svn commit: r209119 - head/sys/sys

[mdf at FreeBSD.org]

On Sun, Jun 13, 2010 at 10:10 AM, Pawel Jakub Dawidek <pjd at freebsd.org> wrote:
> On Sun, Jun 13, 2010 at 02:39:55AM +0000, Lawrence Stewart wrote:
>> Author: lstewart
>> Date: Sun Jun 13 02:39:55 2010
>> New Revision: 209119
>> URL: http://svn.freebsd.org/changeset/base/209119
>>
>> Log:
>>   Add a utility macro to simplify calculating an aggregate sum from a DPCPU
>>   counter variable.
>>
>>   Sponsored by:       FreeBSD Foundation
>>   Reviewed by:        jhb, rpaulo, rwatson (previous version of patch)
>>   MFC after:  1 week
>>
>> Modified:
>>   head/sys/sys/pcpu.h
>>
>> Modified: head/sys/sys/pcpu.h
>> ==============================================================================
>> --- head/sys/sys/pcpu.h       Sun Jun 13 01:27:29 2010        (r209118)
>> +++ head/sys/sys/pcpu.h       Sun Jun 13 02:39:55 2010        (r209119)
>> @@ -106,6 +106,17 @@ extern uintptr_t dpcpu_off[];
>>  #define      DPCPU_ID_GET(i, n)      (*DPCPU_ID_PTR(i, n))
>>  #define      DPCPU_ID_SET(i, n, v)   (*DPCPU_ID_PTR(i, n) = v)
>>
>> +/*
>> + * Utility macros.
>> + */
>> +#define DPCPU_SUM(n, var, sum)                                               \
>> +do {                                                                 \
>> +     (sum) = 0;                                                      \
>> +     u_int i;                                                        \
>> +     CPU_FOREACH(i)                                                  \
>> +             (sum) += (DPCPU_ID_PTR(i, n))->var;                     \
>> +} while (0)
>
> I'd suggest first swapping variable declaration and '(sum) = 0;'.
> Also using 'i' as a counter in macro can easly lead to name collision.
> If you need to do it, I'd suggest '_i' or something.
> Maybe it would be better to make it an inline function rather than macro?

(Relevant but almost a thread hijack):

At Isilon we've run into a lot of problems with variable declarations
in macros, especially with -Wshadow turned on.  We ended up
backporting __COUNTER__ from later versions of gcc and then using it
to make unique variable names.

 - is the backport (or a fresh implementation) something that could be
done within the scope of the GPL license?
 - is it something FreeBSD would be interested in?
 - is __COUNTER__ supported by clang?
 - if not, could it be?

-Wshadow found several nasty bugs in our code, and apart from a few
spurious warnings it has been handy to have when building our
filesystem.

Thanks,
matthew