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

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(a)redhat.com&gt; >> --- >> 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.

But changing the LIBEV_CFLAGS seems more fit for the purpose.