Re: [PATCH] gdbsupport: better detection of -Wmissing-prototypes support

[Andrew Burgess <andrew.burgess@embecosm.com>]

* Andrew Burgess <andrew.burgess@embecosm.com> [2021-09-24 13:29:33 +0100]:

> When building with GCC 9.3.1 I notice lots of warnings like this while
> building GDB:
> 
>   cc1plus: warning: command line option ‘-Wmissing-prototypes’ is valid for C/ObjC but not for C++
> 
> This is a little strange as the configure macro, AM_GDB_WARNINGS,
> should figure out which warning flags are valid, and which are not.
> 
> It turns out that there's a weird bug in some older version of GCC;
> when performing only a compile, the -Wmissing-prototypes flag will
> only ever produce a warning, even when -Werror is passed.  If a full
> compile and link is performed then an error is produced (assuming
> -Werror is passed).
> 
> Of course, the AM_GDB_WARNINGS macro only tests each flag during a
> compile, not a compile and link, so the macro figures that the
> -Wmissing-prototypes flag is valid (even though a warning is emitted),
> and adds it to the set of flags to use.
> 
> In this commit I special case the -Wmissing-prototypes check inside
> AM_GDB_WARNINGS, and for that flag only, we now perform a full compile
> and link.  This means that the configure script can correctly detect
> that the flag is not supported, and so the flag is no longer used.

It has been pointed out to me on IRC (thanks Simon), that this issue
is not GCC, but ccache:

  https://github.com/ccache/ccache/issues/738#issuecomment-740133374

So this patch certainly shouldn't go in as it is currently written.

It's still pretty annoying seeing those warnings though, so I wonder
if we can consider having a work around for this ccache issue in the
GDB configure scripts?

Maybe something as simple as setting CCACHE_DISABLE in the environment
prior to running the AM_GDB_WARNINGS checks?

All ideas welcome...

