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Re: [Libguestfs] [libnbd PATCH 3/5] libev: Check for old compiler



On 5/6/21 6:30 AM, Martin Kletzander wrote:
> At some point GCC added some optimisations and checks that started emitting
> warnings on harmless code:
> 
>   ...dereferencing type-punned pointer will break strict-aliasing rules...
> 
> This was later fixed, but can still break the build (or report warnings) on old
> GCCs (e.g. 7.5.0) when building the libev example.
> 
> Signed-off-by: Martin Kletzander <mkletzan redhat com>
> ---
>  configure.ac | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/configure.ac b/configure.ac
> index 19b7bfdb5d2a..3b51354346cc 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -247,6 +247,30 @@ PKG_CHECK_MODULES([LIBEV], [libev], [
>          AC_MSG_WARN([ev.h not found, some examples will not be compiled])
>      ])
>  ])
> +
> +AS_IF([test "x$LIBEV_LIBS" != "x"], [
> +    old_CFLAGS="$CFLAGS"
> +    CFLAGS="-Werror=strict-aliasing -O2"
> +    AC_MSG_CHECKING([if the compiler is new enough for good aliasing rules])
> +    AC_COMPILE_IFELSE([
> +        AC_LANG_PROGRAM([
> +            #include <ev.h>
> +
> +            static void cb (struct ev_loop *l, ev_timer *t, int e) { }
> +            static ev_timer timer;
> +        ], [
> +            ev_timer_init (&timer, cb, 0, .1);
> +        ])
> +    ], [
> +        AC_MSG_RESULT([yes])
> +    ], [
> +        AC_MSG_RESULT([no])
> +        AC_SUBST([LIBEV_CFLAGS], [""])
> +        AC_SUBST([LIBEV_LIBS], [""])
> +        AC_MSG_WARN([compiler is probably too old to compile with libev without errors, some examples will not be compiled])

Do we really need to skip compiling the example altogether, or can we
just modify CFLAGS to add -Wno-strict-aliasing to prevent the failure?

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org


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