Re: [PATCH v4 07/13] minmax: Introduce {min,max}_array()

[Herve Codina]

Hi Andy,

On Wed, 14 Jun 2023 14:51:43 +0300
Andy Shevchenko <andy.shevchenko@xxxxxxxxx> wrote:

> On Wed, Jun 14, 2023 at 12:42 PM Herve Codina <herve.codina@xxxxxxxxxxx> wrote:
> > On Wed, 14 Jun 2023 12:02:57 +0300
> > Andy Shevchenko <andy.shevchenko@xxxxxxxxx> wrote:  
> > > On Wed, Jun 14, 2023 at 10:49 AM Herve Codina <herve.codina@xxxxxxxxxxx> wrote:  
> 
> ...
> 
> > > > +       typeof(__array[0] + 0) __element = __array[--__len];    \  
> > >
> > > Do we need the ' + 0' part?  
> >
> > Yes.
> >
> > __array can be an array of const items and it is legitimate to get the
> > minimum value from const items.
> >
> > typeof(__array[0]) keeps the const qualifier but we need to assign __element
> > in the loop.
> > One way to drop the const qualifier is to get the type from a rvalue computed
> > from __array[0]. This rvalue has to have the exact same type with only the const
> > dropped.
> > '__array[0] + 0' was a perfect canditate.  
> 
> Seems like this also deserves a comment. But if the series is accepted
> as is, it may be done as a follow up.
> 

Finally not so simple ...
I did some deeper tests and the macros need to be fixed.

I hope this one (with comments added) is correct:
--- 8 ---
/*
 * Do not check the array parameter using __must_be_array().
 * In the following legit use-case where the "array" passed is a simple pointer,
 * __must_be_array() will return a failure.
 * --- 8< ---
 * int *buff
 * ...
 * min = min_array(buff, nb_items);
 * --- 8< ---
 *
 * The first typeof(&(array)[0]) is needed in order to support arrays of both
 * 'int *buff' and 'int buf[N]' types.
 *
 * typeof(__array[0] + 0) used for __element is needed as the array can be an
 * array of const items.
 * In order to discard the const qualifier use an arithmetic operation (rvalue).
 * This arithmetic operation discard the const but also can lead to an integer
 * promotion. For instance, a const s8 __array[0] lead to an int __element due
 * to the promotion.
 * In this case, simple min() or max() operation fails (type mismatch).
 * Use min_t() or max_t() (op_t parameter) enforcing the type in order to avoid
 * the min() or max() failure.
 */
#define __minmax_array(op_t, array, len) ({			\
	typeof(&(array)[0]) __array = (array);			\
	typeof(len) __len = (len);				\
	typeof(__array[0] + 0) __element = __array[--__len];	\
	while (__len--)						\
		__element = op_t(typeof(__array[0]), __element, __array[__len]); \
	__element; })

/**
 * min_array - return minimum of values present in an array
 * @array: array
 * @len: array length
 *
 * Note that @len must not be zero (empty array).
 */
#define min_array(array, len) __minmax_array(min_t, array, len)

/**
 * max_array - return maximum of values present in an array
 * @array: array
 * @len: array length
 *
 * Note that @len must not be zero (empty array).
 */
#define max_array(array, len) __minmax_array(max_t, array, len)

--- 8< ---

Tested ok from user-space on my x86_64 using the following types for *buff
and buff[N]:
- signed/unsigned char
- signed/unsigned short
- signed/unsigned int
- signed/unsigned long
- signed/unsigned long long
- float, double, long double (even if not used in the kernel)

Can you give me your feedback on this last version ?

If you are ok, it will be present in the next iteration of the series.
Otherwise, as a last ressort, I will drop the {min,max}_array() and switch
back to the specific min/max computation in IIO inkern.c

Sorry for this back and forth and this last minute detected bug.

Best regards,
Hervé

-- 
Hervé Codina, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com