Playing with the procedure call mechanism.

* src/objcodes.c (do-pair):  New experiment.
* src/vm_engine.h (ALIGN_AS_NON_IMMEDIATE):  New macro.
  (POP_LIST_ON_STACK):  New experimental macro.
* src/vm_engine.c (call):  In the procedure call case, I tried using the above
  macro.

git-archimport-id: lcourtes@laas.fr--2004-libre/guile-vm--revival--0.6--patch-8

src/vm_engine.h

@@ -241,6 +241,76 @@ do						\
   PUSH (l);					\
 } while (0)
 
+
+/* Below is a (slightly broken) experiment to avoid calling `scm_cell' and to
+   allocate cells on the stack.  This is a significant improvement for
+   programs which call a lot of procedures, since the procedure call
+   mechanism uses POP_LIST which normally uses `scm_cons'.
+
+   What it does is that it creates a list whose cells are allocated on the
+   VM's stack instead of being allocated on the heap via `scm_cell'.  This is
+   much faster.  However, if the callee does something like:
+
+     (lambda (. args)
+       (set! the-args args))
+
+   then terrible things may happen since the list of arguments may be
+   overwritten later on.  */
+
+
+/* Awful hack that aligns PTR so that it can be considered as a non-immediate
+   value by Guile.  */
+#define ALIGN_AS_NON_IMMEDIATE(_ptr)		\
+{						\
+  if ((scm_t_bits)(_ptr) & 6)			\
+    {						\
+      size_t _incr;				\
+						\
+      _incr = (scm_t_bits)(_ptr) & 6;		\
+      _incr = (~_incr) & 7;			\
+      (_ptr) += _incr;				\
+    }						\
+}
+
+#define POP_LIST_ON_STACK(n)			\
+do						\
+{						\
+  int i;					\
+  if (n == 0)					\
+    {						\
+      sp -= n;					\
+      PUSH (SCM_EOL);				\
+    }						\
+  else						\
+    {						\
+      SCM *list_head, *list;			\
+						\
+      list_head = sp + 1;			\
+      ALIGN_AS_NON_IMMEDIATE (list_head);	\
+      list = list_head;				\
+						\
+      sp -= n;					\
+      for (i = 1; i <= n; i++)			\
+	{					\
+	  /* The cell's car and cdr.  */	\
+	  *(list) = sp[i];			\
+	  *(list + 1) = PTR2SCM (list + 2);	\
+	  list += 2;				\
+	}					\
+						\
+      /* The last pair's cdr is '().  */	\
+      list--;					\
+      *list = SCM_EOL;				\
+      /* Push the SCM object that points */	\
+      /* to the first cell.  */			\
+      PUSH (PTR2SCM (list_head));		\
+    }						\
+}						\
+while (0)
+
+/* end of the experiment */
+
+
 #define POP_LIST_MARK()				\
 do {						\
   SCM o;					\