When a value has been optimized away by the compiler, set its value to

"<optimized out>".  This will make it less confusing for people using
the debugger.

- Added constant_dead() to the flow graph.
- Added Symbols::OptimizedOut().

R=fschneider@google.com

Review URL: https://codereview.chromium.org//27339003

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge/dart@28671 260f80e4-7a28-3924-810f-c04153c831b5

runtime/vm/debugger_api_impl_test.cc

@@ -453,13 +453,10 @@ static void InspectStackTest(bool optimize) {
   dart_args[1] = Dart_NewInteger(kLowThreshold);
   EXPECT_VALID(Dart_Invoke(script_lib, NewString("test"), 2, dart_args));
   if (optimize) {
-    // Note that several variables have the value 'null' in the
-    // optimized case.  This is because these values were determined
-    // to be dead by the optimizing compiler and their values were not
-    // preserved by the deopt information.
     EXPECT_STREQ("[0] breakpointNow { }\n"
-                 "[1] helper { a = 5 b = 99 stop = null }\n"
-                 "[2] anotherMiddleMan { one = null two = null stop = null }\n"
+                 "[1] helper { a = 5 b = 99 stop = <optimized out> }\n"
+                 "[2] anotherMiddleMan { one = <optimized out> "
+                 "two = <optimized out> stop = <optimized out> }\n"
                  "[3] middleMan { x = 5 limit = 100 stop = true value = 24255"
                  " i = 99 }\n"
                  "[4] test { stop = true limit = 100 }\n",
@@ -549,10 +546,10 @@ static void InspectStackWithClosureTest(bool optimize) {
   EXPECT_VALID(Dart_Invoke(script_lib, NewString("test"), 2, dart_args));
   if (optimize) {
     EXPECT_STREQ("[0] breakpointNow { }\n"
-                 "[1] helper { a = 50 b = 99 stop = null }\n"
+                 "[1] helper { a = 50 b = 99 stop = <optimized out> }\n"
                  "[2] <anonymous closure> { x = 5 i = 99 stop = true"
-                 " value = null }\n"
-                 "[3] anotherMiddleMan { func = null }\n"
+                 " value = <optimized out> }\n"
+                 "[3] anotherMiddleMan { func = <optimized out> }\n"
                  "[4] middleMan { x = 5 limit = 100 stop = true"
                  " value = 242550 i = 99 }\n"
                  "[5] test { stop = true limit = 100 }\n",

runtime/vm/flow_graph.cc

@@ -34,6 +34,8 @@ FlowGraph::FlowGraph(const FlowGraphBuilder& builder,
     postorder_(),
     reverse_postorder_(),
     optimized_block_order_(),
+    constant_null_(NULL),
+    constant_dead_(NULL),
     block_effects_(NULL),
     licm_allowed_(true),
     use_far_branches_(false),
@@ -706,6 +708,7 @@ void FlowGraph::Rename(GrowableArray<PhiInstr*>* live_phis,
 
   // Add global constants to the initial definitions.
   constant_null_ = GetConstant(Object::ZoneHandle());
+  constant_dead_ = GetConstant(Symbols::OptimizedOut());
 
   // Add parameters to the initial definitions and renaming environment.
   if (inlining_parameters != NULL) {
@@ -804,7 +807,7 @@ void FlowGraph::RenameRecursive(BlockEntryInstr* block_entry,
   BitVector* live_in = variable_liveness->GetLiveInSet(block_entry);
   for (intptr_t i = 0; i < variable_count(); i++) {
     if (!live_in->Contains(i)) {
-      (*env)[i] = constant_null();
+      (*env)[i] = constant_dead();
     }
   }
 
@@ -875,7 +878,7 @@ void FlowGraph::RenameRecursive(BlockEntryInstr* block_entry,
           if (variable_liveness->IsStoreAlive(block_entry, store)) {
             (*env)[index] = result;
           } else {
-            (*env)[index] = constant_null();
+            (*env)[index] = constant_dead();
           }
         } else if (load != NULL) {
           // The graph construction ensures we do not have an unused LoadLocal
@@ -891,7 +894,7 @@ void FlowGraph::RenameRecursive(BlockEntryInstr* block_entry,
           }
 
           if (variable_liveness->IsLastLoad(block_entry, load)) {
-            (*env)[index] = constant_null();
+            (*env)[index] = constant_dead();
           }
         } else if (push != NULL) {
           result = push->value()->definition();

runtime/vm/flow_graph.h

@@ -114,6 +114,10 @@ class FlowGraph : public ZoneAllocated {
     return constant_null_;
   }
 
+  ConstantInstr* constant_dead() const {
+    return constant_dead_;
+  }
+
   intptr_t alloc_ssa_temp_index() { return current_ssa_temp_index_++; }
 
   intptr_t InstructionCount() const;
@@ -269,6 +273,7 @@ class FlowGraph : public ZoneAllocated {
   GrowableArray<BlockEntryInstr*> reverse_postorder_;
   GrowableArray<BlockEntryInstr*> optimized_block_order_;
   ConstantInstr* constant_null_;
+  ConstantInstr* constant_dead_;
 
   BlockEffects* block_effects_;
   bool licm_allowed_;

runtime/vm/symbols.h

@@ -288,6 +288,7 @@ class ObjectPointerVisitor;
   V(_LocalTypeVariableMirrorImpl, "_LocalTypeVariableMirrorImpl")              \
   V(hashCode, "get:hashCode")                                                  \
   V(_leftShiftWithMask32, "_leftShiftWithMask32")                              \
+  V(OptimizedOut, "<optimized out>")                                           \
 
 
 // Contains a list of frequently used strings in a canonicalized form. This