Use a local variable to save/restore the context on entry/exit, instead of

runtime/vm/code_generator_x64.cc

Index: runtime/vm/code_generator_x64.cc
===================================================================
--- runtime/vm/code_generator_x64.cc	(revision 5648)
+++ runtime/vm/code_generator_x64.cc	(working copy)
@@ -612,11 +612,9 @@
   // Unchain the context(s) up to context level 0.
   intptr_t current_context_level = context_level();
   ASSERT(current_context_level >= 0);
-  if (!parsed_function_.function().IsClosureFunction()) {
-    if (current_context_level > 0) {
-      // CTX on entry was saved on the stack, but not linked as context parent.
-      __ popq(CTX);
-    }
+  if (parsed_function_.saved_context_var() != NULL) {
+    // CTX on entry was saved, but not linked as context parent.
+    GenerateLoadVariable(CTX, *parsed_function_.saved_context_var());
   } else {
     while (current_context_level-- > 0) {
       __ movq(CTX, FieldAddress(CTX, Context::parent_offset()));
@@ -814,10 +812,11 @@
     // If this node_sequence is the body of the function being compiled, and if
     // this function is not a closure, do not link the current context as the
     // parent of the newly allocated context, as it is not accessible. Instead,
-    // save it on the stack and restore it on exit.
+    // save it in a pre-allocated variable and restore it on exit.
     if ((node_sequence == parsed_function_.node_sequence()) &&
-         !parsed_function_.function().IsClosureFunction()) {
-      __ pushq(CTX);
+        (parsed_function_.saved_context_var() != NULL)) {
+      GenerateStoreVariable(
+          *parsed_function_.saved_context_var(), CTX, kNoRegister);
       const Immediate raw_null =
           Immediate(reinterpret_cast<intptr_t>(Object::null()));
       __ movq(CTX, raw_null);
@@ -868,23 +867,25 @@
     state()->set_root_node(child_node);
     child_node->Visit(this);
   }
-  if (num_context_variables > 0) {
-    // Unchain the previously allocated context.
-    if ((node_sequence == parsed_function_.node_sequence()) &&
-         !parsed_function_.function().IsClosureFunction()) {
-      __ popq(CTX);
-    } else {
-      __ movq(CTX, FieldAddress(CTX, Context::parent_offset()));
-    }
+
+  // Unchain the previously allocated context.
+  if ((node_sequence == parsed_function_.node_sequence()) &&
+      (parsed_function_.saved_context_var() != NULL)) {
+    ASSERT(num_context_variables > 0);
+    GenerateLoadVariable(CTX, *parsed_function_.saved_context_var());
+  } else if (num_context_variables > 0) {
+    __ movq(CTX, FieldAddress(CTX, Context::parent_offset()));
   }
+
   // If this node sequence is labeled, a break out of the sequence will have
   // taken care of unchaining the context.
   if (node_sequence->label() != NULL) {
     __ Bind(node_sequence->label()->break_label());
-    if ((num_context_variables > 0) &&
-        (node_sequence == parsed_function_.node_sequence()) &&
-         !parsed_function_.function().IsClosureFunction()) {
-      __ popq(CTX);
+
+    // The context saved on entry must be restored.
+    if ((node_sequence == parsed_function_.node_sequence()) &&
+        (parsed_function_.saved_context_var() != NULL)) {
+      GenerateLoadVariable(CTX, *parsed_function_.saved_context_var());
     }
   }
   set_context_level(previous_context_level);
@@ -2546,12 +2547,6 @@
   __ movq(RSP, RBP);
   __ subq(RSP, Immediate(locals_space_size()));
 
-  if ((context_level() > 0) &&
-      !parsed_function_.function().IsClosureFunction()) {
-    // CTX was saved on entry.
-    __ subq(RSP, Immediate(kWordSize));
-  }
-
   // The JumpToExceptionHandler trampoline code sets up
   // - the exception object in RAX (kExceptionObjectReg)
   // - the stacktrace object in register RDX (kStackTraceObjectReg)