[Bug c++/113582] New: incorrect warning about unused label

[Bug c++/113582] New: incorrect warning about unused label

[nmmm at nmmm dot nu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113582

            Bug ID: 113582
           Summary: incorrect warning about unused label
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: nmmm at nmmm dot nu
  Target Milestone: ---

This code:

template<bool B>
void do_something(){
        #pragma GCC diagnostic push
        #pragma GCC diagnostic ignored "-Wunused-label"

        start:

        if constexpr(B)
                goto start;

        #pragma GCC diagnostic pop
}

int main(){
        do_something<0>();
}

Generates a warning:

warning: label ‘start’ defined but not used [-Wunused-label]

pragma GCC diagnostic did not work as well.

[Bug c++/113582] incorrect warning about unused label

[nmmm at nmmm dot nu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113582

--- Comment #1 from Nikolay Mihaylov <nmmm at nmmm dot nu> ---
If you move the pragma outside the templated function, no warning is shown:

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-label"

template<bool B>
void do_something(){

        start:

        if constexpr(B)
                goto start;

}

#pragma GCC diagnostic pop

[Bug c++/113582] incorrect warning about unused label

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113582

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-01-24
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed.

[Bug c++/113582] incorrect warning about unused label with `pragma GCC diagnostic` around the unused label

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113582

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|incorrect warning about     |incorrect warning about
                   |unused label with `pragma   |unused label with `pragma
                   |GCC diagnostic` inside a    |GCC diagnostic` around the
                   |template                    |unused label

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I originally thought this was due to a template but nope, it is to due with
where the warning happens. Which is at the end of the function.


That is this does not work:
```
void do_something(){
        #pragma GCC diagnostic push
        #pragma GCC diagnostic ignored "-Wunused-label"
        start:;
        #pragma GCC diagnostic pop
}
```

But this causes the warning not to happen:
```
void do_something(){
        start:;
        #pragma GCC diagnostic push
        #pragma GCC diagnostic ignored "-Wunused-label"
}
        #pragma GCC diagnostic pop
```

Note another way to remove the warning is just to mark the lable as unused with
an attribute like:
```
[[gnu::unused]]start:
```
Which is slightly shorter than using the pragma too.

[Bug c++/113582] incorrect warning about unused label with `pragma GCC diagnostic` around the unused label

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113582

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
           Assignee|unassigned at gcc dot gnu.org      |mpolacek at gcc dot gnu.org

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Hack/fix:

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 3748ccd49ff..224d47f2f90 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -13093,7 +13093,11 @@ cp_parser_label_for_labeled_statement (cp_parser*
parser, tree attributes)
       /* Anything else must be an ordinary label.  */
       label = finish_label_stmt (cp_parser_identifier (parser));
       if (label && TREE_CODE (label) == LABEL_DECL)
-   FALLTHROUGH_LABEL_P (label) = fallthrough_p;
+   {
+     FALLTHROUGH_LABEL_P (label) = fallthrough_p;
+     if (!warning_enabled_at (input_location, OPT_Wunused_label))
+       suppress_warning (label, OPT_Wunused_label);
+   }
       break;
     }

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 74013533b0f..b6686d8543f 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -18796,11 +18796,13 @@ tsubst_stmt (tree t, tree args, tsubst_flags_t
complain, tree in_decl)
     case LABEL_EXPR:
       {
    tree decl = LABEL_EXPR_LABEL (t);
-   tree label;
-
-   label = finish_label_stmt (DECL_NAME (decl));
+   tree label = finish_label_stmt (DECL_NAME (decl));
    if (TREE_CODE (label) == LABEL_DECL)
-     FALLTHROUGH_LABEL_P (label) = FALLTHROUGH_LABEL_P (decl);
+     {
+       FALLTHROUGH_LABEL_P (label) = FALLTHROUGH_LABEL_P (decl);
+       if (warning_suppressed_p (decl, OPT_Wunused_label))
+         TREE_USED (label) = true;
+     }
    if (DECL_ATTRIBUTES (decl) != NULL_TREE)
      cplus_decl_attributes (&label, DECL_ATTRIBUTES (decl), 0);
       }

[Bug c++/113582] incorrect warning about unused label with `pragma GCC diagnostic` around the unused label

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113582

--- Comment #5 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
The reason I'm not doing simply

  TREE_USED (label) = TREE_USED (decl);

in the pt.cc/LABEL_EXPR hunk is that I think we still want the warning in the
second function, what with the goto being a discarded statement.

// PR c++/113582

template<bool B> void
do_something ()
{
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-label"
start:
  if constexpr(B)
    goto start;
#pragma GCC diagnostic pop
}

template<bool B> void
do_something2 ()
{
start: // { dg-warning "defined but not used" }
  if constexpr(B)
    goto start;
}

void
g ()
{
  do_something<0>();
  do_something2<0>();
}

[Bug c++/113582] incorrect warning about unused label with `pragma GCC diagnostic` around the unused label

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113582

--- Comment #6 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Patch approved for GCC 15:
https://gcc.gnu.org/pipermail/gcc-patches/2024-January/643999.html

[Bug c++/113582] incorrect warning about unused label with `pragma GCC diagnostic` around the unused label

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113582

--- Comment #7 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The trunk branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>:

https://gcc.gnu.org/g:d9318caed3bbff8136d13e00dcfc020a59d10f78

commit r15-329-gd9318caed3bbff8136d13e00dcfc020a59d10f78
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Jan 24 18:06:48 2024 -0500

    c++: #pragma doesn't disable -Wunused-label [PR113582]

    The PR complains that

      void do_something(){
        #pragma GCC diagnostic push
        #pragma GCC diagnostic ignored "-Wunused-label"
        start:;
        #pragma GCC diagnostic pop
      } #1

    doesn't work.  That's because we warn_for_unused_label only while we're
    in finish_function, meaning we're at #1 where we're outside the #pragma
    region.  We can use suppress_warning + warning_suppressed_p to fix this.

    Note that I'm not using TREE_USED.  Propagating it in
tsubst_stmt/LABEL_EXPR
    from decl to label would mean that we don't warn in do_something2, but
    I think we want the warning there: we're in a template and the goto is
    a discarded statement.

            PR c++/113582

    gcc/c-family/ChangeLog:

            * c-warn.cc (warn_for_unused_label): Don't warn if -Wunused-label
has
            been suppressed for the label.

    gcc/cp/ChangeLog:

            * parser.cc (cp_parser_label_for_labeled_statement):
suppress_warning
            if it's not enabled at input_location.
            * pt.cc (tsubst_stmt): Call copy_warning.

    gcc/testsuite/ChangeLog:

            * g++.dg/warn/Wunused-label-4.C: New test.

[Bug c++/113582] incorrect warning about unused label with `pragma GCC diagnostic` around the unused label

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113582

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
             Status|ASSIGNED                    |RESOLVED

--- Comment #8 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Fixed in GCC 15.