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gtype: avoid "-Wcast-align" warning with optimized G_TYPE_CHECK_INSTANCE_CAST()

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We can get a "-Wcast-align", if the target type that we cast to ("ct") has a
larger alignment than GTypeInstance.

That can happen on i686 architecture, if the GObject type has larger
alignment than the parent struct (or GObject). Since on i686, embeding
a "long long" or a "long double" in a struct still does not increase
the alignment beyond 4 bytes, this usually only happens when using the
__attribute__() to increase the alignment (or to have a field that has
the alignment increased).

It can happen on x86_64 when having a "long double" field.

The compiler warning is hard to avoid but not very useful, because it purely
operates on the pointer types at compile time. G_TYPE_CHECK_INSTANCE_CAST()
instead asserts (in non-optimized mode) that the pointer really points
to the expected GTypeInstance (and if that's the case, then the alignment
should be suitable already).

This is like in commit ed553e8e30 ('gtype: Eliminate -Wcast-align warnings
with G_TYPE_CHECK_INSTANCE_CAST'). But also fix the optimized code path.

With the unpatched G_TYPE_CHECK_INSTANCE_CAST() macro, the unit test would
now show the problem (with gcc-9.3.1-2.fc30.i686 or
gcc-12.2.1-4.fc37.x86_64):

  $ export G_DISABLE_CAST_CHECKS=1
  $ export CFLAGS='-Wcast-align=strict'
  $ meson build
  $ ninja -C build
  ...
  In file included from ../gobject/gobject.h:26,
                   from ../gobject/gbinding.h:31,
                   from ../glib/glib-object.h:24,
                   from ../gobject/tests/objects-refcount1.c:2:
  ../gobject/tests/objects-refcount1.c: In function ‘my_test_dispose’:
  ../gobject/gtype.h:2523:42: warning: cast increases required alignment of target type [-Wcast-align]
   2523 | #  define _G_TYPE_CIC(ip, gt, ct)       ((ct*) ip)
        |                                          ^
  ../gobject/gtype.h:517:66: note: in expansion of macro ‘_G_TYPE_CIC’
    517 | #define G_TYPE_CHECK_INSTANCE_CAST(instance, g_type, c_type)    (_G_TYPE_CIC ((instance), (g_type), c_type))
        |                                                                  ^~~~~~~~~~~
  ../gobject/tests/objects-refcount1.c:9:37: note: in expansion of macro ‘G_TYPE_CHECK_INSTANCE_CAST’
      9 | #define MY_TEST(test)              (G_TYPE_CHECK_INSTANCE_CAST ((test), G_TYPE_TEST, GTest))
        |                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~
  ../gobject/tests/objects-refcount1.c:96:10: note: in expansion of macro ‘MY_TEST’
     96 |   test = MY_TEST (object);
        |          ^~~~~~~
This commit is contained in:
Thomas Haller 2022-12-15 18:29:08 +01:00 committed by Marco Trevisan (Treviño)
parent ded3099afc
commit 938a1caf89
2 changed files with 20 additions and 3 deletions
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4
gobject/gtype.h
View File

@ -2520,8 +2520,8 @@ const gchar * g_type_name_from_class (GTypeClass *g_class);
/* --- implementation bits --- */
#if defined(G_DISABLE_CAST_CHECKS) || defined(__OPTIMIZE__)
# define _G_TYPE_CIC(ip, gt, ct) ((ct*) ip)
# define _G_TYPE_CCC(cp, gt, ct) ((ct*) cp)
# define _G_TYPE_CIC(ip, gt, ct) ((ct*) (void *) ip)
# define _G_TYPE_CCC(cp, gt, ct) ((ct*) (void *) cp)
#else
# define _G_TYPE_CIC(ip, gt, ct) \
((ct*) (void *) g_type_check_instance_cast ((GTypeInstance*) ip, gt))

19
gobject/tests/objects-refcount1.c
View File

@ -15,10 +15,27 @@
typedef struct _GTest GTest;
typedef struct _GTestClass GTestClass;
#if G_GNUC_CHECK_VERSION (4, 0)
/* Increase the alignment of GTest to check whether
* G_TYPE_CHECK_INSTANCE_CAST() would trigger a "-Wcast-align=strict" warning.
* That would happen, when trying to cast a "GObject*" to "GTest*", if latter
* has larger alignment.
*
* Note that merely adding a int64 field to GTest does not increase the
* alignment above 4 bytes on i386, hence use the __attribute__((__aligned__())).
*/
#define _GTest_increase_alignment __attribute__((__aligned__(__alignof(gint64))))
#else
#define _GTest_increase_alignment
#endif
struct _GTest
{
GObject object;
};
/* See _GTest_increase_alignment. */
long double increase_alignment2;
} _GTest_increase_alignment;
struct _GTestClass
{