Explorations in adding atomic compare and swap

VarHandles were introduced in openjdk9, so the code needs to be
conditionalized somehow.  It is currently being developed on
openjdk17.

<#92>

src/org/armedbear/lisp/Atomic.java

@@ -0,0 +1,44 @@
+package org.armedbear.lisp;
+
+import static org.armedbear.lisp.Lisp.*;
+
+import java.lang.invoke.MethodHandles;
+import java.lang.invoke.VarHandle;
+
+public class Atomic
+{
+  static MethodHandles.Lookup Lookup
+    = MethodHandles.lookup();
+
+  static boolean compareAndSwap(LispObject place, LispObject expectedValue, LispObject newValue) {
+    VarHandle vh = null; 
+    if (place instanceof Fixnum) {
+      try {
+        vh = Lookup.findVarHandle(Fixnum.class, "value", int.class);
+      } catch (ReflectiveOperationException e) {
+        java_error(e);
+        return false;  // unreached
+      }
+    }
+    if (vh == null) {
+      simple_error("Unable to find VarHandle for place ~a", place);
+      return false; // unreached
+    }
+    return vh.compareAndSet(expectedValue, newValue);
+  }
+
+  public static final Primitive _CAS = new pf_cas();
+  @DocString(name="%cas",
+             args="place expected-value new-value",
+             returns="generalized boolean")
+  private static final class pf_cas extends Primitive { // Maybe a special operator?  Or do that Lisp-side?
+    pf_cas() {
+      super(Symbol._CAS);
+    }
+    @Override
+    public LispObject execute(LispObject place, LispObject expectedValue, LispObject newValue) {
+      boolean result = compareAndSwap (place, expectedValue, newValue);
+      return (result == false) ? NIL : T; // TODO this has to exist somewhere as a static method
+    }
+  }
+}

src/org/armedbear/lisp/Autoload.java

@@ -692,5 +692,7 @@ public LispObject execute(LispObject arg)
         autoload(PACKAGE_EXT, "autoload-setf-expander", "AutoloadGeneralizedReference", true);
         autoload(PACKAGE_EXT, "autoload-setf-function", "AutoloadGeneralizedReference", true);
         autoload(PACKAGE_EXT, "autoload-ref-p", "AutoloadGeneralizedReference", true);
+
+        autoload(Symbol._CAS, "Atomic");
     }
 }

src/org/armedbear/lisp/Symbol.java

@@ -2973,6 +2973,8 @@ public String toString() {
     PACKAGE_EXT.addExternalSymbol("WEAK-REFERENCE");
   public static final Symbol ADD_PACKAGE_LOCAL_NICKNAME =
     PACKAGE_EXT.addExternalSymbol("ADD-PACKAGE-LOCAL-NICKNAME");
+  public static final Symbol _CAS =
+    PACKAGE_EXT.addExternalSymbol("%CAS");
 
   // MOP.
   public static final Symbol CLASS_LAYOUT =