Messages in this thread | | | Date | Fri, 12 Sep 2014 15:40:13 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 2/2] kernel.h: use __COUNTER__ in min and max macros to avoid -Wshadow warnings |
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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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