Implement simple dead store elimination.

runtime/vm/compiler.cc

 // Copyright (c) 2012, 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/compiler.h"
 
 #include "vm/assembler.h"
 
 #include "vm/ast_printer.h"
 #include "vm/block_scheduler.h"
@@ skipping 420 matching lines @@
           if (DominatorBasedCSE::Optimize(flow_graph)) {
             DEBUG_ASSERT(flow_graph->VerifyUseLists());
             // Do another round of CSE to take secondary effects into account:
             // e.g. when eliminating dependent loads (a.x[0] + a.x[0])
             // TODO(fschneider): Change to a one-pass optimization pass.
             DominatorBasedCSE::Optimize(flow_graph);
             DEBUG_ASSERT(flow_graph->VerifyUseLists());
           }
         }
 
-        // Optimize (a << b) & c patterns, merge operations.
-        // Run after CSE in order to have more opportunity to merge
-        // instructions that have same inputs.
-        optimizer.TryOptimizePatterns();
-        DEBUG_ASSERT(flow_graph->VerifyUseLists());
-
+        // Run loop-invariant code motion right after load elimination since it
+        // depends on the numbering of loads from the previous load-elimination.
         if (FLAG_loop_invariant_code_motion) {
           LICM licm(flow_graph);
           licm.Optimize();
           DEBUG_ASSERT(flow_graph->VerifyUseLists());
         }
         flow_graph->RemoveRedefinitions();
 
+        // Optimize (a << b) & c patterns, merge operations.
+        // Run after CSE in order to have more opportunity to merge
+        // instructions that have same inputs.
+        optimizer.TryOptimizePatterns();
+        DEBUG_ASSERT(flow_graph->VerifyUseLists());
+
+        DeadStoreElimination::Optimize(flow_graph);
+
         if (FLAG_range_analysis) {
           // Propagate types after store-load-forwarding. Some phis may have
           // become smi phis that can be processed by range analysis.
           FlowGraphTypePropagator::Propagate(flow_graph);
           DEBUG_ASSERT(flow_graph->VerifyUseLists());
 
           // We have to perform range analysis after LICM because it
           // optimistically moves CheckSmi through phis into loop preheaders
           // making some phis smi.
           optimizer.InferSmiRanges();
@@ skipping 499 matching lines @@
     const Object& result =
       Object::Handle(isolate->object_store()->sticky_error());
     isolate->object_store()->clear_sticky_error();
     return result.raw();
   }
   UNREACHABLE();
   return Object::null();
 }
 
 }  // namespace dart