[Bug c/52106] New: missing -Wunused-but-set-variable warning with the a = b = ... construct

[Bug c/52106] New: missing -Wunused-but-set-variable warning with the a = b = ... construct

[vincent-gcc at vinc17 dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52106

             Bug #: 52106
           Summary: missing -Wunused-but-set-variable warning with the a =
                    b = ... construct
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vincent-gcc@vinc17.net


The following code

int foo (void)
{
  int a, b, c, d;
  a = b = 0;
  c = d = 0;
  return a + d;
}

gives:

$ gcc-snapshot -Wall -c tst.c
tst.c: In function 'foo':
tst.c:3:13: warning: variable 'c' set but not used [-Wunused-but-set-variable]

One would expect the same warning for variable 'b'. Even though it appears to
be used to assign variable 'a', it shouldn't count in such a construct. Indeed
a variable assignment is useless if the variable isn't used in a later
expression (if the goal is to convert the value, then a cast would be more
readable).

Tested with:
gcc (Debian 20120128-1) 4.7.0 20120128 (experimental) [trunk revision 183664]

[Bug c/52106] missing -Wunused-but-set-variable warning with the a = b = ... construct

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52106

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
                 CC|                            |jakub at gcc dot gnu.org
         Resolution|                            |INVALID

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-02-03 12:53:42 UTC ---
The variable is used, so it isn't unused-but-set.  It is intentional that the
warning behaves the way it does.

[Bug c/52106] warning for useless assignments

[vincent-gcc at vinc17 dot net]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52106

Vincent Lefèvre <vincent-gcc at vinc17 dot net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |
            Summary|missing                     |warning for useless
                   |-Wunused-but-set-variable   |assignments
                   |warning with the a = b =    |
                   |... construct               |

--- Comment #2 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> 2012-02-03 13:28:39 UTC ---
Then there should be another warning. So, this is a request for a warning for
assignments that are obviously useless (thus maybe a typo in the code or old
code that could be removed...). It should cover variables set but not used
outside the full expression where they are set. The code

int foo (void)
{
  int a, b;
  a = b = 0;
  return a;
}

gives an example. It could be written:

int foo (void)
{
  int a;
  a = 0;
  return a;
}

"a = b = ...;" is generally used as a shortcut for "a = ...; b = ...;", and if
it isn't (because the behavior would be different at compile time or at run
time), there should be a warning against this construct.

[Bug c/52106] warning for useless assignments

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52106

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2012-02-03
     Ever Confirmed|0                           |1

[Bug c/52106] [C11] missing -Wunused-but-set-variable warning with the a = b = ... construct

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52106

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|warning for useless         |[C11] missing
                   |assignments                 |-Wunused-but-set-variable
                   |                            |warning with the a = b =
                   |                            |... construct

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> 2012-02-03 17:18:13 UTC ---
>  a = b = 0;

In C99 and before is equivalent to b = 0; a = b;
In C11 (which we don't implement this semantics yet), it is a = 0; b = 0;