Relax TransitionElementsKind DependsOn/Changes dependencies.

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:
 //
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@@ skipping 1158 matching lines @@
   present_flags_.RemoveAll();
   for (int i = 0; i < array_size_; ++i) {
     HValue* value = array_[i].value;
     if (value != NULL) {
       // Clear list of collisions first, so we know if it becomes empty.
       int kept = kNil;  // List of kept elements.
       int next;
       for (int current = array_[i].next; current != kNil; current = next) {
         next = lists_[current].next;
         HValue* value = lists_[current].value;
         if (value->gvn_flags().ContainsAnyOf(depends_flags)) {

[fschneider, 2012/02/16 13:50:51]

You'd also need to handle TransitionElementsKind specially here.

Generally, I think it would be cleaner to treat them specially when doing loop
code motion, rather than here by removing all depends-flags from
TransitionElementsKind.

There is already special code in ProcessLoopBlock for transitions, and this way
you can avoid having code for special treatment in two different places.

           // Drop it.
           count_--;
           lists_[current].next = free_list_head_;
           free_list_head_ = current;
         } else {
           // Keep it.
           lists_[current].next = kept;
           kept = current;
           present_flags_.Add(value->gvn_flags());
         }
       }
       array_[i].next = kept;
 
       // Now possibly drop directly indexed element.
       value = array_[i].value;
-      if (value->gvn_flags().ContainsAnyOf(depends_flags)) {  // Drop it.
+      // Remove a value from the value map is one of its dependencies has been
+      // changed, unless it's a TransitionElementsKind, in which case it's safe
+      // to keep it. TransitionElementsKinds that are dominated by another
+      // equivalent transition, even if there is an intervening side-effect.
+      // TransitionElementsKind only has a one-time side effect, so all
+      // subsequent Transitions are no-ops, even if there are other side effects
+      // in between.
+      if (value->gvn_flags().ContainsAnyOf(depends_flags) &&
+          !value->IsTransitionElementsKind()) {  // Drop it.
         count_--;
         int head = array_[i].next;
         if (head == kNil) {
           array_[i].value = NULL;
         } else {
           array_[i].value = lists_[head].value;
           array_[i].next = lists_[head].next;
           lists_[head].next = free_list_head_;
           free_list_head_ = head;
         }
@@ skipping 3241 matching lines @@
         map->FindTransitionedMap(&possible_transitioned_maps);
     transition_target.Add(transitioned_map);
   }
 
   int num_untransitionable_maps = 0;
   Handle<Map> untransitionable_map;
   for (int i = 0; i < maps->length(); ++i) {
     Handle<Map> map = maps->at(i);
     ASSERT(map->IsMap());
     if (!transition_target.at(i).is_null()) {
-      object = AddInstruction(new(zone()) HTransitionElementsKind(
+      AddInstruction(new(zone()) HTransitionElementsKind(
           object, map, transition_target.at(i)));
     } else {
       type_todo[map->elements_kind()] = true;
       if (map->elements_kind() >= FIRST_EXTERNAL_ARRAY_ELEMENTS_KIND) {
         todo_external_array = true;
       }
       num_untransitionable_maps++;
       untransitionable_map = map;
     }
   }
@@ skipping 3239 matching lines @@
     }
   }
 
 #ifdef DEBUG
   if (graph_ != NULL) graph_->Verify(false);  // No full verify.
   if (allocator_ != NULL) allocator_->Verify();
 #endif
 }
 
 } }  // namespace v8::internal