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);
| ^~~~~~~
938a1caf89