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, 8 insertions, 7 deletions

diff --git a/src/process.c b/src/process.c
index f25d10dbd67..89a5f3e0386 100644
--- a/src/process.c
+++ b/src/process.c
@@ -1188,25 +1188,26 @@ Returns nil if format of ADDRESS is invalid.  */)
   if (VECTORP (address))  /* AF_INET or AF_INET6 */
     {
       register struct Lisp_Vector *p = XVECTOR (address);
+      EMACS_UINT size = p->header.size;
       Lisp_Object args[10];
       int nargs, i;
 
-      if (p->size == 4 || (p->size == 5 && !NILP (omit_port)))
+      if (size == 4 || (size == 5 && !NILP (omit_port)))
         {
           args[0] = build_string ("%d.%d.%d.%d");
           nargs = 4;
         }
-      else if (p->size == 5)
+      else if (size == 5)
         {
           args[0] = build_string ("%d.%d.%d.%d:%d");
           nargs = 5;
         }
-      else if (p->size == 8 || (p->size == 9 && !NILP (omit_port)))
+      else if (size == 8 || (size == 9 && !NILP (omit_port)))
         {
           args[0] = build_string ("%x:%x:%x:%x:%x:%x:%x:%x");
           nargs = 8;
         }
-      else if (p->size == 9)
+      else if (size == 9)
         {
           args[0] = build_string ("[%x:%x:%x:%x:%x:%x:%x:%x]:%d");
           nargs = 9;
@@ -2064,13 +2065,13 @@ get_lisp_to_sockaddr_size (Lisp_Object address, int *familyp)
   if (VECTORP (address))
     {
       p = XVECTOR (address);
-      if (p->size == 5)
+      if (p->header.size == 5)
         {
           *familyp = AF_INET;
           return sizeof (struct sockaddr_in);
         }
 #ifdef AF_INET6
-      else if (p->size == 9)
+      else if (p->header.size == 9)
         {
           *familyp = AF_INET6;
           return sizeof (struct sockaddr_in6);
@@ -2089,7 +2090,7 @@ get_lisp_to_sockaddr_size (Lisp_Object address, int *familyp)
       struct sockaddr *sa;
       *familyp = XINT (XCAR (address));
       p = XVECTOR (XCDR (address));
-      return p->size + sizeof (sa->sa_family);
+      return p->header.size + sizeof (sa->sa_family);
     }
   return 0;
 }