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



On Fri, May 7, 2021 at 4:24 PM Martin Kletzander <mkletzan redhat com> wrote:
>
> On Fri, May 07, 2021 at 07:14:32AM -0500, Eric Blake wrote:
> >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?
> >
>
> This is one of the results that I was not sure about.  We could just add
> no-strict-aliasing for that one particular example of course.  I figured
> it is just an example and it only causes an issue on CentOS 7, OpenSUSE
> Leap 15.2, Ubuntu 18.04 and some similarly old systems if I remember
> correctly.

Do we really support Centos 7? libnbd is available since rhel 8 (2, 3?).
Why support older systems?

Time spent on older systems means less time working features,
performance, support for new systems, etc.

> But changing the LIBEV_CFLAGS seems more fit for the purpose.

Sounds like the best option.

Nir


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