lisp.h: Fix a problem with aliasing and vector headers.

GCC 4.6.0 optimizes based on type-based alias analysis.  For
example, if b is of type struct buffer * and v of type struct
Lisp_Vector *, then gcc -O2 was incorrectly assuming that &b->size
!= &v->size, and therefore "v->size = 1; b->size = 2; return
v->size;" must therefore return 1.  This assumption is incorrect
for Emacs, since it type-puns struct Lisp_Vector * with many other
types.  To fix this problem, this patch adds a new type struct
vector_header that documents the constraints on layout of vectors
and pseudovectors, and helps optimizing compilers not get fooled
by Emacs's type punning.  It also adds the macros XSETTYPED_PVECTYPE
XSETTYPED_PSEUDOVECTOR, TYPED_PSEUDOVECTORP, for similar reasons.
* lisp.h (XVECTOR_SIZE): New convenience macro.  All previous uses of
XVECTOR (foo)->size replaced to use this macro, to avoid the hassle
of writing XVECTOR (foo)->header.size.
(XVECTOR_HEADER_SIZE): New macro, for use in XSETPSEUDOVECTOR.
(XSETTYPED_PVECTYPE): New macro, specifying the name of the size
member.
(XSETPVECTYPE): Rewrite in terms of new macro.
(XSETPVECTYPESIZE): New macro, specifying both type and size.
This is a bit clearer, and further avoids the possibility of
undesirable aliasing.
(XSETTYPED_PSEUDOVECTOR): New macro, specifying the size.
(XSETPSEUDOVECTOR): Rewrite in terms of XSETTYPED_PSEUDOVECTOR
and XVECTOR_HEADER_SIZE.
(XSETSUBR): Rewrite in terms of XSETTYPED_PSEUDOVECTOR and XSIZE,
since Lisp_Subr is a special case (no "next" field).
(ASIZE): Rewrite in terms of XVECTOR_SIZE.
(struct vector_header): New type.
(TYPED_PSEUDOVECTORP): New macro, also specifying the C type of the
object, to help avoid aliasing.
(PSEUDOVECTORP): Rewrite in terms of TYPED_PSEUDOVECTORP.
(SUBRP): Likewise, since Lisp_Subr is a special case.
* lisp.h (struct Lisp_Vector, struct Lisp_Char_Table):
(struct Lisp_Sub_Char_Table, struct Lisp_Bool_Vector):
(struct Lisp_Hash_Table): Combine first two members into a single
struct vector_header member.  All uses of "size" and "next" members
changed to be "header.size" and "header.next".
* buffer.h (struct buffer): Likewise.
* font.h (struct font_spec, struct font_entity, struct font): Likewise.
* frame.h (struct frame): Likewise.
* process.h (struct Lisp_Process): Likewise.
* termhooks.h (struct terminal): Likewise.
* window.c (struct save_window_data, struct saved_window): Likewise.
* window.h (struct window): Likewise.
* alloc.c (allocate_buffer, Fmake_bool_vector, allocate_pseudovector):
Use XSETPVECTYPESIZE, not XSETPVECTYPE, to avoid aliasing problems.
* buffer.c (init_buffer_once): Likewise.
* lread.c (defsubr): Use XSETTYPED_PVECTYPE, since Lisp_Subr is a
special case.
* process.c (Fformat_network_address): Use local var for size,
for brevity.

1 files changed, 9 insertions, 9 deletions

diff --git a/src/buffer.c b/src/buffer.c
index c649836adb8..6ced4fbae87 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -43,7 +43,7 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 struct buffer *current_buffer;          /* the current buffer */
 
 /* First buffer in chain of all buffers (in reverse order of creation).
-   Threaded through ->next.  */
+   Threaded through ->header.next.  */
 
 struct buffer *all_buffers;
 
@@ -359,7 +359,7 @@ even if it is dead.  The return value is never nil.  */)
   b->prevent_redisplay_optimizations_p = 1;
 
   /* Put this on the chain of all buffers including killed ones.  */
-  b->next = all_buffers;
+  b->header.next.buffer = all_buffers;
   all_buffers = b;
 
   /* An ordinary buffer normally doesn't need markers
@@ -588,7 +588,7 @@ CLONE nil means the indirect buffer's state is reset to default values.  */)
   BVAR (b, width_table) = Qnil;
 
   /* Put this on the chain of all buffers including killed ones.  */
-  b->next = all_buffers;
+  b->header.next.buffer = all_buffers;
   all_buffers = b;
 
   name = Fcopy_sequence (name);
@@ -1458,7 +1458,7 @@ with SIGHUP.  */)
 
       GCPRO1 (buffer);
 
-      for (other = all_buffers; other; other = other->next)
+      for (other = all_buffers; other; other = other->header.next.buffer)
         /* all_buffers contains dead buffers too;
            don't re-kill them.  */
         if (other->base_buffer == b && !NILP (BVAR (other, name)))
@@ -2099,7 +2099,7 @@ DEFUN ("buffer-swap-text", Fbuffer_swap_text, Sbuffer_swap_text,
 
   { /* This is probably harder to make work.  */
     struct buffer *other;
-    for (other = all_buffers; other; other = other->next)
+    for (other = all_buffers; other; other = other->header.next.buffer)
       if (other->base_buffer == other_buffer
           || other->base_buffer == current_buffer)
         error ("One of the buffers to swap has indirect buffers");
@@ -2476,7 +2476,7 @@ current buffer is cleared.  */)
 
   /* Copy this buffer's new multibyte status
      into all of its indirect buffers.  */
-  for (other = all_buffers; other; other = other->next)
+  for (other = all_buffers; other; other = other->header.next.buffer)
     if (other->base_buffer == current_buffer && !NILP (BVAR (other, name)))
       {
         BVAR (other, enable_multibyte_characters)
@@ -4178,7 +4178,7 @@ static int last_overlay_modification_hooks_used;
 static void
 add_overlay_mod_hooklist (Lisp_Object functionlist, Lisp_Object overlay)
 {
-  int oldsize = XVECTOR (last_overlay_modification_hooks)->size;
+  int oldsize = XVECTOR_SIZE (last_overlay_modification_hooks);
 
   if (last_overlay_modification_hooks_used == oldsize)
     last_overlay_modification_hooks = larger_vector
@@ -4973,9 +4973,9 @@ init_buffer_once (void)
   buffer_local_symbols.text = &buffer_local_symbols.own_text;
   BUF_INTERVALS (&buffer_defaults) = 0;
   BUF_INTERVALS (&buffer_local_symbols) = 0;
-  XSETPVECTYPE (&buffer_defaults, PVEC_BUFFER);
+  XSETPVECTYPESIZE (&buffer_defaults, PVEC_BUFFER, 0);
   XSETBUFFER (Vbuffer_defaults, &buffer_defaults);
-  XSETPVECTYPE (&buffer_local_symbols, PVEC_BUFFER);
+  XSETPVECTYPESIZE (&buffer_local_symbols, PVEC_BUFFER, 0);
   XSETBUFFER (Vbuffer_local_symbols, &buffer_local_symbols);
 
   /* Set up the default values of various buffer slots.  */