Re: [IDEA+RFC] Possible solution for min()/max() war

Re: [IDEA+RFC] Possible solution for min()/max() war

[Herbert Rosmanith]

> From: Helge Hafting <helgehaf@idb.hist.no>
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> To: Brad Chapman <kakadu_croc@yahoo.com>, linux-kernel@vger.kernel.org
> Subject: Re: [IDEA+RFC] Possible solution for min()/max() war
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> 
> Brad Chapman wrote:
> > 
> > Then why, IIRC, are the kernel hackers so upset about how
> > the three-arg macros _prevent_ signed/unsigned comparisons?
> > Apparently the ability to compare
> > signed/unsigned variables must have _some_ significance....
> 
> You are misunderstanding this.
> It is possible to write something that _looks like_ a signed to unsigned
> comparison, i.e.:
> int a;
> unsigned int b;
> ...
> x = min(a,b);
> 
> Looks like a signed to unsigned comparison - but it isn't!
> 
> You see, the compiler always does an _implicit_ cast in cases like this.
                             ^^^^^^^^^^^^^^^^^^^^^

if that's your concern, we can easily fix this by still moving min/max
to its same form.


#define type_min(type,x,y) \
        ({ type __x = (x), __y = (y); __x < __y ? __x: __y; })
#define type_max(type,x,y) \
        ({ type __x = (x), __y = (y); __x > __y ? __x: __y; })

#define min(x,y) type_min(typeof(x),x,y)
#define max(x,y) type_max(typeof(x),x,y)

no _implicit_ cast and ...

> One of the arguments gets changed invisibly, and that is what kernel
> developers are so upset about.  You don't really know which one without
> thinking hard about it, and that is a source of many hard-to-find bugs.

... joy, we would even know which one.


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mfg,
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Re: [IDEA+RFC] Possible solution for min()/max() war -- I like this

[Christopher Friesen]

Herbert Rosmanith wrote:

> #define type_min(type,x,y) \
>         ({ type __x = (x), __y = (y); __x < __y ? __x: __y; })
> #define type_max(type,x,y) \
>         ({ type __x = (x), __y = (y); __x > __y ? __x: __y; })
> 
> #define min(x,y) type_min(typeof(x),x,y)
> #define max(x,y) type_max(typeof(x),x,y)
> 
> no _implicit_ cast and ...
> 
> > One of the arguments gets changed invisibly, and that is what kernel
> > developers are so upset about.  You don't really know which one without
> > thinking hard about it, and that is a source of many hard-to-find bugs.
> 
> ... joy, we would even know which one.

I think this makes a lot of sense.  The explicit version could be preferred, but
for the implicit version we at least know that what type will be used for it and
it won't immediately break things that are still using the old-style min/max.


-- 
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