On Fri, Sep 12 2014, Andrew Morton <a...@linux-foundation.org> wrote:
> On Thu, 11 Sep 2014 23:39:36 +0200 Michal Nazarewicz <min...@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 +--<m...@google.com>--<xmpp:min...@jabber.org>--ooO--(_)--Ooo--
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