VM: Improve phi-elimination to improve code inside of try-catch.

runtime/vm/redundancy_elimination.cc

 // Copyright (c) 2016, 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/redundancy_elimination.h"
 
 #include "vm/bit_vector.h"
 #include "vm/flow_graph.h"
 #include "vm/hash_map.h"
 #include "vm/il_printer.h"
@@ skipping 2329 matching lines @@
         a->RemoveFromGraph();
       }
     }
 
     return true;
   }
 
   // Insert the given phi into the graph. Attempt to find an equal one in the
   // target block first.
   // Returns true if the phi was inserted and false if it was replaced.
+  // TODO(fschneider): Generalize this to allow replacing a phi with a non-phi
+  // definition as well so that we don't need Phi::HasReplacement anymore.
   bool EmitPhi(PhiInstr* phi) {
     for (PhiIterator it(phi->block()); !it.Done(); it.Advance()) {
       if (ReplacePhiWith(phi, it.Current())) {
         return false;
       }
     }
 
     phi->mark_alive();
     phi->block()->InsertPhi(phi);
     return true;
@@ skipping 1070 matching lines @@
        it.Advance()) {
     JoinEntryInstr* join = it.Current()->AsJoinEntry();
     if (join != NULL) {
       if (join->phis_ == NULL) continue;
 
       // Eliminate dead phis and compact the phis_ array of the block.
       intptr_t to_index = 0;
       for (intptr_t i = 0; i < join->phis_->length(); ++i) {
         PhiInstr* phi = (*join->phis_)[i];
         if (phi != NULL) {
+          Definition* replacement = NULL;
           if (!phi->is_alive()) {
             phi->ReplaceUsesWith(flow_graph->constant_null());
             phi->UnuseAllInputs();
             (*join->phis_)[i] = NULL;
             if (FLAG_trace_optimization) {
               THR_Print("Removing dead phi v%" Pd "\n", phi->ssa_temp_index());
             }
           } else if (phi->IsRedundant()) {
             phi->ReplaceUsesWith(phi->InputAt(0)->definition());
             phi->UnuseAllInputs();
             (*join->phis_)[i] = NULL;
             if (FLAG_trace_optimization) {
               THR_Print("Removing redundant phi v%" Pd "\n",
-                         phi->ssa_temp_index());
+                        phi->ssa_temp_index());
+            }
+          } else if (phi->HasReplacement(&replacement)) {
+            phi->ReplaceUsesWith(replacement);
+            phi->UnuseAllInputs();
+            (*join->phis_)[i] = NULL;
+            if (FLAG_trace_optimization) {
+              THR_Print("Replace redundant phi v%" Pd " with v%" Pd "\n",
+                        phi->ssa_temp_index(),
+                        replacement->ssa_temp_index());
             }
           } else {
             (*join->phis_)[to_index++] = phi;
           }
         }
       }
       if (to_index == 0) {
         join->phis_ = NULL;
       } else {
         join->phis_->TruncateTo(to_index);
       }
     }
   }
 }
 
 
 }  // namespace dart