Libguestfs - May 2021 - [libnbd PATCH 3/5] libev: Check for old compiler

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])
+    ])
+    CFLAGS="$old_CFLAGS"
+])
 AM_CONDITIONAL([HAVE_LIBEV], [test "x$LIBEV_LIBS" != "x"])
 
 dnl FUSE 3 is optional to build the nbdfuse program.
-- 
2.31.1

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?

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

Patch already discussed extensively.  I'm happy with what seems to
have been decided (adding -Wno-strict-aliasing I think).

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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live CD or over the network (PXE) and turn machines into KVM guests.
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