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/fns.c b/src/fns.c
index 20190399f9a..a128b9292c6 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -3681,9 +3681,9 @@ copy_hash_table (struct Lisp_Hash_Table *h1)
   struct Lisp_Vector *next;
 
   h2 = allocate_hash_table ();
-  next = h2->vec_next;
+  next = h2->header.next.vector;
   memcpy (h2, h1, sizeof *h2);
-  h2->vec_next = next;
+  h2->header.next.vector = next;
   h2->key_and_value = Fcopy_sequence (h1->key_and_value);
   h2->hash = Fcopy_sequence (h1->hash);
   h2->next = Fcopy_sequence (h1->next);
@@ -4026,7 +4026,7 @@ sweep_weak_hash_tables (void)
       marked = 0;
       for (h = weak_hash_tables; h; h = h->next_weak)
 	{
-	  if (h->size & ARRAY_MARK_FLAG)
+	  if (h->header.size & ARRAY_MARK_FLAG)
 	    marked |= sweep_weak_table (h, 0);
 	}
     }
@@ -4037,7 +4037,7 @@ sweep_weak_hash_tables (void)
     {
       next = h->next_weak;
 
-      if (h->size & ARRAY_MARK_FLAG)
+      if (h->header.size & ARRAY_MARK_FLAG)
 	{
 	  /* TABLE is marked as used.  Sweep its contents.  */
 	  if (h->count > 0)
@@ -4153,7 +4153,7 @@ sxhash_bool_vector (Lisp_Object vec)
   unsigned hash = XBOOL_VECTOR (vec)->size;
   int i, n;
 
-  n = min (SXHASH_MAX_LEN, XBOOL_VECTOR (vec)->vector_size);
+  n = min (SXHASH_MAX_LEN, XBOOL_VECTOR (vec)->header.size);
   for (i = 0; i < n; ++i)
     hash = SXHASH_COMBINE (hash, XBOOL_VECTOR (vec)->data[i]);