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

[Martin Kletzander]

On Mon, May 10, 2021 at 12:03:36AM +0300, Nir Soffer wrote:
>On Fri, May 7, 2021 at 4:24 PM Martin Kletzander <mkletzan at 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 at 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?
>

Strictly speaking I do not think there is any description of what is
supported for libnbd.  I just wanted as much as possible to start with,
but centos7 is now already out.

>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.
>

I did that, we can remove it later.

>Nir
>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 833 bytes
Desc: not available
URL: <http://listman.redhat.com/archives/libguestfs/attachments/20210512/a02edd6a/attachment.sig>