dependant: improve conditional-store

In case of real multiprocessing wrapping a body in without-interrupts
is useless -- another thread may modify the place concurrently without
invoking any interrupts. For that we use a lock by default.

Clasp, ECL and SBCL have CAS operators which we use.

dependent.lisp

@@ -805,25 +805,34 @@
 ;; This should use GET-SETF-METHOD to avoid evaluating subforms multiple times.
 ;; It doesn't because CLtL doesn't pass the environment to GET-SETF-METHOD.
 
-;; FIXME: both sbcl and ecl has compare-and-swap these days.
-;; FIXME: Verify for clasp 
+#-(or (and clasp threads) ecl sbcl)
+(progn
+  (defvar *conditional-store-lock*
+    (make-process-lock "conditional store"))
+  (defmacro conditional-store (place old-value new-value)
+    `(holding-lock (*conditional-store-lock*)
+       (if (eq ,place ,old-value)
+           (prog1 t
+             (setf ,place ,new-value))
+           nil))))
 
-#-sbcl
+#+(and clasp threads)
 (defmacro conditional-store (place old-value new-value)
-  `(without-interrupts
-     (cond ((eq ,place ,old-value)
-            (setf ,place ,new-value)
-            t))))
+  (let ((ov (gensym)))
+    `(let ((,ov ,old-value))
+       (eq ,ov (mp:cas ,place ,ov ,new-value)))))
+
+#+ecl
+(defmacro conditional-store (place old-value new-value)
+  (let ((ov (gensym)))
+    `(let ((,ov ,old-value))
+       (eq ,ov (mp:compare-and-swap ,place ,ov ,new-value)))))
 
 #+sbcl
-(progn
-  (defvar *conditional-store-lock*
-    (sb-thread:make-mutex :name "conditional store"))
-  (defmacro conditional-store (place old-value new-value)
-    `(sb-thread:with-mutex (*conditional-store-lock*)
-       (cond ((eq ,place ,old-value)
-              (setf ,place ,new-value)
-              t)))))
+(defmacro conditional-store (place old-value new-value)
+  (let ((ov (gensym)))
+    `(let ((,ov ,old-value))
+       (eq ,ov (sb-ext:compare-and-swap ,place ,ov ,new-value)))))
 
 ;;;----------------------------------------------------------------------------
 ;;; IO Error Recovery