--- a/src/cpu/sparc/vm/sharedRuntime_sparc.cpp	Tue Aug 25 13:56:32 2015 +0200
+++ b/src/cpu/sparc/vm/sharedRuntime_sparc.cpp	Tue Aug 25 14:34:50 2015 +0200
@@ -3542,6 +3542,7 @@
   masm->block_comment("BEGIN JVMCI");
   int implicit_exception_uncommon_trap_offset = __ offset() - start;
   __ ld_ptr(G2_thread, in_bytes(JavaThread::jvmci_implicit_exception_pc_offset()), O7);
+  DEBUG_ONLY(__ st(G0, Address(G2_thread, in_bytes(JavaThread::jvmci_implicit_exception_pc_offset())));)
   __ add(O7, -8, O7);
 
   int uncommon_trap_offset = __ offset() - start;

--- a/src/cpu/x86/vm/sharedRuntime_x86_64.cpp	Tue Aug 25 13:56:32 2015 +0200
+++ b/src/cpu/x86/vm/sharedRuntime_x86_64.cpp	Tue Aug 25 14:34:50 2015 +0200
@@ -3397,6 +3397,7 @@
   int implicit_exception_uncommon_trap_offset = __ pc() - start;
 
   __ pushptr(Address(r15_thread, in_bytes(JavaThread::jvmci_implicit_exception_pc_offset())));
+  DEBUG_ONLY(__ movptr(Address(r15_thread, in_bytes(JavaThread::jvmci_implicit_exception_pc_offset())), (int32_t)NULL_WORD);)
 
   int uncommon_trap_offset = __ pc() - start;
 

--- a/src/share/vm/runtime/javaCalls.cpp	Tue Aug 25 13:56:32 2015 +0200
+++ b/src/share/vm/runtime/javaCalls.cpp	Tue Aug 25 14:34:50 2015 +0200
@@ -327,8 +327,9 @@
   CHECK_UNHANDLED_OOPS_ONLY(thread->clear_unhandled_oops();)
 
 #if INCLUDE_JVMCI
-  nmethod* nm = args->alternative_target();
-  if (nm == NULL) {
+  // Gets the nmethod (if any) that should be called instead of normal target
+  nmethod* alternative_target = args->alternative_target();
+  if (alternative_target == NULL) {
 #endif
 // Verify the arguments
 
@@ -402,9 +403,9 @@
   }
 
 #if INCLUDE_JVMCI
-  if (nm != NULL) {
-    if (nm->is_alive()) {
-      thread->set_jvmci_alternate_call_target(nm->verified_entry_point());
+  if (alternative_target != NULL) {
+    if (alternative_target->is_alive()) {
+      thread->set_jvmci_alternate_call_target(alternative_target->verified_entry_point());
       entry_point = method->adapter()->get_i2c_entry();
     } else {
       THROW(vmSymbols::jdk_internal_jvmci_code_InvalidInstalledCodeException());

--- a/src/share/vm/runtime/thread.cpp	Tue Aug 25 13:56:32 2015 +0200
+++ b/src/share/vm/runtime/thread.cpp	Tue Aug 25 14:34:50 2015 +0200
@@ -1488,8 +1488,10 @@
   _pending_deoptimization = -1;
   _pending_failed_speculation = NULL;
   _pending_transfer_to_interpreter = false;
-  _jvmci_alternate_call_target = NULL;
-  _jvmci_implicit_exception_pc = NULL;
+  _jvmci._alternate_call_target = NULL;
+  // TODO: If _jvmci becomes a union, then this assignment
+  // should be converted to an assertion or guarantee
+  _jvmci._implicit_exception_pc = NULL;
   if (JVMCICounterSize > 0) {
     _jvmci_counters = NEW_C_HEAP_ARRAY(jlong, JVMCICounterSize, mtInternal);
     memset(_jvmci_counters, 0, sizeof(jlong) * JVMCICounterSize);

--- a/src/share/vm/runtime/thread.hpp	Tue Aug 25 13:56:32 2015 +0200
+++ b/src/share/vm/runtime/thread.hpp	Tue Aug 25 14:34:50 2015 +0200
@@ -921,9 +921,18 @@
   oop       _pending_failed_speculation;
   bool      _pending_monitorenter;
   bool      _pending_transfer_to_interpreter;
-  address   _jvmci_alternate_call_target;
-  address   _jvmci_implicit_exception_pc;    // pc at which the most recent implicit exception occurred
+  // These fields are mutually exclusive in terms of live ranges
+  // so this could be a union instead of a struct.
+  struct {
+    // Communicates the pc at which the most recent implicit exception occurred
+    // from the signal handler to a deoptimization stub.
+    address   _implicit_exception_pc;
 
+    // Communicates an alternative call target to an i2c stub from a JavaCall.
+    address   _alternate_call_target;    //
+  } _jvmci;
+
+  // Support for high precision, thread sensitive counters in JVMCI compiled code.
   jlong*    _jvmci_counters;
 
  public:
@@ -1314,8 +1323,8 @@
   void set_pending_deoptimization(int reason)     { _pending_deoptimization = reason; }
   void set_pending_failed_speculation(oop failed_speculation) { _pending_failed_speculation = failed_speculation; }
   void set_pending_transfer_to_interpreter(bool b) { _pending_transfer_to_interpreter = b; }
-  void set_jvmci_alternate_call_target(address a) { _jvmci_alternate_call_target = a; }
-  void set_jvmci_implicit_exception_pc(address a) { _jvmci_implicit_exception_pc = a; }
+  void set_jvmci_alternate_call_target(address a) { assert(_jvmci._alternate_call_target == NULL, "must be"); _jvmci._alternate_call_target = a; }
+  void set_jvmci_implicit_exception_pc(address a) { assert(_jvmci._implicit_exception_pc == NULL, "must be"); _jvmci._implicit_exception_pc = a; }
 #endif
 
   // Exception handling for compiled methods
@@ -1415,8 +1424,8 @@
   static ByteSize pending_deoptimization_offset() { return byte_offset_of(JavaThread, _pending_deoptimization); }
   static ByteSize pending_monitorenter_offset()  { return byte_offset_of(JavaThread, _pending_monitorenter); }
   static ByteSize pending_failed_speculation_offset() { return byte_offset_of(JavaThread, _pending_failed_speculation); }
-  static ByteSize jvmci_alternate_call_target_offset() { return byte_offset_of(JavaThread, _jvmci_alternate_call_target); }
-  static ByteSize jvmci_implicit_exception_pc_offset() { return byte_offset_of(JavaThread, _jvmci_implicit_exception_pc); }
+  static ByteSize jvmci_alternate_call_target_offset() { return byte_offset_of(JavaThread, _jvmci._alternate_call_target); }
+  static ByteSize jvmci_implicit_exception_pc_offset() { return byte_offset_of(JavaThread, _jvmci._implicit_exception_pc); }
   static ByteSize jvmci_counters_offset()        { return byte_offset_of(JavaThread, _jvmci_counters      ); }
 #endif
   static ByteSize exception_oop_offset()         { return byte_offset_of(JavaThread, _exception_oop       ); }