[Bug c/51896] New: Should gcc warn if a variable initializer inside a switch statement is never used?

[Bug c/51896] New: Should gcc warn if a variable initializer inside a switch statement is never used?

[kaie at kuix dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51896

             Bug #: 51896
           Summary: Should gcc warn if a variable initializer inside a
                    switch statement is never used?
    Classification: Unclassified
           Product: gcc
           Version: 4.6.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: kaie@kuix.de


While maintaining existing code, we encountered a snippet like this:

--------------- begin test.c -----------------------
#include "stdio.h"
#include "stdlib.h"

int main(int argc, char *argv[])
{
  int *a = (int*)0xaaaaaaaa;
  printf("a is: %p\n", a);

  int r = rand() % 2;

  switch (r)
  {
    int *b = (int*)0xbbbbbbbb;

    case 2: break;
    default:
      printf("b is: %p\n", b);
  }

  return 0;
} 
--------------- end test.c -----------------------

$ gcc -o test -Wall test.c
test.c: In function ‘main’:
test.c:17:13: warning: ‘b’ may be used uninitialized in this function
[-Wuninitialized]

$ ./test 
a is: 0xaaaaaaaa
b is: 0x434b5fc4


Proposal: 

As gcc decides to ignore value 0xbbbbbbbb, it should print a warning message,
informing the user, e.g.:

  test.c: warning: ignoring initialization value ‘0xbbbbbbbb’

[Bug c/51896] Should gcc warn if a variable initializer inside a switch statement is never used?

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51896

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-01-19
     Ever Confirmed|0                           |1
           Severity|normal                      |enhancement

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-01-19 10:04:01 UTC ---
Confirmed.  This is similar to

int 
main(int argc, char *argv[])
{
  goto _default;
    {
      int *b = (int*)0xbbbbbbbb;

      _default:
      printf("b is: %p\n", b);
    }

  return 0;
}

thus, when you jump across an init expression and still use the variable.