bug 465032 - operationCount as the first field in JSContext. r=brendan

js/src/jscntxt.cpp

@@ -277,7 +277,7 @@ js_NewContext(JSRuntime *rt, size_t stackChunkSize)
         }
         JS_WAIT_CONDVAR(rt->stateChange, JS_NO_TIMEOUT);
     }
-    JS_APPEND_LINK(&cx->links, &rt->contextList);
+    JS_APPEND_LINK(&cx->link, &rt->contextList);
     JS_UNLOCK_GC(rt);
 
     /*
@@ -391,7 +391,7 @@ js_DestroyContext(JSContext *cx, JSDestroyContextMode mode)
     /* Remove cx from context list first. */
     JS_LOCK_GC(rt);
     JS_ASSERT(rt->state == JSRTS_UP || rt->state == JSRTS_LAUNCHING);
-    JS_REMOVE_LINK(&cx->links);
+    JS_REMOVE_LINK(&cx->link);
     last = (rt->contextList.next == &rt->contextList);
     if (last)
         rt->state = JSRTS_LANDING;
@@ -519,7 +519,7 @@ js_ValidContextPointer(JSRuntime *rt, JSContext *cx)
     JSCList *cl;
 
     for (cl = rt->contextList.next; cl != &rt->contextList; cl = cl->next) {
-        if (cl == &cx->links)
+        if (cl == &cx->link)
             return JS_TRUE;
     }
     JS_RUNTIME_METER(rt, deadContexts);
@@ -533,8 +533,8 @@ js_ContextIterator(JSRuntime *rt, JSBool unlocked, JSContext **iterp)
 
     if (unlocked)
         JS_LOCK_GC(rt);
-    cx = (JSContext *) (cx ? cx->links.next : rt->contextList.next);
-    if (&cx->links == &rt->contextList)
+    cx = js_ContextFromLinkField(cx ? cx->link.next : rt->contextList.next);
+    if (&cx->link == &rt->contextList)
         cx = NULL;
     *iterp = cx;
     if (unlocked)

js/src/jscntxt.h

@@ -734,15 +734,15 @@ JS_STATIC_ASSERT(sizeof(JSTempValueUnion) == sizeof(void *));
 #define JSRESOLVE_INFER         0xffff  /* infer bits from current bytecode */
 
 struct JSContext {
-    /* JSRuntime contextList linkage. */
-    JSCList             links;
-
     /*
-     * Operation count. It is declared early in the structure as a frequently
-     * accessed field.
+     * Operation count. It is declared as the first field in the struct to
+     * ensure the fastest possible access.
      */
     int32               operationCount;
 
+    /* JSRuntime contextList linkage. */
+    JSCList             link;
+
 #if JS_HAS_XML_SUPPORT
     /*
      * Bit-set formed from binary exponentials of the XML_* tiny-ids defined
@@ -1047,6 +1047,13 @@ js_DestroyContext(JSContext *cx, JSDestroyContextMode mode);
 extern JSBool
 js_ValidContextPointer(JSRuntime *rt, JSContext *cx);
 
+static JS_INLINE JSContext *
+js_ContextFromLinkField(JSCList *link)
+{
+    JS_ASSERT(link);
+    return (JSContext *) ((uint8 *) link - offsetof(JSContext, link));
+}
+
 /*
  * If unlocked, acquire and release rt->gcLock around *iterp update; otherwise
  * the caller must be holding rt->gcLock.