Messages in this thread | | | From | "Rustad, Mark D" <> | Subject | Re: [PATCH 2/2] kernel.h: use __COUNTER__ in min and max macros to avoid -Wshadow warnings | Date | Fri, 12 Sep 2014 23:48:07 +0000 |
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Michal,
On Sep 12, 2014, at 4:37 PM, Michal Nazarewicz <mina86@mina86.com> wrote:
> 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.
It does have that going for it.
>> 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.
Well, I learned something from it. Thank you for teaching this old dog a new trick.
>> Has anyone found a bug using it? The number of warnings in default >> builds is already way too high :(
-- Mark Rustad, Networking Division, Intel Corporation
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