gistfile1.txt

diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index 47060f4..8b2e769 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -738,7 +738,8 @@ js::PerThreadData::PerThreadData(JSRuntime *runtime)
 {}
 
 JSRuntime::JSRuntime(JSUseHelperThreads useHelperThreads)
-  : mainThread(this),
+  : RuntimeFriendFields(&mainThread),
+    mainThread(this),
     atomsCompartment(NULL),
 #ifdef JS_THREADSAFE
     ownerThread_(NULL),
diff --git a/js/src/jsfriendapi.cpp b/js/src/jsfriendapi.cpp
index f20a2f2..bc58947 100644
--- a/js/src/jsfriendapi.cpp
+++ b/js/src/jsfriendapi.cpp
@@ -22,9 +22,6 @@
 using namespace js;
 using namespace JS;
 
-// Required by PerThreadDataFriendFields::getMainThread()
-JS_STATIC_ASSERT(offsetof(JSRuntime, mainThread) == JS_PERTHREADDATAOFFSET);
-
 PerThreadDataFriendFields::PerThreadDataFriendFields()
 {
 #if defined(JSGC_ROOT_ANALYSIS) || defined(JSGC_USE_EXACT_ROOTING)
diff --git a/js/src/jspubtd.h b/js/src/jspubtd.h
index e699e68..cf4d641 100644
--- a/js/src/jspubtd.h
+++ b/js/src/jspubtd.h
@@ -299,26 +299,24 @@ struct ContextFriendFields {
 #endif
 };
 
+class PerThreadData;
+
 struct RuntimeFriendFields {
     /*
      * If non-zero, we were been asked to call the operation callback as soon
      * as possible.
      */
     volatile int32_t    interrupt;
+    PerThreadData      *mainThreadPtr;
 
-    RuntimeFriendFields()
-      : interrupt(0) { }
+    RuntimeFriendFields(PerThreadData *mainThread)
+      : interrupt(0), mainThreadPtr(mainThread) { }
 
     static const RuntimeFriendFields *get(const JSRuntime *rt) {
         return reinterpret_cast<const RuntimeFriendFields *>(rt);
     }
 };
 
-class PerThreadData;
-
-#define JS_PERTHREADDATAOFFSET \
-    ((sizeof(RuntimeFriendFields) + (sizeof(uintptr_t) - 1)) & sizeof(uintptr_t))
-
 struct PerThreadDataFriendFields
 {
     PerThreadDataFriendFields();
@@ -341,15 +339,13 @@ struct PerThreadDataFriendFields
     static PerThreadDataFriendFields *getMainThread(JSRuntime *rt) {
         // mainThread must always appear directly after |RuntimeFriendFields|.
         // Tested by a JS_STATIC_ASSERT in |jsfriendapi.cpp|
-        return reinterpret_cast<PerThreadDataFriendFields *>(
-            reinterpret_cast<char*>(rt) + JS_PERTHREADDATAOFFSET);
+        return reinterpret_cast<PerThreadDataFriendFields *>(RuntimeFriendFields::get(rt)->mainThreadPtr);
     }
 
     static const PerThreadDataFriendFields *getMainThread(const JSRuntime *rt) {
         // mainThread must always appear directly after |RuntimeFriendFields|.
         // Tested by a JS_STATIC_ASSERT in |jsfriendapi.cpp|
-        return reinterpret_cast<const PerThreadDataFriendFields *>(
-            reinterpret_cast<const char*>(rt) + JS_PERTHREADDATAOFFSET);
+        return reinterpret_cast<const PerThreadDataFriendFields *>(RuntimeFriendFields::get(rt)->mainThreadPtr);
     }
 };