Revert r13176.

src/hydrogen.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 3308 matching lines @@
   if (FLAG_use_range) {
     HRangeAnalysis rangeAnalysis(this);
     rangeAnalysis.Analyze();
   }
   ComputeMinusZeroChecks();
 
   // Eliminate redundant stack checks on backwards branches.
   HStackCheckEliminator sce(this);
   sce.Process();
 
-  if (FLAG_array_bounds_checks_elimination) EliminateRedundantBoundsChecks();
-  if (FLAG_array_index_dehoisting) DehoistSimpleArrayIndexComputations();
+  EliminateRedundantBoundsChecks();
+  DehoistSimpleArrayIndexComputations();
   if (FLAG_dead_code_elimination) DeadCodeElimination();
 
   return true;
 }
 
 
 // We try to "factor up" HBoundsCheck instructions towards the root of the
 // dominator tree.
 // For now we handle checks where the index is like "exp + int32value".
 // If in the dominator tree we check "exp + v1" and later (dominated)
@@ skipping 136 matching lines @@
                        Key()->IndexBase(),
                        new_check->index()->representation(),
                        new_offset);
         lower_check_->SetOperandAt(0, added_lower_index_);
       }
     } else {
       ASSERT(false);
     }
 
     if (!keep_new_check) {
-      new_check->DeleteAndReplaceWith(new_check->index());
+      new_check->DeleteAndReplaceWith(NULL);
     }
   }
 
   void RemoveZeroOperations() {
     RemoveZeroAdd(&added_lower_index_, &added_lower_offset_);
     RemoveZeroAdd(&added_upper_index_, &added_upper_offset_);
   }
 
   BoundsCheckBbData(BoundsCheckKey* key,
                     int32_t lower_offset,
@@ skipping 86 matching lines @@
     Remove(key, key->Hash());
   }
 
   explicit BoundsCheckTable(Zone* zone)
       : ZoneHashMap(BoundsCheckKeyMatch, ZoneHashMap::kDefaultHashMapCapacity,
                     ZoneAllocationPolicy(zone)) { }
 };
 
 
 // Eliminates checks in bb and recursively in the dominated blocks.
+// Also replace the results of check instructions with the original value, if
+// the result is used. This is safe now, since we don't do code motion after
+// this point. It enables better register allocation since the value produced
+// by check instructions is really a copy of the original value.
 void HGraph::EliminateRedundantBoundsChecks(HBasicBlock* bb,
                                             BoundsCheckTable* table) {
   BoundsCheckBbData* bb_data_list = NULL;
 
   for (HInstruction* i = bb->first(); i != NULL; i = i->next()) {
     if (!i->IsBoundsCheck()) continue;
+
     HBoundsCheck* check = HBoundsCheck::cast(i);
+    check->ReplaceAllUsesWith(check->index());
+
+    if (!FLAG_array_bounds_checks_elimination) continue;
 
     int32_t offset;
     BoundsCheckKey* key =
         BoundsCheckKey::Create(zone(), check, &offset);
     if (key == NULL) continue;
     BoundsCheckBbData** data_p = table->LookupOrInsert(key, zone());
     BoundsCheckBbData* data = *data_p;
     if (data == NULL) {
       bb_data_list = new(zone()) BoundsCheckBbData(key,
                                                    offset,
                                                    offset,
                                                    bb,
                                                    check,
                                                    check,
                                                    bb_data_list,
                                                    NULL);
       *data_p = bb_data_list;
     } else if (data->OffsetIsCovered(offset)) {
-      check->DeleteAndReplaceWith(check->index());
+      check->DeleteAndReplaceWith(NULL);
     } else if (data->BasicBlock() == bb) {
       data->CoverCheck(check, offset);
     } else {
       int32_t new_lower_offset = offset < data->LowerOffset()
           ? offset
           : data->LowerOffset();
       int32_t new_upper_offset = offset > data->UpperOffset()
           ? offset
           : data->UpperOffset();
       bb_data_list = new(zone()) BoundsCheckBbData(key,
@@ skipping 74 matching lines @@
   if (index->HasNoUses()) {
     index->DeleteAndReplaceWith(NULL);
   }
   ASSERT(value >= 0);
   array_operation->SetIndexOffset(static_cast<uint32_t>(value));
   array_operation->SetDehoisted(true);
 }
 
 
 void HGraph::DehoistSimpleArrayIndexComputations() {
+  if (!FLAG_array_index_dehoisting) return;
+
   HPhase phase("H_Dehoist index computations", this);
   for (int i = 0; i < blocks()->length(); ++i) {
     for (HInstruction* instr = blocks()->at(i)->first();
         instr != NULL;
         instr = instr->next()) {
       ArrayInstructionInterface* array_instruction = NULL;
       if (instr->IsLoadKeyed()) {
         HLoadKeyed* op = HLoadKeyed::cast(instr);
         array_instruction = static_cast<ArrayInstructionInterface*>(op);
       } else if (instr->IsStoreKeyed()) {
@@ skipping 6358 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify(false);  // No full verify.
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal