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SubjectRe: [PATCH 2/2] kernel.h: use __COUNTER__ in min and max macros to avoid -Wshadow warnings
On Thu, 11 Sep 2014 23:39:36 +0200 Michal Nazarewicz <mina86@mina86.com> wrote:

> Because min and max macros use the same variable names no matter
> how many times they are called (or how deep the nesting of their
> calls), each time min or max calls are nested, the same variables
> are declared. This is especially noisy after min3 and max3 have
> been changed to nest min/max calls.
>
> Using __COUNTER__ solves the problem since each variable will get
> a unique number aadded to it. The code will still work even if
> the compiler does not support __COUNTER__, but then the protection
> from shadow warning won't work.
>
> The same applies to min_t and max_t macros.
>
> ...
>
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -695,15 +695,27 @@ static inline void ftrace_dump(enum ftrace_dump_mode oops_dump_mode) { }
> #endif /* CONFIG_TRACING */
>
> /*
> + * Preprocessor magic generating unique identifiers to avoid -Wshadow warnings
> + * used by min, max, min_t and max_t macros. cnt is __COUNTER__, op is the
> + * comparison operator; tx (ty) is type of the first (second) argument,
> + * xx (yy) is name of a temporary variable to hold the first (second) argument,
> + * and x (y) is the first (second) argument.
> + */
> +#define _min_max_var(cnt, base) _mm_ ## cnt ## base
> +#define _min_max__(op, tx, xx, x, ty, yy, y) ({ \
> + tx xx = (x); \
> + ty yy = (y); \
> + (void) (&xx == &yy); \
> + xx op yy ? xx : yy; })
> +#define _min_max_(cnt, op, tx, x, ty, y) \
> + _min_max__(op, tx, _min_max_var(cnt, a), x, ty, _min_max_var(cnt, b), y)
> +#define _min_max(...) _min_max_(__COUNTER__, __VA_ARGS__)

The fact that __COUNTER__ is used in compiler-gcc4.h but not in
compiler-gcc3.h makes me suspicious about its availability?

I do think that [1/2] made the code significantly worse-looking and
this one is getting crazy. How useful is W=2 anyway? Has anyone found
a bug using it? The number of warnings in default builds is already way
too high :(



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