--- a/src/hotspot/share/c1/c1_ValueStack.cpp	Tue Mar 12 09:59:58 2019 -0400
+++ b/src/hotspot/share/c1/c1_ValueStack.cpp	Tue Mar 12 15:29:59 2019 +0100
@@ -37,7 +37,7 @@
 , _kind(Parsing)
 , _locals(scope->method()->max_locals(), scope->method()->max_locals(), NULL)
 , _stack(scope->method()->max_stack())
-, _locks()
+, _locks(NULL)
 {
   verify();
 }
@@ -50,7 +50,7 @@
   , _kind(kind)
   , _locals()
   , _stack()
-  , _locks(copy_from->locks_size())
+  , _locks(copy_from->locks_size() == 0 ? NULL : new Values(copy_from->locks_size()))
 {
   assert(kind != EmptyExceptionState || !Compilation::current()->env()->should_retain_local_variables(), "need locals");
   if (kind != EmptyExceptionState) {
@@ -70,7 +70,9 @@
     _stack.appendAll(&copy_from->_stack);
   }
 
-  _locks.appendAll(&copy_from->_locks);
+  if (copy_from->locks_size() > 0) {
+    _locks->appendAll(copy_from->_locks);
+  }
 
   verify();
 }
@@ -90,8 +92,11 @@
   for_each_stack_value(this, index, value) {
     if (value->type()->tag() != s->stack_at(index)->type()->tag()) return false;
   }
-  for_each_lock_value(this, index, value) {
-    if (value != s->lock_at(index)) return false;
+  for (int i = 0; i < locks_size(); i++) {
+    value = lock_at(i);
+    if (value != NULL && value != s->lock_at(i)) {
+      return false;
+    }
   }
   return true;
 }
@@ -113,7 +118,7 @@
 
 
 // apply function to all values of a list; factored out from values_do(f)
-void ValueStack::apply(Values list, ValueVisitor* f) {
+void ValueStack::apply(const Values& list, ValueVisitor* f) {
   for (int i = 0; i < list.length(); i++) {
     Value* va = list.adr_at(i);
     Value v0 = *va;
@@ -135,7 +140,9 @@
   for_each_state(state) {
     apply(state->_locals, f);
     apply(state->_stack, f);
-    apply(state->_locks, f);
+    if (state->_locks != NULL) {
+      apply(*state->_locks, f);
+    }
   }
 }
 
@@ -160,7 +167,10 @@
 }
 
 int ValueStack::lock(Value obj) {
-  _locks.push(obj);
+  if (_locks == NULL) {
+    _locks = new Values();
+  }
+  _locks->push(obj);
   int num_locks = total_locks_size();
   scope()->set_min_number_of_locks(num_locks);
   return num_locks - 1;
@@ -168,7 +178,8 @@
 
 
 int ValueStack::unlock() {
-  _locks.pop();
+  assert(locks_size() > 0, "sanity");
+  _locks->pop();
   return total_locks_size();
 }
 

--- a/src/hotspot/share/c1/c1_ValueStack.hpp	Tue Mar 12 09:59:58 2019 -0400
+++ b/src/hotspot/share/c1/c1_ValueStack.hpp	Tue Mar 12 15:29:59 2019 +0100
@@ -47,7 +47,7 @@
 
   Values   _locals;                              // the locals
   Values   _stack;                               // the expression stack
-  Values   _locks;                               // the monitor stack (holding the locked values)
+  Values*  _locks;                               // the monitor stack (holding the locked values)
 
   Value check(ValueTag tag, Value t) {
     assert(tag == t->type()->tag() || tag == objectTag && t->type()->tag() == addressTag, "types must correspond");
@@ -60,7 +60,7 @@
   }
 
   // helper routine
-  static void apply(Values list, ValueVisitor* f);
+  static void apply(const Values& list, ValueVisitor* f);
 
   // for simplified copying
   ValueStack(ValueStack* copy_from, Kind kind, int bci);
@@ -90,9 +90,9 @@
 
   int locals_size() const                        { return _locals.length(); }
   int stack_size() const                         { return _stack.length(); }
-  int locks_size() const                         { return _locks.length(); }
+  int locks_size() const                         { return _locks == NULL ? 0 : _locks->length(); }
   bool stack_is_empty() const                    { return _stack.is_empty(); }
-  bool no_active_locks() const                   { return _locks.is_empty(); }
+  bool no_active_locks() const                   { return _locks == NULL || _locks->is_empty(); }
   int total_locks_size() const;
 
   // locals access
@@ -201,7 +201,7 @@
   // locks access
   int lock  (Value obj);
   int unlock();
-  Value lock_at(int i) const                     { return _locks.at(i); }
+  Value lock_at(int i) const                     { return _locks->at(i); }
 
   // SSA form IR support
   void setup_phi_for_stack(BlockBegin* b, int index);
@@ -261,14 +261,6 @@
        index += value->type()->size())
 
 
-#define for_each_lock_value(state, index, value)                                               \
-  int temp_var = state->locks_size();                                                          \
-  for (index = 0;                                                                              \
-       index < temp_var && (value = state->lock_at(index), true);                              \
-       index++)                                                                                \
-    if (value != NULL)
-
-
 // Macro definition for simple iteration of all state values of a ValueStack
 // Because the code cannot be executed in a single loop, the code must be passed
 // as a macro parameter.