Re: [PATCH] Fix -Wno-unknown-warning support detection

[Simon Marchi <simon.marchi@ericsson.com>]

On 15-12-16 05:57 PM, Pedro Alves wrote:
> Ref: https://sourceware.org/ml/gdb/2015-12/msg00024.html
> 
> We have code in configure.ac that tries to detect whether the compiler
> supports each warning and suppress it if not, but that doesn't work
> with "-Wno-" options, because gcc doesn't error out for
> -Wno-unknown-warning unless other diagnostics are being produced.
> 
> See https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html.
> 
> Handle this by checking whether -Wfoo works when we actually want
> -Wno-foo.
> 
> gdb/ChangeLog:
> 2015-12-16  Pedro Alves  <palves@redhat.com>
> 
> 	* configure.ac (compiler warning flags): When testing a
> 	-Wno-foo option, check whether -Wfoo works instead.
> 	* configure: Regenerate.
> 
> gdb/gdbserver/ChangeLog:
> 2015-12-16  Pedro Alves  <palves@redhat.com>
> 
> 	* configure.ac (compiler warning flags): When testing a
> 	-Wno-foo option, check whether -Wfoo works instead.
> 	* configure: Regenerate.
> ---
>  gdb/configure              | 13 +++++++++++--
>  gdb/configure.ac           | 13 +++++++++++--
>  gdb/gdbserver/configure    | 13 +++++++++++--
>  gdb/gdbserver/configure.ac | 13 +++++++++++--
>  4 files changed, 44 insertions(+), 8 deletions(-)
> 
> diff --git a/gdb/configure b/gdb/configure
> index 249a399..4c308cb 100755
> --- a/gdb/configure
> +++ b/gdb/configure
> @@ -14376,14 +14376,23 @@ $as_echo_n "checking compiler warning flags... " >&6; }
>      # Separate out the -Werror flag as some files just cannot be
>      # compiled with it enabled.
>      for w in ${build_warnings}; do
> +	# GCC does not complain about -Wno-unknown-warning.  Invert
> +	# and test -Wunknown-warning instead.
> +	case $w in
> +	-Wno-*)
> +		wtest=`echo $w | sed 's/-Wno-/-W/g'` ;;
> +	*)
> +		wtest=$w ;;
> +	esac
> +
>  	case $w in
>  	-Werr*) WERROR_CFLAGS=-Werror ;;
>  	*)
>  	    # Check whether GCC accepts it.
>  	    saved_CFLAGS="$CFLAGS"
> -	    CFLAGS="$CFLAGS $w"
> +	    CFLAGS="$CFLAGS $wtest"
>  	    saved_CXXFLAGS="$CXXFLAGS"
> -	    CXXFLAGS="$CXXFLAGS $w"
> +	    CXXFLAGS="$CXXFLAGS $wtest"
>  	    cat confdefs.h - <<_ACEOF >conftest.$ac_ext
>  /* end confdefs.h.  */
>  
> diff --git a/gdb/configure.ac b/gdb/configure.ac
> index ebd797b..b9d8f8b 100644
> --- a/gdb/configure.ac
> +++ b/gdb/configure.ac
> @@ -2007,14 +2007,23 @@ then
>      # Separate out the -Werror flag as some files just cannot be
>      # compiled with it enabled.
>      for w in ${build_warnings}; do
> +	# GCC does not complain about -Wno-unknown-warning.  Invert
> +	# and test -Wunknown-warning instead.
> +	case $w in
> +	-Wno-*)
