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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 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_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.

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

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