diff --git a/tests/tracks.impure.lisp b/tests/tracks.impure.lisp
index bcd3077f5..baa66776e 100644
--- a/tests/tracks.impure.lisp
+++ b/tests/tracks.impure.lisp
@@ -1,3 +1,8 @@
+
+;;;
+;;; an inhomogeneous long list, mixed from different tracks
+;;;
+
 (defun test-long-list-with-mixed-tracks (n)
   (let ((l nil))
     (dotimes (i (* n 256))
@@ -15,3 +20,118 @@
 
 (with-test (:name (:tracks :basic :lists))
   (test-long-list-with-mixed-tracks 100))
+
+(sb-ext:gc :full t)
+
+
+;;;
+;;; TRACK-SELECTION encoding, preferably as fixnum
+;;;
+
+
+(defun %try-encode-tracks/fixnum-bits (tracks)
+  ;; assumes non-nil tracks, returns nil on failure
+  (loop
+    :with n-bits := (1- sb-vm:n-fixnum-bits)
+    :with result := most-negative-fixnum
+    :for tr :in tracks
+    :do
+       (if (< tr n-bits)
+           (setf (logbitp tr result) t)
+           (return nil)) ;; cannot encode
+    :finally
+       (return result)))
+
+(defun %try-encode-tracks/fixnum-bytes (tracks)
+  ;; assumes non-nil tracks, returns nil on failure
+  (loop
+    :with result := 0
+    :for tr :in (sort tracks '<)
+    :for shift :from 0 :by sb-vm:+track-bits+
+    :do (incf result (ash tr shift))
+    :finally
+       (when (typep result 'fixnum)
+         (return result))))
+
+(defun encode-tracks/bit-vector (tracks)
+  (let ((bv (make-array sb-vm:+tracks-end+ :element-type 'bit)))
+    (dolist (tr tracks)
+      (setf (aref bv tr) 1))
+    bv))
+
+(defun encode-tracks (tracks)
+  (when tracks
+    (or (%try-encode-tracks/fixnum-bits tracks)
+        (%try-encode-tracks/fixnum-bytes tracks)
+        (encode-tracks/bit-vector tracks))))
+
+(with-test (:name (:tracks :selection-encoding :fixnum-bytes))
+  ;; small sets of arbitrary tracks, as a non-negative fixnum
+  (assert (= 5                  (%try-encode-tracks/fixnum-bytes '(5))))
+  (assert (= #x1305             (%try-encode-tracks/fixnum-bytes '(#x05 #x13))))
+  #+64-bit
+  (assert (= #x3f06050403020100 (%try-encode-tracks/fixnum-bytes '(#x0 #x1 #x2 #x3 #x4 #x5 #x6 #x3f))))
+  (assert (null                 (%try-encode-tracks/fixnum-bytes '(#x0 #x1 #x2 #x3 #x4 #x5 #x6 #x40))))
+  (assert (null                 (%try-encode-tracks/fixnum-bytes '(#x0 #x1 #x2 #x3 #x4 #x5 #x6 #x7 #x8)))))
+
+(with-test (:name (:tracks :selection-encoding :fixnum-bits))
+  ;; medium sets of low-numbered tracks, as a negative fixnum
+  (assert (= (+ most-negative-fixnum #b100000)            (%try-encode-tracks/fixnum-bits '(5))))
+  (assert (= (+ most-negative-fixnum #x80020)             (%try-encode-tracks/fixnum-bits '(#x05 #x13))))
+  #+64-bit
+  (assert (= (+ most-negative-fixnum #x200000000000007f)  (%try-encode-tracks/fixnum-bits '(#x0 #x1 #x2 #x3 #x4 #x5 #x6 #x3d))))
+  (assert (null                                           (%try-encode-tracks/fixnum-bits '(#x0 #x1 #x2 #x3 #x4 #x5 #x6 #x40))))
+  (assert (= (+ most-negative-fixnum #x1ff)               (%try-encode-tracks/fixnum-bits '(#x0 #x1 #x2 #x3 #x4 #x5 #x6 #x7 #x8)))))
+
+
+;;;
+;;; WALK-DYNAMIC-SPACE with TRACK-SELECTION arg
+;;;
+
+(defun count-track-objects (track-selection)
+  (let ((count 0))
+    (sb-sys:without-gcing ()
+      (sb-vm::walk-dynamic-space
+       (lambda (&rest args)
+	 (declare (ignore args))
+	 (incf count))
+       #b111111 0 0 track-selection))
+    count))
+
+(defun test-track-count (tr expected-count)
+  (flet ((_test/0 (track-selection)
+           (unless (zerop (count-track-objects track-selection))
+             (format *trace-output* "~&; track-selection = ~S~%" track-selection))
+           (assert (zerop (count-track-objects track-selection))))
+         (_test (track-selection)
+           (unless (= expected-count (count-track-objects track-selection))
+             (format *trace-output* "~&; track-selection = ~S~%" track-selection))
+           (assert (= expected-count (count-track-objects track-selection)))))
+    (let* ((range-with-tr (loop :for i :from 1 :upto 7 :collect i))
+           (range-without-tr (set-difference range-with-tr (list tr))))
+      (_test tr)                         ;; single track, simple fixnum
+      (_test/0 (1+ tr))
+      (_test (lambda (x) (eql x tr)))    ;; test predicate, simple function
+      (_test/0 (lambda (x) (member x `(,(1- tr) ,(1+ tr)))))
+      (_test `(,tr))                     ;; list of tracks
+      (_test/0 range-without-tr)
+      (_test `((,tr . ,tr)))             ;; ... or track ranges
+      (_test/0 `((1 . ,(1- tr)) (,(1+ tr) . 252)))
+      (_test (+ (ash tr sb-vm:+track-bits+) 1)) ;; equivalent to `(,tr 1)
+      (_test/0 (encode-tracks range-without-tr))
+      (_test (%try-encode-tracks/fixnum-bits `(,tr)))
+      (_test/0 (%try-encode-tracks/fixnum-bits range-without-tr))
+      (_test (encode-tracks/bit-vector `(,tr)))
+      (_test/0 (encode-tracks/bit-vector range-without-tr)))))
+
+(with-test (:name (:tracks :walk-dynamic-space 1))
+  (let* ((tr 5)
+         (foo (sb-vm:with-track (tr)
+                (make-array 1000))))
+    (test-track-count 5 1)
+    foo))
+
+(sb-ext:gc :full t)
+
+(with-test (:name (:tracks :walk-dynamic-space 2))
+  (test-track-count 5 0))