[KJ] [RFC] Regarding min/max

[KJ] [RFC] Regarding min/max

[Richard Knutsson]

Hi

Regarding converting:

	a = x < y ? x : y;

with
	a = min(x, y);
and etc.

What about variables in structs? It seem to not like the multiple lines in min()/max(),
so why not introduce something like:
#define __min(x,y)	((x) < (y) ? (x) : (y))
(and use __min() in min() also)? Then there would be no problem to clean up in the structs
+ the underscores should make people aware it does not type-check.

Just a thought
/Richard Knutsson


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Re: [KJ] [RFC] Regarding min/max

[Randy Dunlap]

On Sat, 03 Feb 2007 02:23:14 +0100 Richard Knutsson wrote:

> Hi
> 
> Regarding converting:
> 
> 	a = x < y ? x : y;
> 
> with
> 	a = min(x, y);
> and etc.
> 
> What about variables in structs? It seem to not like the multiple lines in min()/max(),

Can you give an example of this "not like"?
Thanks.

> so why not introduce something like:
> #define __min(x,y)	((x) < (y) ? (x) : (y))
> (and use __min() in min() also)? Then there would be no problem to clean up in the structs
> + the underscores should make people aware it does not type-check.



---
~Randy
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Re: [KJ] [RFC] Regarding min/max

[Robert P. J. Day]

On Sat, 3 Feb 2007, Richard Knutsson wrote:

> Hi
>
> Regarding converting:
>
> 	a = x < y ? x : y;
>
> with
> 	a = min(x, y);
> and etc.
>
> What about variables in structs? It seem to not like the multiple
> lines in min()/max(), so why not introduce something like: #define
> __min(x,y) ((x) < (y) ? (x) : (y)) (and use __min() in min() also)?
> Then there would be no problem to clean up in the structs + the
> underscores should make people aware it does not type-check.
>
> Just a thought
> /Richard Knutsson

as hard as it may be to believe, i'm going to sit this one out.  :-)
i don't really have any strong opinions on how to implement min/max
routines, but it *is* obvious that there's a lot of that kind of
cleanup that could be done.

rday

-- 
====================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

http://www.fsdev.dreamhosters.com/wiki/index.php?title=Main_Page
====================================
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Re: [KJ] [RFC] Regarding min/max

[Jaco Kroon]

Robert P. J. Day wrote:
> On Sat, 3 Feb 2007, Richard Knutsson wrote:
>>Hi
>>
>>Regarding converting:
>>
>>	a = x < y ? x : y;
>>
>>with
>>	a = min(x, y);
>>and etc.
>>
>>What about variables in structs? It seem to not like the multiple
>>lines in min()/max(), so why not introduce something like: #define
>>__min(x,y) ((x) < (y) ? (x) : (y)) (and use __min() in min() also)?
>>Then there would be no problem to clean up in the structs + the
>>underscores should make people aware it does not type-check.
>>
>>Just a thought
>>/Richard Knutsson

from include/linux/kernel.h:

/*
 * min()/max() macros that also do
 * strict type-checking.. See the
 * "unnecessary" pointer comparison.
 */
#define min(x,y) ({ \
    typeof(x) _x = (x); \
    typeof(y) _y = (y); \
    (void) (&_x = &_y);        \
    _x < _y ? _x : _y; })

#define max(x,y) ({ \
    typeof(x) _x = (x); \
    typeof(y) _y = (y); \
    (void) (&_x = &_y);        \
    _x > _y ? _x : _y; })

So one just need to convert to using these :).

Jaco
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Re: [KJ] [RFC] Regarding min/max

[Richard Knutsson]

Jaco Kroon wrote:
> Robert P. J. Day wrote:
>   
>> On Sat, 3 Feb 2007, Richard Knutsson wrote:
>>     
>>> Hi
>>>
>>> Regarding converting:
>>>
>>> 	a = x < y ? x : y;
>>>
>>> with
>>> 	a = min(x, y);
>>> and etc.
>>>
>>> What about variables in structs? It seem to not like the multiple
>>> lines in min()/max(), so why not introduce something like: #define
>>> __min(x,y) ((x) < (y) ? (x) : (y)) (and use __min() in min() also)?
>>> Then there would be no problem to clean up in the structs + the
>>> underscores should make people aware it does not type-check.
>>>
>>> Just a thought
>>> /Richard Knutsson
>>>       
>
> from include/linux/kernel.h:
>
> /*
>  * min()/max() macros that also do
>  * strict type-checking.. See the
>  * "unnecessary" pointer comparison.
>  */
> #define min(x,y) ({ \
>     typeof(x) _x = (x); \
>     typeof(y) _y = (y); \
>     (void) (&_x = &_y);        \
>     _x < _y ? _x : _y; })
>
> #define max(x,y) ({ \
>     typeof(x) _x = (x); \
>     typeof(y) _y = (y); \
>     (void) (&_x = &_y);        \
>     _x > _y ? _x : _y; })
>
> So one just need to convert to using these :).
>
> Jaco
>   
Hmm, seems I pushed the "Reply" instead of "Reply all"-button when 
answering Randy.
I tried to recreate the compile-failure (was some time ago since it 
happened) when using min()/max() in a struct but it happily compiled it 
so (thankfully) you are correct, just need to use min()/max(). :)

Richard Knutsson

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Re: [KJ] [RFC] Regarding min/max

[Srdjan Todorovic]

Hi,

On Sat, 3 Feb 2007, Jaco Kroon wrote:

> from include/linux/kernel.h:
> 
> /*
>  * min()/max() macros that also do
>  * strict type-checking.. See the
>  * "unnecessary" pointer comparison.
>  */
> #define min(x,y) ({ \
>     typeof(x) _x = (x); \
>     typeof(y) _y = (y); \
>     (void) (&_x = &_y);        \
>     _x < _y ? _x : _y; })

I don't quite understand that macro, specificaly the (void) (&_x = &_y); line.
The comment says that this macro does strict type-checking. From what I
can see, it does a comparison on the two automatic variables, but throws
away the result. So thus is essentially a no-op.

If x and y are two different types, the compiler would complain with a
warning/error because there was no cast. Is that the strict type-checking
that the comment mentions, and did I understand this correctly?

Thanks,

Srdjan
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Re: [KJ] [RFC] Regarding min/max

[Jaco Kroon]

Srdjan Todorovic wrote:
> Hi,
> 
> On Sat, 3 Feb 2007, Jaco Kroon wrote:
> 
> 
>>from include/linux/kernel.h:
>>
>>/*
>> * min()/max() macros that also do
>> * strict type-checking.. See the
>> * "unnecessary" pointer comparison.
>> */
>>#define min(x,y) ({ \
>>    typeof(x) _x = (x); \
>>    typeof(y) _y = (y); \
>>    (void) (&_x = &_y);        \
>>    _x < _y ? _x : _y; })
> 
> I don't quite understand that macro, specificaly the (void) (&_x = &_y); line.
> The comment says that this macro does strict type-checking. From what I
> can see, it does a comparison on the two automatic variables, but throws
> away the result. So thus is essentially a no-op.

Exactly.  But it verifies that a pointer to each of the two can be
compared, and that can only happen if the pointer types are identical.
Whilst a int can automatically be "upgraded" to long (for example), int*
and long* are two distinct types.  So the pointer comparison is thrown
away, since that really isn't the comparison we're after, then _x < _y
actually compares the data and returns the appropriate one.


> If x and y are two different types, the compiler would complain with a
> warning/error because there was no cast. Is that the strict type-checking
> that the comment mentions, and did I understand this correctly?

Jaco
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