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, 5 insertions, 5 deletions

diff --git a/src/keymap.c b/src/keymap.c
index 8713bcf1279..110447b19ff 100644
--- a/src/keymap.c
+++ b/src/keymap.c
@@ -359,7 +359,7 @@ Return PARENT.  PARENT should be nil or another keymap.  */)
                                 XCDR (XCAR (list)));
 
       if (VECTORP (XCAR (list)))
-        for (i = 0; i < XVECTOR (XCAR (list))->size; i++)
+        for (i = 0; i < XVECTOR_SIZE (XCAR (list)); i++)
           if (CONSP (XVECTOR (XCAR (list))->contents[i]))
             fix_submap_inheritance (keymap, make_number (i),
                                     XVECTOR (XCAR (list))->contents[i]);
@@ -2226,7 +2226,7 @@ spaces are put between sequence elements, etc.  */)
   if (STRINGP (list))
     size = SCHARS (list);
   else if (VECTORP (list))
-    size = XVECTOR (list)->size;
+    size = XVECTOR_SIZE (list);
   else if (CONSP (list))
     size = XINT (Flength (list));
   else
@@ -3125,7 +3125,7 @@ key             binding\n\
 
           elt = XCAR (list);
           elt_prefix = Fcar (elt);
-          if (XVECTOR (elt_prefix)->size >= 1)
+          if (XVECTOR_SIZE (elt_prefix) >= 1)
             {
               tem = Faref (elt_prefix, make_number (0));
               if (EQ (tem, Qmenu_bar))
@@ -3168,7 +3168,7 @@ key             binding\n\
           /* If the sequence by which we reach this keymap is zero-length,
              then the shadow map for this keymap is just SHADOW.  */
           if ((STRINGP (elt_prefix) && SCHARS (elt_prefix) == 0)
-              || (VECTORP (elt_prefix) && XVECTOR (elt_prefix)->size == 0))
+              || (VECTORP (elt_prefix) && XVECTOR_SIZE (elt_prefix) == 0))
             ;
           /* If the sequence by which we reach this keymap actually has
              some elements, then the sequence's definition in SHADOW is
@@ -3592,7 +3592,7 @@ describe_vector (Lisp_Object vector, Lisp_Object prefix, Lisp_Object args,
   if (CHAR_TABLE_P (vector))
     stop = MAX_5_BYTE_CHAR + 1, to = MAX_CHAR + 1;
   else
-    stop = to = XVECTOR (vector)->size;
+    stop = to = XVECTOR_SIZE (vector);
 
   for (i = from; ; i++)
     {