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/print.c b/src/print.c
index 2c9aa9dfd8..8f559045f8 100644
--- a/src/print.c
+++ b/src/print.c
@@ -1198,7 +1198,7 @@ print_preprocess (Lisp_Object obj)
 	  goto loop;
 
 	case Lisp_Vectorlike:
-	  size = XVECTOR (obj)->size;
+	  size = XVECTOR_SIZE (obj);
 	  if (size & PSEUDOVECTOR_FLAG)
 	    size &= PSEUDOVECTOR_SIZE_MASK;
 	  for (i = 0; i < size; i++)
@@ -1786,7 +1786,7 @@ print_object (Lisp_Object obj, register Lisp_Object printcharfun, int escapeflag
 	      strout (SDATA (SYMBOL_NAME (h->weak)), -1, -1, printcharfun);
 	      PRINTCHAR (' ');
 	      sprintf (buf, "%ld/%ld", (long) h->count,
-		       (long) XVECTOR (h->next)->size);
+		       (long) XVECTOR_SIZE (h->next));
 	      strout (buf, -1, -1, printcharfun);
 	    }
 	  sprintf (buf, " 0x%lx", (unsigned long) h);
@@ -1797,7 +1797,7 @@ print_object (Lisp_Object obj, register Lisp_Object printcharfun, int escapeflag
 	    #s(hash-table size 2 test equal data (k1 v1 k2 v2)) */
 	  /* Always print the size. */
 	  sprintf (buf, "#s(hash-table size %ld",
-		   (long) XVECTOR (h->next)->size);
+		   (long) XVECTOR_SIZE (h->next));
 	  strout (buf, -1, -1, printcharfun);
 
 	  if (!NILP (h->test))
@@ -1909,7 +1909,7 @@ print_object (Lisp_Object obj, register Lisp_Object printcharfun, int escapeflag
 	}
       else
 	{
-	  EMACS_INT size = XVECTOR (obj)->size;
+	  EMACS_INT size = XVECTOR_SIZE (obj);
 	  if (COMPILEDP (obj))
 	    {
 	      PRINTCHAR ('#');
@@ -2025,7 +2025,7 @@ print_object (Lisp_Object obj, register Lisp_Object printcharfun, int escapeflag
 	if (MISCP (obj))
 	  sprintf (buf, "(MISC 0x%04x)", (int) XMISCTYPE (obj));
 	else if (VECTORLIKEP (obj))
-	  sprintf (buf, "(PVEC 0x%08lx)", (unsigned long) XVECTOR (obj)->size);
+	  sprintf (buf, "(PVEC 0x%08lx)", (unsigned long) XVECTOR_SIZE (obj));
 	else
 	  sprintf (buf, "(0x%02x)", (int) XTYPE (obj));
 	strout (buf, -1, -1, printcharfun);