard that sourceline?  If yes then why
> > bother?  Why not omit that sourceline?
> > 
> > Pat LaVarre
> > 
> > ---
> > 
> > http://lwn.net/Articles/driver-porting/
> > 
> > Miscellaneous changes ...
> > http://lwn.net/Articles/22196/
> > 
> > ...
> > The min() and max() macros
> > 
> > A common step in the development of most C programmers, shortly after
> > they learn to include stdio.h seems to be the definition of macros like:
> > 
> >     #define max(a,b) ((a) > (b) ? (a) : (b))
> > 
> > In 2.5, it was noted that a number of kernel developers had seemingly
> > not moved beyond that stage, and there was an unbelievable number of
> > min() and max() definitions sprinkled throughout the kernel source.
> > These definitions were not only redundant - they were unsafe. A max()
> > macro as defined above has the usual problem with side effects; it also
> > is not type-safe, especially when signed and unsigned values are used. 
> > 
> > Linus initially added his own definition of min() and max() which added
> > a third argument - an explicit type. That change upset people badly
> > enough that some put substantial amounts of time into developing
> > two-argument versions that are type and side-effect safe. The result now
> > lives in <linux/kernel.h>, and should be used in preference to any
> > locally-defined version.
> > ...
> > 
> > http://lxr.linux.no/source/include/linux/kernel.h?v=2.6.0-test7#L157
> > 
> > ...
> > 157 /*
> > 158  * min()/max() macros that also do
> > 159  * strict type-checking.. See the
> > 160  * "unnecessary" pointer comparison.
> > 161  */
> > 162 #define min(x,y) ({ \
> > 163         const typeof(x) _x = (x);       \
> > 164         const typeof(y) _y = (y);       \
> > 165         (void) (&_x == &_y);            \
> > 166         _x < _y ? _x : _y; })
> > 167 
> > 168 #define max(x,y) ({ \
> > 169         const typeof(x) _x = (x);       \
> > 170         const typeof(y) _y = (y);       \
> > 171         (void) (&_x == &_y);            \
> > 172         _x > _y ? _x : _y; })
> > ...
> > 
> > 
> > --
> > Kernelnewbies: Help each other learn about the Linux kernel.
> > Archive:       http://mail.nl.linux.org/kernelnewbies/
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> > 
> --
> Kernelnewbies: Help each other learn about the Linux kernel.
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> 
-------------------------------------------------------------------------------
						 Jan 'Bulb' Hudec <bulb@ucw.cz>
--
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