[glib] glib: Fix strict-aliasing warnings with g_clear_pointer()

[Philip Withnall <pwithnall src gnome org>]

commit 97d24b93ab05762ec53785b5ec1c68e8d660b054
Author: Philip Withnall <withnall endlessm com>
Date:   Thu Dec 21 17:47:29 2017 +0000

    glib: Fix strict-aliasing warnings with g_clear_pointer()
    
    gpointer* cannot be aliased with arbitrary types. In order to fix
    -Wstrict-aliasing=2 warnings with the g_clear_pointer() macro, we need
    to cast through char*, which is allowed to alias with anything.
    
    Even if we don’t make GLib strict-aliasing safe, it’s important to
    ensure this macro is safe, since it could be used from projects which do
    compile with -fstrict-aliasing.
    
    Signed-off-by: Philip Withnall <withnall endlessm com>
    
    https://bugzilla.gnome.org/show_bug.cgi?id=791622

 glib/gmem.h |    9 +++++----
 1 files changed, 5 insertions(+), 4 deletions(-)
---
diff --git a/glib/gmem.h b/glib/gmem.h
index 462d49e..9530512 100644
--- a/glib/gmem.h
+++ b/glib/gmem.h
@@ -113,16 +113,17 @@ gpointer g_try_realloc_n  (gpointer        mem,
 #define g_clear_pointer(pp, destroy) \
   G_STMT_START {                                                               \
     G_STATIC_ASSERT (sizeof *(pp) == sizeof (gpointer));                       \
-    /* Only one access, please */                                              \
-    gpointer *_pp = (gpointer *) (pp);                                         \
+    /* Only one access, please; work around type aliasing */                   \
+    union { char *in; gpointer *out; } _pp;                                    \
     gpointer _p;                                                               \
     /* This assignment is needed to avoid a gcc warning */                     \
     GDestroyNotify _destroy = (GDestroyNotify) (destroy);                      \
                                                                                \
-    _p = *_pp;                                                                 \
+    _pp.in = (char *) (pp);                                                    \
+    _p = *_pp.out;                                                             \
     if (_p)                                                                   \
       {                                                                       \
-        *_pp = NULL;                                                          \
+        *_pp.out = NULL;                                                       \
         _destroy (_p);                                                         \
       }                                                                        \
   } G_STMT_END