Re: gnulib-common.m4: Make _GL_UNUSED_LABEL effective for clang

[Bruno Haible]

Paul Eggert wrote:
> Also, your example used C code, but this fix doesn't affect C code. So 
> can we assume the real problem was with C++ code?

Yes. I tried the example both with C and C++.

================================ foo.cc ================================
int foo (int x)
{
  x += 2;
  if (x & 1)
    goto a;
b:
  x *= 3;
a:
  return x;
}
========================================================================
$ clang -Wall -S foo.cc
foo.cc:6:1: warning: unused label 'b' [-Wunused-label]
b:
^~
1 warning generated.
The same definition as for GCC works, however, with clang:

================================ foo.cc ================================
int foo (int x)
{
  x += 2;
  if (x & 1)
    goto a;
b:
  __attribute__ ((__unused__));
  x *= 3;
a:
  return x;
}
========================================================================
$ clang -Wall -S foo.cc
(no warning)

> On 8/22/21 1:17 PM, Bruno Haible wrote:
> > -#if !defined __cplusplus || _GL_GNUC_PREREQ (4, 5)
> > +#if (!defined __cplusplus || _GL_GNUC_PREREQ (4, 5)) || defined __clang__
> 
> I got a bit confused by the extra parentheses, thinking that somehow GCC 
> and Clang differ with respect to C++. Perhaps remove the extra parens?

And I got confused by this boolean expression already before, and understood
it only after looking at the ChangeLog entry from 2014-06-01. This indicates
that a comment is needed. This one is clearer for me. For you as well?


2021-08-22  Bruno Haible  <bruno@clisp.org>

        gnulib-common.m4: Clarify logic behind _GL_UNUSED_LABEL.
        Reported by Paul Eggert.
        * m4/gnulib-common.m4 (gl_COMMON_BODY): Add comment and write the
        condition in a way that is close to the comment.

diff --git a/m4/gnulib-common.m4 b/m4/gnulib-common.m4
index 8a10e5bc1..639fa8567 100644
--- a/m4/gnulib-common.m4
+++ b/m4/gnulib-common.m4
@@ -418,7 +418,9 @@ AC_DEFUN([gl_COMMON_BODY], [
    immediately preceding label is not used.  The compiler should not warn
    if the label is not used.  */
 /* Applies to: label (both in C and C++).  */
-#if (!defined __cplusplus || _GL_GNUC_PREREQ (4, 5)) || defined __clang__
+/* Note that g++ < 4.5 does not support the '__attribute__ ((__unused__)) ;'
+   syntax.  But clang does.  */
+#if !(defined __cplusplus && !_GL_GNUC_PREREQ (4, 5)) || defined __clang__
 # define _GL_UNUSED_LABEL _GL_ATTRIBUTE_UNUSED
 #else
 # define _GL_UNUSED_LABEL