[Bug c/100670] New: unused attribute ignored on function definition

[Bug c/100670] New: unused attribute ignored on function definition

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100670

            Bug ID: 100670
           Summary: unused attribute ignored on function definition
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

Consider the following test-case:
...
$ cat foo.c
typedef void *void_ptr;

static TYPE __attribute__((unused))
foo (void)
{
  return 0;
}
...

Let's try with type int:
...
$ gcc-11 -c -Wall -O0 -g -Werror foo.c -DTYPE="int"
$
...
No warning.

Ok, now with void *:
...
$ gcc-11 -c -Wall -O0 -g -Werror foo.c -DTYPE="void *"
foo.c:4:1: error: ‘foo’ defined but not used [-Werror=unused-function]
    4 | foo (void)
      | ^~~
cc1: all warnings being treated as errors
...
Hmm, a warning.

Now with void_ptr:
...
$ gcc-11 -c -Wall -O0 -g -Werror foo.c -DTYPE="void_ptr"
$
...
again no warning.

My expectation was that there would be no warning for the "void *" case.

I cannot tell from the gcc attributes syntax documentation what is the correct
behaviour.

If this is not a compiler bug, then I suggest this is changed into a
documentation PR.

Note that there is at least one example that uses this style (
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
):
...
void __attribute__ ((visibility ("protected")))
f () { /* Do something. */; }
...

[Bug c/100670] unused attribute ignored on function definition

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100670

--- Comment #1 from Tom de Vries <vries at gcc dot gnu.org> ---
Note btw that clang does not generate a warning:
...
$ clang -c -Wall -O0 -g -Werror foo.c -DTYPE="void *"
$ 
...
which means the attribute works, because if we remove the attribute we have
instead:
...
$ clang -c -Wall -O0 -g -Werror foo.c -DTYPE="void *"
foo.c:4:1: error: unused function 'foo' [-Werror,-Wunused-function]
foo (void)
^
1 error generated.
...

[Bug c/100670] unused attribute ignored on function definition

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100670

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jsm28 at gcc dot gnu.org
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2021-05-19
             Status|UNCONFIRMED                 |NEW
           Keywords|                            |documentation

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
I think you need to use

static __attribute__((unused)) TYPE

thus documentation bug.  Attribute parsing is somewhat odd, esp. when
pointer types are involved.