Bug 700915: Diagnostic for GC helper thread crashes. (r=billm)

js/src/jsgc.h

@@ -1492,6 +1492,10 @@ class GCHelperThread {
         backgroundAllocation = false;
     }
 
+    PRThread *getThread() const {
+        return thread;
+    }
+
     /*
      * Outside the GC lock may give true answer when in fact the sweeping has
      * been done.

js/src/vm/RegExpObject-inl.h

@@ -359,6 +359,11 @@ RegExpPrivate::incref(JSContext *cx)
 inline void
 RegExpPrivate::decref(JSContext *cx)
 {
+#ifdef JS_THREADSAFE
+    JS_OPT_ASSERT_IF(cx->runtime->gcHelperThread.getThread(),
+                     PR_GetCurrentThread() != cx->runtime->gcHelperThread.getThread());
+#endif
+
     if (--refCount != 0)
         return;