[aarch64-port-dev ] Query regarding backport of memory order changes to JDK7

[Andrew Dinn]

I'm backporting to jdk7 some recent jdk8 changes which appear to be
needed for memory correctness. I'm a tad baffled by one line in the
change from this checkin

  id      7215
  sha1 id f5e4bc9d2b26
  author  aph
  Add frame anchor fences

A diff for src/cpu/aarch64/vm/javaFrameAnchor_aarch64.hpp is included below.

Is there a reason why the call to OrderAccess::release() in
set_last_Java_fp comes before the assignment of _last_Java_fp?

regards,


Andrew Dinn
-----------
@@ -42,7 +42,7 @@
   void clear(void) {
     // clearing _last_Java_sp must be first
     _last_Java_sp = NULL;
-    // fence?
+    OrderAccess::release();
     _last_Java_fp = NULL;
     _last_Java_pc = NULL;
   }
@@ -55,9 +55,10 @@
     // To act like previous version (pd_cache_state) don't NULL
_last_Java_sp
     // unless the value is changing
     //
-    if (_last_Java_sp != src->_last_Java_sp)
+    if (_last_Java_sp != src->_last_Java_sp) {
       _last_Java_sp = NULL;
-
+      OrderAccess::release();
+    }
     _last_Java_fp = src->_last_Java_fp;
     _last_Java_pc = src->_last_Java_pc;
     // Must be last so profiler will always see valid frame if
has_last_frame() is true
@@ -79,10 +80,10 @@

 public:

-  void set_last_Java_sp(intptr_t* sp)            { _last_Java_sp = sp; }
+  void set_last_Java_sp(intptr_t* sp)            { _last_Java_sp = sp;
OrderAccess::release(); }

   intptr_t*   last_Java_fp(void)                     { return
_last_Java_fp; }
   // Assert (last_Java_sp == NULL || fp == NULL)
-  void set_last_Java_fp(intptr_t* fp)                { _last_Java_fp =
fp; }
+  void set_last_Java_fp(intptr_t* fp)                {
OrderAccess::release(); _last_Java_fp = fp; }

 #endif // CPU_AARCH64_VM_JAVAFRAMEANCHOR_AARCH64_HPP