> +		wtest=`echo $w | sed 's/-Wno-/-W/g'` ;;
> +	*)
> +		wtest=$w ;;
> +	esac
> +
>  	case $w in
>  	-Werr*) WERROR_CFLAGS=-Werror ;;
>  	*)
>  	    # Check whether GCC accepts it.
>  	    saved_CFLAGS="$CFLAGS"
> -	    CFLAGS="$CFLAGS $w"
> +	    CFLAGS="$CFLAGS $wtest"
>  	    saved_CXXFLAGS="$CXXFLAGS"
> -	    CXXFLAGS="$CXXFLAGS $w"
> +	    CXXFLAGS="$CXXFLAGS $wtest"
>  	    AC_TRY_COMPILE([],[],WARN_CFLAGS="${WARN_CFLAGS} $w",)
>  	    CFLAGS="$saved_CFLAGS"
>  	    CXXFLAGS="$saved_CXXFLAGS"
> diff --git a/gdb/gdbserver/configure b/gdb/gdbserver/configure
> index ccb9639..ab03cac 100755
> --- a/gdb/gdbserver/configure
> +++ b/gdb/gdbserver/configure
> @@ -6148,14 +6148,23 @@ $as_echo_n "checking compiler warning flags... " >&6; }
>      # Separate out the -Werror flag as some files just cannot be
>      # compiled with it enabled.
>      for w in ${build_warnings}; do
> +	# GCC does not complain about -Wno-unknown-warning.  Invert
> +	# and test -Wunknown-warning instead.
> +	case $w in
> +	-Wno-*)
> +		wtest=`echo $w | sed 's/-Wno-/-W/g'` ;;
> +	*)
> +		wtest=$w ;;
> +	esac
> +
>  	case $w in
>  	-Werr*) WERROR_CFLAGS=-Werror ;;
>  	*)
>  	    # Check whether GCC accepts it.
>  	    saved_CFLAGS="$CFLAGS"
> -	    CFLAGS="$CFLAGS $w"
> +	    CFLAGS="$CFLAGS $wtest"
>  	    saved_CXXFLAGS="$CXXFLAGS"
> -	    CXXFLAGS="$CXXFLAGS $w"
> +	    CXXFLAGS="$CXXFLAGS $wtest"
>  	    cat confdefs.h - <<_ACEOF >conftest.$ac_ext
>  /* end confdefs.h.  */
>  
> diff --git a/gdb/gdbserver/configure.ac b/gdb/gdbserver/configure.ac
> index 5524a05..6caf91c 100644
> --- a/gdb/gdbserver/configure.ac
> +++ b/gdb/gdbserver/configure.ac
> @@ -193,14 +193,23 @@ then
>      # Separate out the -Werror flag as some files just cannot be
>      # compiled with it enabled.
>      for w in ${build_warnings}; do
> +	# GCC does not complain about -Wno-unknown-warning.  Invert
> +	# and test -Wunknown-warning instead.
> +	case $w in
> +	-Wno-*)
> +		wtest=`echo $w | sed 's/-Wno-/-W/g'` ;;
> +	*)
> +		wtest=$w ;;
> +	esac
> +
>  	case $w in
>  	-Werr*) WERROR_CFLAGS=-Werror ;;
>  	*)
>  	    # Check whether GCC accepts it.
>  	    saved_CFLAGS="$CFLAGS"
> -	    CFLAGS="$CFLAGS $w"
> +	    CFLAGS="$CFLAGS $wtest"
>  	    saved_CXXFLAGS="$CXXFLAGS"
> -	    CXXFLAGS="$CXXFLAGS $w"
> +	    CXXFLAGS="$CXXFLAGS $wtest"
>  	    AC_TRY_COMPILE([],[],WARN_CFLAGS="${WARN_CFLAGS} $w",)
>  	    CFLAGS="$saved_CFLAGS"
>  	    CXXFLAGS="$saved_CXXFLAGS"
> 

It looks good.  Note that you could have done

  wtest=`echo $w | sed 's/-Wno-/-W/g'`

unconditionally.  If it doesn't match, it won't replace anything:

$ echo -Wlol | sed 's/-Wno-/-W/g'
-Wlol
$ echo -Wno-lol | sed 's/-Wno-/-W/g'
-Wlol