Nouveau - Mar 2022 - [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr

On Mon, Feb 28, 2022 at 12:37:15PM -0800, Linus Torvalds
wrote:
> On Mon, Feb 28, 2022 at 12:16 PM Matthew Wilcox <willy at
infradead.org> wrote:
> >
> > Then we can never use -Wshadow ;-(  I'd love to be able to turn it
on;
> > it catches real bugs.
> 
> Oh, we already can never use -Wshadow regardless of things like this.
> That bridge hasn't just been burned, it never existed in the first
> place.
> 
> The whole '-Wshadow' thing simply cannot work with local variables
in
> macros - something that we've used since day 1.
> 
> Try this (as a "p.c" file):
> 
>         #define min(a,b) ({                     \
>                 typeof(a) __a = (a);            \
>                 typeof(b) __b = (b);            \
>                 __a < __b ? __a : __b; })
> 
>         int min3(int a, int b, int c)
>         {
>                 return min(a,min(b,c));
>         }
> 
> and now do "gcc -O2 -S t.c".
> 
> Then try it with -Wshadow.
#define ___PASTE(a, b)	a##b
#define __PASTE(a, b) ___PASTE(a, b)
#define _min(a, b, u) ({         \
        typeof(a) __PASTE(__a,u) = (a);            \
        typeof(b) __PASTE(__b,u) = (b);            \
        __PASTE(__a,u) < __PASTE(__b,u) ? __PASTE(__a,u) : __PASTE(__b,u); })

#define min(a, b) _min(a, b, __COUNTER__)

int min3(int a, int b, int c)
{
        return min(a,min(b,c));
}

(probably there's a more elegant way to do this)

On Mon, Feb 28, 2022 at 3:26 PM Matthew Wilcox <willy at infradead.org>
wrote:
>
> #define ___PASTE(a, b)  a##b
> #define __PASTE(a, b) ___PASTE(a, b)
> #define _min(a, b, u) ({         \
Yeah, except that's ugly beyond belief, plus it's literally not what
we do in the kernel.

Really. The "-Wshadow doesn't work on the kernel" is not some new
issue, because you have to do completely insane things to the source
code to enable it.

Just compare your uglier-than-sin version to my straightforward one.
One does the usual and obvious "use a private variable to avoid the
classic multi-use of a macro argument". And the other one is an
abomination.

              Linus