Implement loop variable assignment analysis.

src/compiler/graph-builder.cc

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/compiler/graph-builder.h"
 
 #include "src/compiler.h"
 #include "src/compiler/generic-graph.h"
 #include "src/compiler/generic-node.h"
 #include "src/compiler/generic-node-inl.h"
@@ skipping 133 matching lines @@
   UpdateEffectDependency(effect);
 
   // Introduce Phi nodes for values that have differing input at merge points,
   // potentially extending an existing Phi node if possible.
   for (int i = 0; i < static_cast<int>(values_.size()); ++i) {
     values_[i] = builder_->MergeValue(values_[i], other->values_[i], control);
   }
 }
 
 
-void StructuredGraphBuilder::Environment::PrepareForLoop() {
+void StructuredGraphBuilder::Environment::PrepareForLoop(BitVector* assigned) {
   Node* control = GetControlDependency();
-  for (int i = 0; i < static_cast<int>(values()->size()); ++i) {
-    Node* phi = builder_->NewPhi(1, values()->at(i), control);
-    values()->at(i) = phi;
+  int size = static_cast<int>(values()->size());
+  if (assigned == NULL) {
+    // Assume that everything is updated in the loop.
+    for (int i = 0; i < size; ++i) {
+      Node* phi = builder_->NewPhi(1, values()->at(i), control);
+      values()->at(i) = phi;
+    }
+  } else {
+    // Only build phis for those locals assigned in this loop.
+    for (int i = 0; i < size; ++i) {
+      if (i < assigned->length() && !assigned->Contains(i)) continue;
+      Node* phi = builder_->NewPhi(1, values()->at(i), control);
+      values()->at(i) = phi;
+    }
   }
   Node* effect = builder_->NewEffectPhi(1, GetEffectDependency(), control);
   UpdateEffectDependency(effect);
 }
 
 
 Node* StructuredGraphBuilder::NewPhi(int count, Node* input, Node* control) {
   const Operator* phi_op = common()->Phi(kMachAnyTagged, count);
   Node** buffer = local_zone()->NewArray<Node*>(count + 1);
   MemsetPointer(buffer, input, count);
@@ skipping 72 matching lines @@
   if (!dead_control_.is_set()) {
     Node* dead_node = graph()->NewNode(common_->Dead());
     dead_control_.set(dead_node);
     return dead_node;
   }
   return dead_control_.get();
 }
 }
 }
 }  // namespace v8::internal::compiler