[Bug c/82963] New: -Waddress too trigger happy

[mhocko at kernel dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82963

            Bug ID: 82963
           Summary: -Waddress too trigger happy
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mhocko at kernel dot org
  Target Milestone: ---

Hi,
in the kernel we have a uggly^Wmacro to help printing numa mask defined as
follows
#define nodemask_pr_args(maskp) MAX_NUMNODES : (maskp)->bits

I have updated it to allow NULL maskp as follows
#define nodemask_pr_args(maskp) (maskp) ? MAX_NUMNODES : 0, (maskp) ?
(maskp)->bits : NULL

but this has triggered warnings on usage where it is clear that maskp is never
NULL. E.g.
In file included from include/linux/mmzone.h:17:0,                              
                 from include/linux/mempolicy.h:10,                             
                 from mm/mempolicy.c:70:                                        
mm/mempolicy.c: In function 'mpol_to_str':                                      
include/linux/nodemask.h:107:41: warning: the address of 'nodes' will always
evaluate as 'true' [-Waddress]                                                  
 #define nodemask_pr_args(maskp) (maskp) ? MAX_NUMNODES : 0, (maskp) ?
(maskp)->bits : NULL                                                            
                                         ^                                      
mm/mempolicy.c:2817:11: note: in expansion of macro 'nodemask_pr_args'
           nodemask_pr_args(&nodes));
           ^
include/linux/nodemask.h:107:69: warning: the address of 'nodes' will always
evaluate as 'true' [-Waddress]
 #define nodemask_pr_args(maskp) (maskp) ? MAX_NUMNODES : 0, (maskp) ?
(maskp)->bits : NULL
                                                                     ^
mm/mempolicy.c:2817:11: note: in expansion of macro 'nodemask_pr_args'
           nodemask_pr_args(&nodes));

While the warning is correct and the given mask will always resolve to the
success path of the ternary operator I really fail to see why we should warn
about this fact. I really do not see any potential problem which could be
caused by this fact.

Moreover the warning itself is quite inconsistent. E.g. the following warns
about the explicit &m but not for n. So I believe this is more of a suboptimal
warning implementation than real intention.

#include <stdio.h>

#define MAX_NUMNODES 10
struct mask {
        void *bits;
};
#define nodemask_pr_args(maskp) (maskp) ? MAX_NUMNODES : 0, (maskp) ?
(maskp)->bits : NULL

int foo(void)
{
        struct mask m;
        struct mask *n = &m;

        printf("%*p\n", nodemask_pr_args(&m));
        printf("%*p\n", nodemask_pr_args(n));

        return 0;
}