gmacros: Don’t use __alignof__ in G_ALIGNOF implementation

It has different semantics from _Alignof and our G_STRUCT_OFFSET
fallback. See the comments in the diff for details.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://gitlab.gnome.org/GNOME/glib/issues/1055

2 files changed, 10 insertions, 3 deletions

diff --git a/glib/docs.c b/glib/docs.c
index 30f6e6a56..9e5269236 100644
--- a/glib/docs.c
+++ b/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
  */
 
diff --git a/glib/gmacros.h b/glib/gmacros.h
index f8db7e83b..2f3d84746 100644
--- a/glib/gmacros.h
+++ b/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