thanks,
Andrewx


> ---
>  gdb/configure         | 25 +++++++++++++++++++++++++
>  gdbserver/configure   | 25 +++++++++++++++++++++++++
>  gdbsupport/configure  | 25 +++++++++++++++++++++++++
>  gdbsupport/warning.m4 | 14 ++++++++++++++
>  4 files changed, 89 insertions(+)
> 
> diff --git a/gdb/configure b/gdb/configure
> index f0b1af4a6ea..1bd9a7698b7 100755
> --- a/gdb/configure
> +++ b/gdb/configure
> @@ -17016,6 +17016,31 @@ if ac_fn_cxx_try_compile "$LINENO"; then :
>    WARN_CFLAGS="${WARN_CFLAGS} $w"
>  fi
>  rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
> +            elif test "x$w" = "x-Wmissing-prototypes"; then
> +              # On some versions of GCC, e.g. 9.3.1, even when -Werror
> +              # is passed, the -Wmissing-prototypes flag will only
> +              # ever produce a warning when compiling, but, will
> +              # produce an error when a full link is performed.  This
> +              # bug is fixed in later versions of GCC.
> +              #
> +              # To avoid this bug, and detect if -Wmissing-prototypes
> +              # is supported, we do a compile and link here.
> +	      cat confdefs.h - <<_ACEOF >conftest.$ac_ext
> +/* end confdefs.h.  */
> +
> +int
> +main ()
> +{
> +
> +  ;
> +  return 0;
> +}
> +_ACEOF
> +if ac_fn_cxx_try_link "$LINENO"; then :
> +  WARN_CFLAGS="${WARN_CFLAGS} $w"
> +fi
> +rm -f core conftest.err conftest.$ac_objext \
> +    conftest$ac_exeext conftest.$ac_ext
>  	    else
>  	      cat confdefs.h - <<_ACEOF >conftest.$ac_ext
>  /* end confdefs.h.  */
> diff --git a/gdbserver/configure b/gdbserver/configure
> index b227167e270..c12354f583c 100755
> --- a/gdbserver/configure
> +++ b/gdbserver/configure
> @@ -9766,6 +9766,31 @@ if ac_fn_cxx_try_compile "$LINENO"; then :
>    WARN_CFLAGS="${WARN_CFLAGS} $w"
>  fi
>  rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
> +            elif test "x$w" = "x-Wmissing-prototypes"; then
> +              # On some versions of GCC, e.g. 9.3.1, even when -Werror
> +              # is passed, the -Wmissing-prototypes flag will only
> +              # ever produce a warning when compiling, but, will
> +              # produce an error when a full link is performed.  This
> +              # bug is fixed in later versions of GCC.
> +              #
> +              # To avoid this bug, and detect if -Wmissing-prototypes
> +              # is supported, we do a compile and link here.
> +	      cat confdefs.h - <<_ACEOF >conftest.$ac_ext
> +/* end confdefs.h.  */
> +
> +int
> +main ()
> +{
> +
> +  ;
> +  return 0;
> +}
> +_ACEOF
> +if ac_fn_cxx_try_link "$LINENO"; then :
> +  WARN_CFLAGS="${WARN_CFLAGS} $w"
> +fi
> +rm -f core conftest.err conftest.$ac_objext \
> +    conftest$ac_exeext conftest.$ac_ext
>  	    else
>  	      cat confdefs.h - <<_ACEOF >conftest.$ac_ext
>  /* end confdefs.h.  */
> diff --git a/gdbsupport/configure b/gdbsupport/configure
> index a9dd02c5b72..ff8eb517d9e 100755
> --- a/gdbsupport/configure
> +++ b/gdbsupport/configure
> @@ -10251,6 +10251,31 @@ if ac_fn_cxx_try_compile "$LINENO"; then :
>    WARN_CFLAGS="${WARN_CFLAGS} $w"
>  fi
>  rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
> +            elif test "x$w" = "x-Wmissing-prototypes"; then
> +              # On some versions of GCC, e.g. 9.3.1, even when -Werror
> +              # is passed, the -Wmissing-prototypes flag will only
> +              # ever produce a warning when compiling, but, will
> +              # produce an error when a full link is performed.  This
> +              # bug is fixed in later versions of GCC.
> +              #
> +              # To avoid this bug, and detect if -Wmissing-prototypes
> +              # is supported, we do a compile and link here.
> +	      cat confdefs.h - <<_ACEOF >conftest.$ac_ext
> +/* end confdefs.h.  */
> +
> +int
> +main ()
> +{
> +
> +  ;
> +  return 0;
> +}
> +_ACEOF
> +if ac_fn_cxx_try_link "$LINENO"; then :
> +  WARN_CFLAGS="${WARN_CFLAGS} $w"
> +fi
> +rm -f core conftest.err conftest.$ac_objext \
> +    conftest$ac_exeext conftest.$ac_ext
>  	    else
>  	      cat confdefs.h - <<_ACEOF >conftest.$ac_ext
>  /* end confdefs.h.  */
> diff --git a/gdbsupport/warning.m4 b/gdbsupport/warning.m4
> index 46036fa461e..7bb8176b50f 100644
> --- a/gdbsupport/warning.m4
> +++ b/gdbsupport/warning.m4
> @@ -150,6 +150,20 @@ then
>  		[WARN_CFLAGS="${WARN_CFLAGS} $w"],
>  		[]
>  	      )
> +            elif test "x$w" = "x-Wmissing-prototypes"; then
> +              # On some versions of GCC, e.g. 9.3.1, even when -Werror
> +              # is passed, the -Wmissing-prototypes flag will only
> +              # ever produce a warning when compiling, but, will
> +              # produce an error when a full link is performed.  This
> +              # bug is fixed in later versions of GCC.
> +              #
> +              # To avoid this bug, and detect if -Wmissing-prototypes
> +              # is supported, we do a compile and link here.
> +	      AC_LINK_IFELSE(
> +		[AC_LANG_PROGRAM([],[])],
> +		[WARN_CFLAGS="${WARN_CFLAGS} $w"],
> +		[]
> +	      )
>  	    else
>  	      AC_COMPILE_IFELSE(
>  		[AC_LANG_PROGRAM([], [])],
> -- 
> 2.25.4
>