[ChangeLog]

* configure.in (GC_LISP_OBJECT_ALIGNMENT): Remove.
This is now done by src/alloc.c.
[src/ChangeLog]
* alloc.c: (GC_LISP_OBJECT_ARGUMENT):
Use offsetof, not __alignof__ or sizeof.  __alignof__ gives
the wrong answer on the x86 with GCC.

2 files changed, 6 insertions, 9 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index 3432652dbe5..c3e03c211f5 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -14,6 +14,9 @@
         Use int, not EMACS_INT, where int is wide enough.
         (inhibit_garbage_collection, Fgarbage_collect):
         Use ptrdiff_t, not int, to avoid needless 32-bit limit on 64-bit hosts.
+        (GC_LISP_OBJECT_ARGUMENT):
+        Use offsetof, not __alignof__ or sizeof.  __alignof__ gives
+        the wrong answer on the x86 with GCC.
         * bidi.c (bidi_mirror_char): Use EMACS_INT, not int, where
         int might not be wide enough.
         (bidi_cache_search, bidi_cache_find, bidi_init_it)
diff --git a/src/alloc.c b/src/alloc.c
index bfb40e6c09a..100a5e593fa 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -4455,10 +4455,7 @@ dump_zombies (void)
    pass starting at the start of the stack + 2.  Likewise, if the
    minimal alignment of Lisp_Objects on the stack is 1, four passes
    would be necessary, each one starting with one byte more offset
-   from the stack start.
-
-   The current code assumes by default that Lisp_Objects are aligned
-   equally on the stack.  */
+   from the stack start.  */
 
 static void
 mark_stack (void)
@@ -4522,11 +4519,8 @@ mark_stack (void)
      that's not the case, something has to be done here to iterate
      over the stack segments.  */
 #ifndef GC_LISP_OBJECT_ALIGNMENT
-#ifdef __GNUC__
-#define GC_LISP_OBJECT_ALIGNMENT __alignof__ (Lisp_Object)
-#else
-#define GC_LISP_OBJECT_ALIGNMENT sizeof (Lisp_Object)
-#endif
+# define GC_LISP_OBJECT_ALIGNMENT \
+    offsetof (struct {char a; Lisp_Object b;}, b)
 #endif
   for (i = 0; i < sizeof (Lisp_Object); i += GC_LISP_OBJECT_ALIGNMENT)
     mark_memory (stack_base, end, i);