Dumper/loader can avoid building primordial layouts bottom up.

make-target-2.sh

@@ -62,7 +62,9 @@ echo //checking for leftover cold-init symbols
   (sb-vm::map-allocated-objects
    (lambda (obj type size)
      (declare (ignore type size))
-     (when (and (symbolp obj) (search "!"  (string obj)))
+     (when (and (symbolp obj)
+                (search "!" (string obj))
+                (not (symbol-package obj)))
        (push obj l)))
    :dynamic)
   (format t "Found ~D:~%" (1- (length l)))

src/code/fop.lisp

@@ -706,3 +706,30 @@ a bug.~@:>")
   (when (eql *skip-until* label)
     (setf *skip-until* nil))
   (values))
+
+;;; Primordial layouts.
+;;; At the rate the opcode space is filling up, it might be wise to
+;;; rethink using 16 each for FOP-CODE and -SYMBOL-IN-PACKAGE-SAVE.
+(macrolet ((frob (&rest specs)
+             `(progn
+                (defun known-layout-fop (name)
+                  (case name
+                    ,@(mapcar (lambda (spec) `((,(cadr spec)) ,(car spec)))
+                              specs)))
+                ,@(mapcar (lambda (spec)
+                            `(!define-fop ,(car spec)
+                                          (,(symbolicate "FOP-LAYOUT-OF-"
+                                                         (cadr spec)))
+                                          (find-layout ',(cadr spec))))
+                          specs))))
+  (frob (#x6c t)
+        (#x6d structure-object)
+        (#x6e structure!object)
+        (#x6f definition-source-location)
+        (#x70 sb!c::debug-fun)
+        (#x71 sb!c::compiled-debug-fun)
+        (#x72 sb!c::debug-info)
+        (#x73 sb!c::compiled-debug-info)
+        (#x74 sb!c::debug-source)
+        (#x75 defstruct-description)
+        (#x76 defstruct-slot-description)))

src/compiler/dump.lisp

@@ -1323,6 +1323,13 @@
     ;; Q: Shouldn't we aver that NAME is the proper name for its classoid?
     (unless name
       (compiler-error "dumping anonymous layout: ~S" obj))
+    ;; The target lisp can save some space in fasls (sometimes),
+    ;; but the cross-compiler can't because we need to construct the
+    ;; cold representation of all layouts, not reference host layouts.
+    #-sb-xc-host
+    (let ((fop (known-layout-fop name)))
+      (when fop
+        (return-from dump-layout (dump-byte fop file))))
     (dump-object name file))
   (sub-dump-object (layout-inherits obj) file)
   (sub-dump-object (layout-depthoid obj) file)