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, 6 insertions, 6 deletions

diff --git a/src/indent.c b/src/indent.c
index 094f6dabd55..a5ac2ab1d0c 100644
--- a/src/indent.c
+++ b/src/indent.c
@@ -93,7 +93,7 @@ character_width (int c, struct Lisp_Char_Table *dp)
   /* Everything can be handled by the display table, if it's
      present and the element is right.  */
   if (dp && (elt = DISP_CHAR_VECTOR (dp, c), VECTORP (elt)))
-    return XVECTOR (elt)->size;
+    return XVECTOR_SIZE (elt);
 
   /* Some characters are special.  */
   if (c == '\n' || c == '\t' || c == '\015')
@@ -121,7 +121,7 @@ disptab_matches_widthtab (struct Lisp_Char_Table *disptab, struct Lisp_Vector *w
 {
   int i;
 
-  if (widthtab->size != 256)
+  if (widthtab->header.size != 256)
     abort ();
 
   for (i = 0; i < 256; i++)
@@ -143,7 +143,7 @@ recompute_width_table (struct buffer *buf, struct Lisp_Char_Table *disptab)
   if (!VECTORP (BVAR (buf, width_table)))
     BVAR (buf, width_table) = Fmake_vector (make_number (256), make_number (0));
   widthtab = XVECTOR (BVAR (buf, width_table));
-  if (widthtab->size != 256)
+  if (widthtab->header.size != 256)
     abort ();
 
   for (i = 0; i < 256; i++)
@@ -284,7 +284,7 @@ skip_invisible (EMACS_INT pos, EMACS_INT *next_boundary_p, EMACS_INT to, Lisp_Ob
     else								\
       {									\
 	if (dp != 0 && VECTORP (DISP_CHAR_VECTOR (dp, ch)))		\
-	  width = XVECTOR (DISP_CHAR_VECTOR (dp, ch))->size;		\
+	  width = XVECTOR_SIZE (DISP_CHAR_VECTOR (dp, ch));		\
 	else								\
 	  width = CHAR_WIDTH (ch);					\
       }									\
@@ -766,7 +766,7 @@ string_display_width (string, beg, end)
 
       c = *--ptr;
       if (dp != 0 && VECTORP (DISP_CHAR_VECTOR (dp, c)))
-	col += XVECTOR (DISP_CHAR_VECTOR (dp, c))->size;
+	col += XVECTOR_SIZE (DISP_CHAR_VECTOR (dp, c));
       else if (c >= 040 && c < 0177)
 	col++;
       else if (c == '\n')
@@ -1127,7 +1127,7 @@ compute_motion (EMACS_INT from, EMACS_INT fromvpos, EMACS_INT fromhpos, int did_
        : !NILP (BVAR (current_buffer, selective_display)) ? -1 : 0);
   int selective_rlen
     = (selective && dp && VECTORP (DISP_INVIS_VECTOR (dp))
-       ? XVECTOR (DISP_INVIS_VECTOR (dp))->size : 0);
+       ? XVECTOR_SIZE (DISP_INVIS_VECTOR (dp)) : 0);
   /* The next location where the `invisible' property changes, or an
      overlay starts or ends.  */
   EMACS_INT next_boundary = from;