add allocate-struct primitive and rtl opcode

* libguile/struct.h:
* libguile/struct.c (scm_allocate_struct): New interface: allocates a
  struct.

* libguile/vm-engine.c (allocate_struct): Instead of make-struct with a
  rest arg, separate allocation from initialization.

libguile/struct.c

@@ -517,6 +517,42 @@ scm_c_make_struct (SCM vtable, size_t n_tail, size_t n_init, scm_t_bits init, ..
   return scm_c_make_structv (vtable, n_tail, n_init, v);
 }
 
+SCM_DEFINE (scm_allocate_struct, "allocate-struct", 2, 0, 0,
+            (SCM vtable, SCM nfields),
+	    "Allocate a new structure with space for @var{nfields} fields.\n\n"
+	    "@var{vtable} must be a vtable structure (@pxref{Vtables}).\n\n"
+	    "@var{nfields} must be a non-negative integer.  Strictly speaking\n"
+	    "@var{nfields} is redundant, as the vtable carries the size\n"
+            "for its instances.  However passing it is useful as a sanity\n"
+            "check, given that one module can inline a constructor in\n"
+            "another.\n\n"
+	    "Fields will be initialized with their default values.")
+#define FUNC_NAME s_scm_allocate_struct
+{
+  SCM ret;
+  size_t c_nfields;
+
+  SCM_VALIDATE_VTABLE (1, vtable);
+  c_nfields = scm_to_size_t (nfields);
+
+  SCM_ASSERT (SCM_STRUCT_DATA_REF (vtable, scm_vtable_index_size) == c_nfields,
+              nfields, 2, FUNC_NAME);
+
+  ret = scm_i_alloc_struct (SCM_STRUCT_DATA (vtable), c_nfields);
+
+  if (SCM_LIKELY (SCM_VTABLE_FLAG_IS_SET (vtable, SCM_VTABLE_FLAG_SIMPLE)))
+    {
+      size_t n;
+      for (n = 0; n < c_nfields; n++)
+        SCM_STRUCT_DATA_SET (ret, n, SCM_UNPACK (SCM_BOOL_F));
+    }
+  else
+    scm_struct_init (ret, SCM_VTABLE_LAYOUT (vtable), 0, 0, NULL);
+
+  return ret;
+}
+#undef FUNC_NAME
+
 SCM_DEFINE (scm_make_struct, "make-struct", 2, 0, 1, 
             (SCM vtable, SCM tail_array_size, SCM init),
 	    "Create a new structure.\n\n"

libguile/struct.h

@@ -174,6 +174,7 @@ SCM_API SCM scm_applicable_struct_with_setter_vtable_vtable;
 SCM_API SCM scm_make_struct_layout (SCM fields);
 SCM_API SCM scm_struct_p (SCM x);
 SCM_API SCM scm_struct_vtable_p (SCM x);
+SCM_INTERNAL SCM scm_allocate_struct (SCM vtable, SCM n_words);
 SCM_API SCM scm_make_struct (SCM vtable, SCM tail_array_size, SCM init);
 SCM_API SCM scm_c_make_struct (SCM vtable, size_t n_tail, size_t n_inits,
                                scm_t_bits init, ...);

libguile/vm-engine.c

@@ -3090,50 +3090,25 @@ RTL_VM_NAME (SCM vm, SCM program, SCM *argv, size_t nargs_)
       RETURN (SCM_STRUCT_VTABLE (obj));
     }
 
-  /* make-struct dst:12 vtable:12 _:8 n-init:24 init0:24 0:8 ...
+  /* allocate-struct dst:8 vtable:8 nfields:8
    *
-   * Make a new struct with VTABLE, and place it in DST.  The struct
-   * will be constructed with N-INIT initializers, which are located in
-   * the locals given by INIT0....  The format of INIT0... is as in the
-   * "call" opcode: unsigned 24-bit values, with 0 in the high byte.
+   * Allocate a new struct with VTABLE, and place it in DST.  The struct
+   * will be constructed with space for NFIELDS fields, which should
+   * correspond to the field count of the VTABLE.
    */
-  VM_DEFINE_OP (102, make_struct, "make-struct", OP2 (U8_U12_U12, X8_R24))
-#if 0
+  VM_DEFINE_OP (102, allocate_struct, "allocate-struct", OP1 (U8_U8_U8_U8) | OP_DST)
     {
-      scm_t_uint16 dst, vtable_r;
-      scm_t_uint32 n_init, n;
-      SCM vtable, ret;
+      scm_t_uint8 dst, vtable, nfields;
+      SCM ret;
 
-      SCM_UNPACK_RTL_12_12 (op, dst, vtable_r);
-      vtable = LOCAL_REF (vtable_r);
-      SCM_UNPACK_RTL_24 (ip[1], n_init);
+      SCM_UNPACK_RTL_8_8_8 (op, dst, vtable, nfields);
 
       SYNC_IP ();
-
-      if (SCM_LIKELY (SCM_STRUCTP (vtable)
-                      && SCM_VTABLE_FLAG_IS_SET (vtable, SCM_VTABLE_FLAG_SIMPLE)
-                      && (SCM_STRUCT_DATA_REF (vtable, scm_vtable_index_size)
-                          == n_init)
-                      && !SCM_VTABLE_INSTANCE_FINALIZER (vtable)))
-        {
-          /* Verily, we are making a simple struct with the right number of
-             initializers, and no finalizer. */
-          ret = scm_words ((scm_t_bits)SCM_STRUCT_DATA (vtable) | scm_tc3_struct,
-                           n_init + 2);
-          SCM_SET_CELL_WORD_1 (ret, (scm_t_bits)SCM_CELL_OBJECT_LOC (ret, 2));
-          
-          for (n = 0; n < n_init; n++)
-            SCM_STRUCT_DATA (ret)[n] = SCM_UNPACK (LOCAL_REF (ip[n + 1]));
-        }
-      else
-        ret = scm_c_make_structvs (vtable, fp, &ip[1], n_init);
-
+      ret = scm_allocate_struct (LOCAL_REF (vtable), SCM_I_MAKINUM (nfields));
       LOCAL_SET (dst, ret);
-      NEXT (n_init + 1);
+
+      NEXT (1);
     }
-#else
-  abort ();
-#endif
 
   /* struct-ref dst:8 src:8 idx:8
    *