[compiler] Introduce a simple store-store elimination, disabled by default.

src/compiler/pipeline.cc

Index: src/compiler/pipeline.cc
diff --git a/src/compiler/pipeline.cc b/src/compiler/pipeline.cc
index ff442dda9ec88a42c4bb7acdf19e6f9c40a0f88c..0457a5795870ad29188f04ced6efe8b058b7d32d 100644
--- a/src/compiler/pipeline.cc
+++ b/src/compiler/pipeline.cc
@@ -9,6 +9,7 @@
 
 #include "src/base/adapters.h"
 #include "src/base/platform/elapsed-timer.h"
+#include "src/compiler/all-nodes.h"
 #include "src/compiler/ast-graph-builder.h"
 #include "src/compiler/ast-loop-assignment-analyzer.h"
 #include "src/compiler/basic-block-instrumentor.h"
@@ -57,6 +58,7 @@
 #include "src/compiler/simplified-lowering.h"
 #include "src/compiler/simplified-operator-reducer.h"
 #include "src/compiler/simplified-operator.h"
+#include "src/compiler/store-store-elimination.h"
 #include "src/compiler/tail-call-optimization.h"
 #include "src/compiler/type-hint-analyzer.h"
 #include "src/compiler/typer.h"
@@ -1028,6 +1030,37 @@ struct EffectControlLinearizationPhase {
   }
 };
 
+struct StoreStoreEliminationPhase {
+  static const char* phase_name() { return "Store-store elimination"; }
+
+  void Run(PipelineData* data, Zone* temp_zone) {
+    StoreStoreElimination store_store_elimination(data->jsgraph(), temp_zone);
+
+    // The store-store elimination does work on chains of certain types of
+    // nodes. The elimination must be invoked on the lowest node in such a
+    // chain; we have a helper function IsEligibleNode that returns true
+    // precisely on the lowest node in such a chain.
+    //
+    // Because the elimination removes nodes from the graph, even remove nodes
+    // that the elimination was not invoked on, we cannot use a normal
+    // AdvancedReducer but we manually find which nodes to invoke the
+    // elimination on. Then in a next step, we invoke the elimination for each
+    // node that was eligible.
+
+    NodeVector eligible(temp_zone);

[Jarin, 2016/06/21 13:28:32]

Could we move this code into the StoreStoreElimination class? E.g., you could
have a Run method there that does all this.

[bgeron, 2016/06/21 17:48:58]

Done.

+
+    for (Node* node : MarkingPreDFS(data->jsgraph()->graph())) {
+      if (StoreStoreElimination::IsEligibleNode(node)) {
+        eligible.push_back(node);
+      }
+    }
+
+    for (Node* node : eligible) {
+      store_store_elimination.ReduceEligibleNode(node);
+    }
+  }
+};
+
 struct MemoryOptimizationPhase {
   static const char* phase_name() { return "memory optimization"; }
 
@@ -1464,6 +1497,9 @@ bool PipelineImpl::OptimizeGraph(Linkage* linkage) {
   Run<EffectControlLinearizationPhase>();
   RunPrintAndVerify("Effect and control linearized", true);
 
+  Run<StoreStoreEliminationPhase>();
+  RunPrintAndVerify("Store-store elimination", true);
+
   Run<BranchEliminationPhase>();
   RunPrintAndVerify("Branch conditions eliminated", true);