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

[Herve Codina]

Hi Andy,

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:
> >
> > Introduce min_array() (resp max_array()) in order to get the
> > minimal (resp maximum) of values present in an array.  
> 
> Reviewed-by: Andy Shevchenko <andy.shevchenko@xxxxxxxxx>
> See a remark below.
> 
> > Signed-off-by: Herve Codina <herve.codina@xxxxxxxxxxx>
> > ---
> >  include/linux/minmax.h | 36 ++++++++++++++++++++++++++++++++++++
> >  1 file changed, 36 insertions(+)
> >
> > diff --git a/include/linux/minmax.h b/include/linux/minmax.h
> > index 396df1121bff..2cd0d34ce921 100644
> > --- a/include/linux/minmax.h
> > +++ b/include/linux/minmax.h
> > @@ -133,6 +133,42 @@
> >   */
> >  #define max_t(type, x, y)      __careful_cmp((type)(x), (type)(y), >)
> >
> > +/*
> > + * 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< ---
> > + */
> > +#define __minmax_array(op, array, len) ({                      \
> > +       typeof(array) __array = (array);                        \
> > +       typeof(len) __len = (len);                              \
> > +       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.

Regards,
Hervé

> 
> > +       while (__len--)                                         \
> > +               __element = op(__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, 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, array, len)
> > +
> >  /**
> >   * clamp_t - return a value clamped to a given range using a given type
> >   * @type: the type of variable to use
> > --
> > 2.40.1
> >  
> 
>