[turbofan] Allocate a small amount of slack space to graph builder environments.

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 107 matching lines @@
     : builder_(builder),
       control_dependency_(control_dependency),
       effect_dependency_(control_dependency),
       values_(zone()) {}
 
 
 StructuredGraphBuilder::Environment::Environment(const Environment& copy)
     : builder_(copy.builder()),
       control_dependency_(copy.control_dependency_),
       effect_dependency_(copy.effect_dependency_),
-      values_(copy.values_) {}
+      values_(copy.zone()) {
+  const size_t kStackEstimate = 7;  // optimum from experimentation!
+  values_.reserve(copy.values_.size() + kStackEstimate);
+  values_.insert(values_.begin(), copy.values_.begin(), copy.values_.end());
+}
 
 
 void StructuredGraphBuilder::Environment::Merge(Environment* other) {
   DCHECK(values_.size() == other->values_.size());
 
   // Nothing to do if the other environment is dead.
   if (other->IsMarkedAsUnreachable()) return;
 
   // Resurrect a dead environment by copying the contents of the other one and
   // placing a singleton merge as the new control dependency.
@@ skipping 129 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