Messages in this thread | | | From | Michal Nazarewicz <> | Subject | Re: [PATCH 2/2] kernel.h: use __COUNTER__ in min and max macros to avoid -Wshadow warnings | Date | Sat, 13 Sep 2014 01:37:40 +0200 |
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On Fri, Sep 12 2014, Andrew Morton <akpm@linux-foundation.org> wrote: > 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?
http://gcc.gnu.org/ml/gcc-patches/2007-05/msg01579.html so looks like it has 7 years. But as the commit message says, the code will still work, even w/o working __COUNTER__.
> I do think that [1/2] made the code significantly worse-looking
Oh? I actually thought [1/2] makes it nicer by having a single place where the min/max logic is implemented.
> and this one is getting crazy. How useful is W=2 anyway?
I actually do agree with that. I didn't have high hopes about getting this patch accepted, but wanted to send it out to show that it can be done, if it's really deemed useful.
> Has anyone found a bug using it? The number of warnings in default > builds is already way too high :(
-- Best regards, _ _ .o. | Liege of Serenely Enlightened Majesty of o' \,=./ `o ..o | Computer Science, Michał “mina86” Nazarewicz (o o) ooo +--<mpn@google.com>--<xmpp:mina86@jabber.org>--ooO--(_)--Ooo-- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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