[Emacs-diffs] emacs-26 d703802: Do not under-align pseudovectors

[Paul Eggert]

branch: emacs-26
commit d7038020aa0a868c74cddaed5733ad6eeb73406b
Author: Paul Eggert <address@hidden>
Commit: Paul Eggert <address@hidden>

    Do not under-align pseudovectors
    
    Problem reported by Fabrice Popineau in:
    https://lists.gnu.org/archive/html/emacs-devel/2017-10/msg00357.html
    Also see diagnosis by Eli Zaretskii in:
    https://lists.gnu.org/archive/html/emacs-devel/2017-10/msg00476.html
    * src/alloc.c (vector_alignment):
    Now a multiple of alignof (max_align_t).
---
 src/alloc.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/src/alloc.c b/src/alloc.c
index 2e6399e..da0c3ad 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -2923,9 +2923,13 @@ set_next_vector (struct Lisp_Vector *v, struct 
Lisp_Vector *p)
 
 enum
   {
-    /* Alignment of struct Lisp_Vector objects.  */
-    vector_alignment = COMMON_MULTIPLE (FLEXALIGNOF (struct Lisp_Vector),
-                                        GCALIGNMENT),
+    /* Alignment of struct Lisp_Vector objects.  Because pseudovectors
+       can contain any C type, align at least as strictly as
+       max_align_t.  On x86 and x86-64 this can waste up to 8 bytes
+       for typical vectors, since alignof (max_align_t) is 16 but
+       typical vectors need only an alignment of 8.  However, it is
+       not worth the hassle to avoid wasting those bytes.  */
+    vector_alignment = COMMON_MULTIPLE (alignof (max_align_t), GCALIGNMENT),
 
     /* Vector size requests are a multiple of this.  */
     roundup_size = COMMON_MULTIPLE (vector_alignment, word_size)