Allow constants to be deleted by reinserting them into the graph as needed.

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/31943002

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17299 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/code-stubs-hydrogen.cc

@@ -150,10 +150,6 @@ bool CodeStubGraphBuilderBase::BuildGraph() {
   next_block->SetJoinId(BailoutId::StubEntry());
   set_current_block(next_block);
 
-  HConstant* undefined_constant =
-      Add<HConstant>(isolate()->factory()->undefined_value());
-  graph()->set_undefined_constant(undefined_constant);
-
   for (int i = 0; i < param_count; ++i) {
     HParameter* param =
         Add<HParameter>(i, HParameter::REGISTER_PARAMETER);

src/hydrogen-dce.cc

@@ -98,11 +98,7 @@ void HDeadCodeEliminationPhase::RemoveDeadInstructions() {
       HInstruction* instr = it.Current();
       if (!instr->CheckFlag(HValue::kIsLive)) {
         // Instruction has not been marked live, so remove it.
-        if (!instr->IsConstant() || instr->block()->block_id() != 0) {
-          // TODO(titzer): Some global constants in block 0 can be used
-          // again later, and can't currently be removed. Fix that.
-          instr->DeleteAndReplaceWith(NULL);
-        }
+        instr->DeleteAndReplaceWith(NULL);
       } else {
         // Clear the liveness flag to leave the graph clean for the next DCE.
         instr->ClearFlag(HValue::kIsLive);

src/hydrogen-instructions.cc

@@ -2576,6 +2576,7 @@ bool HConstant::EmitAtUses() {
   ASSERT(IsLinked());
   if (block()->graph()->has_osr() &&
       block()->graph()->IsStandardConstant(this)) {
+    // TODO(titzer): this seems like a hack that should be fixed by custom OSR.
     return true;
   }
   if (UseCount() == 0) return true;

src/hydrogen.cc

@@ -648,10 +648,21 @@ HConstant* HGraph::GetConstant(SetOncePointer<HConstant>* pointer,
     // Can't pass GetInvalidContext() to HConstant::New, because that will
     // recursively call GetConstant
     HConstant* constant = HConstant::New(zone(), NULL, value);
-    constant->InsertAfter(GetConstantUndefined());
+    constant->InsertAfter(entry_block()->first());
     pointer->set(constant);
+    return constant;
   }
-  return pointer->get();
+  return ReinsertConstantIfNecessary(pointer->get());
+}
+
+
+HConstant* HGraph::ReinsertConstantIfNecessary(HConstant* constant) {
+  if (!constant->IsLinked()) {
+    // The constant was removed from the graph. Reinsert.
+    constant->ClearFlag(HValue::kIsDead);
+    constant->InsertAfter(entry_block()->first());
+  }
+  return constant;
 }
 
 
@@ -681,13 +692,14 @@ HConstant* HGraph::GetConstant##Name() {                                       \
         true,                                                                  \
         false,                                                                 \
         boolean_value);                                                        \
-    constant->InsertAfter(GetConstantUndefined());                             \
+    constant->InsertAfter(entry_block()->first());                             \
     constant_##name##_.set(constant);                                          \
   }                                                                            \
-  return constant_##name##_.get();                                             \
+  return ReinsertConstantIfNecessary(constant_##name##_.get());                \
 }
 
 
+DEFINE_GET_CONSTANT(Undefined, undefined, HType::Tagged(), false)
 DEFINE_GET_CONSTANT(True, true, HType::Boolean(), true)
 DEFINE_GET_CONSTANT(False, false, HType::Boolean(), false)
 DEFINE_GET_CONSTANT(Hole, the_hole, HType::Tagged(), false)
@@ -3233,10 +3245,6 @@ void HOptimizedGraphBuilder::SetUpScope(Scope* scope) {
   HInstruction* context = Add<HContext>();
   environment()->BindContext(context);
 
-  HConstant* undefined_constant = HConstant::cast(Add<HConstant>(
-      isolate()->factory()->undefined_value()));
-  graph()->set_undefined_constant(undefined_constant);
-
   // Create an arguments object containing the initial parameters.  Set the
   // initial values of parameters including "this" having parameter index 0.
   ASSERT_EQ(scope->num_parameters() + 1, environment()->parameter_count());
@@ -3250,6 +3258,7 @@ void HOptimizedGraphBuilder::SetUpScope(Scope* scope) {
   AddInstruction(arguments_object);
   graph()->SetArgumentsObject(arguments_object);
 
+  HConstant* undefined_constant = graph()->GetConstantUndefined();
   // Initialize specials and locals to undefined.
   for (int i = environment()->parameter_count() + 1;
        i < environment()->length();

src/hydrogen.h

@@ -342,10 +342,7 @@ class HGraph V8_FINAL : public ZoneObject {
 
   void CollectPhis();
 
-  void set_undefined_constant(HConstant* constant) {
-    undefined_constant_.set(constant);
-  }
-  HConstant* GetConstantUndefined() const { return undefined_constant_.get(); }
+  HConstant* GetConstantUndefined();
   HConstant* GetConstant0();
   HConstant* GetConstant1();
   HConstant* GetConstantMinus1();
@@ -460,6 +457,7 @@ class HGraph V8_FINAL : public ZoneObject {
   bool IsInsideNoSideEffectsScope() { return no_side_effects_scope_count_ > 0; }
 
  private:
+  HConstant* ReinsertConstantIfNecessary(HConstant* constant);
   HConstant* GetConstant(SetOncePointer<HConstant>* pointer,
                          int32_t integer_value);
 
@@ -479,7 +477,7 @@ class HGraph V8_FINAL : public ZoneObject {
   ZoneList<HValue*> values_;
   ZoneList<HPhi*>* phi_list_;
   ZoneList<HInstruction*>* uint32_instructions_;
-  SetOncePointer<HConstant> undefined_constant_;
+  SetOncePointer<HConstant> constant_undefined_;
   SetOncePointer<HConstant> constant_0_;
   SetOncePointer<HConstant> constant_1_;
   SetOncePointer<HConstant> constant_minus1_;