Merge branch '1055-will-the-alignment-never-end' into 'master'

gmacros: Don’t use __alignof__ in G_ALIGNOF implementation

Closes #1055

See merge request GNOME/glib!559

glib/docs.c

@@ -1870,11 +1870,16 @@
  * G_ALIGNOF
  * @a: a type-name
  *
- * Return the minimum alignment required by the platform ABI for values of the given
+ * Return the minimal alignment required by the platform ABI for values of the given
  * type. The address of a variable or struct member of the given type must always be
  * a multiple of this alignment. For example, most platforms require int variables
  * to be aligned at a 4-byte boundary, so `G_ALIGNOF (int)` is 4 on most platforms.
  *
+ * Note this is not necessarily the same as the value returned by GCC’s
+ * `__alignof__` operator, which returns the preferred alignment for a type.
+ * The preferred alignment may be a stricter alignment than the minimal
+ * alignment.
+ *
  * Since: 2.60
  */
 

glib/gmacros.h

@@ -406,12 +406,14 @@
 #endif
 
 /* Provide G_ALIGNOF alignment macro.
+ *
+ * Note we cannot use the gcc __alignof__ operator here, as that returns the
+ * preferred alignment rather than the minimal alignment. See
+ * https://gitlab.gnome.org/GNOME/glib/merge_requests/538/diffs#note_390790.
  */
 
 #if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L && !defined(__cplusplus)
 #define G_ALIGNOF(type) _Alignof (type)
-#elif defined(__GNUC__)
-#define G_ALIGNOF(type) (__alignof__ (type))
 #else
 #define G_ALIGNOF(type) (G_STRUCT_OFFSET (struct { char a; type b; }, b))
 #endif

glib/tests/Makefile.am

@@ -70,6 +70,7 @@ test_programs = \
 	keyfile				\
 	list				\
 	logging				\
+	macros				\
 	mainloop			\
 	mappedfile			\
 	markup				\

glib/tests/macros.c

@@ -0,0 +1,53 @@
+/* GLib testing framework examples and tests
+ *
+ * Copyright © 2018 Endless Mobile, Inc.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General
+ * Public License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ *
+ * Author: Philip Withnall <withnall@endlessm.com>
+ */
+
+#include <glib.h>
+
+/* Test G_ALIGNOF() gives the same results as the G_STRUCT_OFFSET fallback. This
+ * should be the minimal alignment for the given type.
+ *
+ * This is necessary because the implementation of G_ALIGNOF() varies depending
+ * on the compiler in use. We want all implementations to be consistent.
+ *
+ * In the case that the compiler uses the G_STRUCT_OFFSET fallback, this test
+ * is a no-op. */
+static void
+test_alignof_fallback (void)
+{
+#define check_alignof(type) \
+  g_assert_cmpint (G_ALIGNOF (type), ==, G_STRUCT_OFFSET (struct { char a; type b; }, b))
+
+  check_alignof (char);
+  check_alignof (int);
+  check_alignof (float);
+  check_alignof (double);
+  check_alignof (struct { char a; int b; });
+}
+
+int
+main (int   argc,
+      char *argv[])
+{
+  g_test_init (&argc, &argv, NULL);
+
+  g_test_add_func ("/alignof/fallback", test_alignof_fallback);
+
+  return g_test_run ();
+}

glib/tests/meson.build

@@ -38,6 +38,7 @@ glib_tests = {
   'keyfile' : {},
   'list' : {},
   'logging' : {},
+  'macros' : {},
   'mainloop' : {},
   'mappedfile' : {},
   'markup' : {},