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, 4 insertions, 4 deletions

diff --git a/src/data.c b/src/data.c
index 4e81c80d0e..68ea503d61 100644
--- a/src/data.c
+++ b/src/data.c
@@ -1389,7 +1389,7 @@ for this variable.  */)
 	      {
 		struct buffer *b;
 
-		for (b = all_buffers; b; b = b->next)
+		for (b = all_buffers; b; b = b->header.next.buffer)
 		  if (!PER_BUFFER_VALUE_P (b, idx))
 		    PER_BUFFER_VALUE (b, offset) = value;
 	      }
@@ -2093,9 +2093,9 @@ or a byte-code object.  IDX starts at 0.  */)
     {
       int size = 0;
       if (VECTORP (array))
-	size = XVECTOR (array)->size;
+	size = XVECTOR_SIZE (array);
       else if (COMPILEDP (array))
-	size = XVECTOR (array)->size & PSEUDOVECTOR_SIZE_MASK;
+	size = XVECTOR_SIZE (array) & PSEUDOVECTOR_SIZE_MASK;
       else
 	wrong_type_argument (Qarrayp, array);
 
@@ -2120,7 +2120,7 @@ bool-vector.  IDX starts at 0.  */)
 
   if (VECTORP (array))
     {
-      if (idxval < 0 || idxval >= XVECTOR (array)->size)
+      if (idxval < 0 || idxval >= XVECTOR_SIZE (array))
 	args_out_of_range (array, idx);
       XVECTOR (array)->contents[idxval] = newelt;
     }