Remove dead phis as soon as they are discovered.

runtime/vm/intermediate_language.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/intermediate_language.h"
 
 #include "vm/bit_vector.h"
 #include "vm/dart_entry.h"
 #include "vm/flow_graph_allocator.h"
 #include "vm/flow_graph_builder.h"
@@ skipping 742 matching lines @@
       // Search downwards, bubbling up intermediate predecessors.
       for (; new_index > 0; --new_index) {
         if (join->predecessors_[new_index - 1]->block_id() < new_id) break;
         join->predecessors_[new_index] = join->predecessors_[new_index - 1];
       }
     }
     join->predecessors_[new_index] = new_block;
     // If the new and old predecessor index match there is nothing to update.
     if ((join->phis() == NULL) || (old_index == new_index)) return;
     // Otherwise, reorder the predecessor uses in each phi.
-    for (intptr_t i = 0; i < join->phis()->length(); ++i) {
-      PhiInstr* phi = (*join->phis())[i];
-      if (phi == NULL) continue;
+    for (PhiIterator it(join); !it.Done(); it.Advance()) {
+      PhiInstr* phi = it.Current();
+      ASSERT(phi != NULL);
       ASSERT(pred_count == phi->InputCount());
       // Save the predecessor use.
       Value* pred_use = phi->InputAt(old_index);
       // Move uses between old and new.
       intptr_t step = (old_index < new_index) ? 1 : -1;
       for (intptr_t use_idx = old_index;
            use_idx != new_index;
            use_idx += step) {
         phi->SetInputAt(use_idx, phi->InputAt(use_idx + step));
       }
@@ skipping 10 matching lines @@
   // for variable with index i at position i.
   // TODO(fschneider): Store phis in a more compact way.
   if (phis_ == NULL) {
     phis_ = new ZoneGrowableArray<PhiInstr*>(var_count);
     for (intptr_t i = 0; i < var_count; i++) {
       phis_->Add(NULL);
     }
   }
   ASSERT((*phis_)[var_index] == NULL);
   (*phis_)[var_index] = new PhiInstr(this, PredecessorCount());
-  phi_count_++;
 }
 
 
 void JoinEntryInstr::InsertPhi(PhiInstr* phi) {
   // Lazily initialize the array of phis.
   if (phis_ == NULL) {
     phis_ = new ZoneGrowableArray<PhiInstr*>(1);
   }
   phis_->Add(phi);
-  phi_count_++;
 }
 
 
-void JoinEntryInstr::RemoveDeadPhis() {
+void JoinEntryInstr::RemoveDeadPhis(Definition* replacement) {
   if (phis_ == NULL) return;
 
-  for (intptr_t i = 0; i < phis_->length(); i++) {
-    PhiInstr* phi = (*phis_)[i];
-    if ((phi != NULL) && !phi->is_alive()) {
-      (*phis_)[i] = NULL;
-      phi_count_--;
+  intptr_t to_index = 0;
+  for (intptr_t from_index = 0; from_index < phis_->length(); ++from_index) {
+    PhiInstr* phi = (*phis_)[from_index];
+    if (phi != NULL) {
+      if (phi->is_alive()) {
+        (*phis_)[to_index++] = phi;
+        for (intptr_t i = phi->InputCount() - 1; i >= 0; --i) {
+          Value* input = phi->InputAt(i);
+          input->definition()->AddInputUse(input);
+        }
+      } else {
+        phi->ReplaceUsesWith(replacement);
+      }
     }
   }
-
-  // Check if we removed all phis.
-  if (phi_count_ == 0) phis_ = NULL;
+  if (to_index == 0) {
+    phis_ = NULL;
+  } else {
+    phis_->TruncateTo(to_index);
+  }
 }
 
 
 intptr_t Instruction::SuccessorCount() const {
   return 0;
 }
 
 
 BlockEntryInstr* Instruction::SuccessorAt(intptr_t index) const {
   // Called only if index is in range.  Only control-transfer instructions
@@ skipping 1426 matching lines @@
     default:
       UNREACHABLE();
   }
   return kPowRuntimeEntry;
 }
 
 
 #undef __
 
 }  // namespace dart