Shrink pseudovectors a bit

sizeof (struct Lisp_Marker) was 32 on x86, where 24 would do.
Problem noted by Stefan Monnier in:
https://lists.gnu.org/r/emacs-devel/2018-09/msg00165.html
* src/bignum.h (struct Lisp_Bignum):
* src/frame.h (struct frame):
* src/lisp.h (struct Lisp_Vector, struct Lisp_Bool_Vector)
(struct Lisp_Char_Table, struct Lisp_Hash_Table)
(struct Lisp_Marker, struct Lisp_Overlay)
(struct Lisp_Misc_Ptr, struct Lisp_User_Ptr)
(struct Lisp_Finalizer, struct Lisp_Float)
(struct Lisp_Module_Function):
* src/process.h (struct Lisp_Process):
* src/termhooks.h (struct terminal):
* src/thread.h (struct thread_state, struct Lisp_Mutex)
(struct Lisp_CondVar):
* src/window.c (struct save_window_data):
* src/window.h (struct window):
* src/xterm.h (struct scroll_bar):
* src/xwidget.h (struct xwidget, struct xwidget_view):
Add GCALIGNED_STRUCT attribute.
* src/lisp.h (GCALIGNED_UNION_MEMBER): Renamed from GCALIGNED_UNION.
All uses changed.
(GCALIGNED_STRUCT_MEMBER, GCALIGNED_STRUCT, GCALIGNED): New macros.
All uses of open-coded GCALIGNED changed to use GCALIGNED.
(union vectorlike_header): No longer GC-aligned.
(PSEUDOVECSIZE): Yield 0 for pseudovectors without Lisp
objects that place a member before where the first Lisp object
member would be.

1 files changed, 5 insertions, 3 deletions

diff --git a/src/alloc.c b/src/alloc.c
index 28ca7804ee9..abb98a9eb68 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -641,9 +641,11 @@ buffer_memory_full (ptrdiff_t nbytes)
    implement Lisp objects; since pseudovectors can contain any C type,
    this is max_align_t.  On recent GNU/Linux x86 and x86-64 this can
    often waste up to 8 bytes, 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 this waste.  */
-enum { LISP_ALIGNMENT = alignof (union { max_align_t x; GCALIGNED_UNION }) };
+   typical vectors need only an alignment of 8.  Although shrinking
+   the alignment to 8 would save memory, it cost a 20% hit to Emacs
+   CPU performance on Fedora 28 x86-64 when compiled with gcc -m32.  */
+enum { LISP_ALIGNMENT = alignof (union { max_align_t x;
+                                         GCALIGNED_UNION_MEMBER }) };
 verify (LISP_ALIGNMENT % GCALIGNMENT == 0);
 
 /* True if malloc (N) is known to return storage suitably aligned for