Small improvements in array code.

runtime/vm/flow_graph_optimizer.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/flow_graph_optimizer.h"
 
 #include "vm/bit_vector.h"
 #include "vm/cha.h"
 #include "vm/cpu.h"
 #include "vm/dart_entry.h"
@@ skipping 3790 matching lines @@
   ConstantInstr* bytes_per_element =
       flow_graph()->GetConstant(Smi::Handle(I, Smi::New(element_size)));
   BinarySmiOpInstr* len_in_bytes =
       new(I) BinarySmiOpInstr(Token::kMUL,
                               new(I) Value(length),
                               new(I) Value(bytes_per_element),
                               call->deopt_id(), call->token_pos());
   *cursor = flow_graph()->AppendTo(*cursor, len_in_bytes, call->env(),
                                    FlowGraph::kValue);
 
-  ConstantInstr* length_adjustment =
-      flow_graph()->GetConstant(Smi::Handle(I, Smi::New(
-          Instance::ElementSizeFor(view_cid) - 1)));
   // adjusted_length = len_in_bytes - (element_size - 1).
-  BinarySmiOpInstr* adjusted_length =
-      new(I) BinarySmiOpInstr(Token::kSUB,
-                              new(I) Value(len_in_bytes),
-                              new(I) Value(length_adjustment),
-                              call->deopt_id(), call->token_pos());
-  *cursor = flow_graph()->AppendTo(*cursor, adjusted_length, call->env(),
-                                   FlowGraph::kValue);
+  Definition* adjusted_length = len_in_bytes;
+  intptr_t adjustment = Instance::ElementSizeFor(view_cid) - 1;
+  if (adjustment > 0) {
+    ConstantInstr* length_adjustment =
+        flow_graph()->GetConstant(Smi::Handle(I, Smi::New(adjustment)));
+    adjusted_length =
+        new(I) BinarySmiOpInstr(Token::kSUB,
+                                new(I) Value(len_in_bytes),
+                                new(I) Value(length_adjustment),
+                                call->deopt_id(), call->token_pos());
+    *cursor = flow_graph()->AppendTo(*cursor, adjusted_length, call->env(),
+                                     FlowGraph::kValue);
+  }
 
   // Check adjusted_length > 0.
   ConstantInstr* zero =
       flow_graph()->GetConstant(Smi::Handle(I, Smi::New(0)));
   *cursor = flow_graph()->AppendTo(*cursor,
                                    new(I) CheckArrayBoundInstr(
                                        new(I) Value(adjusted_length),
                                        new(I) Value(zero),
                                        call->deopt_id()),
                                    call->env(),
@@ skipping 5786 matching lines @@
 
   // Insert materializations at environment uses.
   for (intptr_t i = 0; i < exits_collector_.exits().length(); i++) {
     CreateMaterializationAt(
         exits_collector_.exits()[i], alloc, alloc->cls(), *slots);
   }
 }
 
 
 }  // namespace dart