Allocation space decisions are precisely made in hydrogen.

src/hydrogen-instructions.h

 // 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 5223 matching lines @@
 
  private:
   Handle<Object> name_;
   bool for_typeof_;
 };
 
 
 class HAllocate: public HTemplateInstruction<2> {
  public:
   enum Flags {
-    CAN_ALLOCATE_IN_NEW_SPACE = 1 << 0,
-    CAN_ALLOCATE_IN_OLD_DATA_SPACE = 1 << 1,
-    CAN_ALLOCATE_IN_OLD_POINTER_SPACE = 1 << 2,
+    ALLOCATE_IN_NEW_SPACE = 1 << 0,
+    ALLOCATE_IN_OLD_DATA_SPACE = 1 << 1,
+    ALLOCATE_IN_OLD_POINTER_SPACE = 1 << 2,
     ALLOCATE_DOUBLE_ALIGNED = 1 << 3,
     PREFILL_WITH_FILLER = 1 << 4
   };
 
   HAllocate(HValue* context, HValue* size, HType type, Flags flags)
       : flags_(flags) {
+    ASSERT(((flags & kAllocationFlagsMask) == ALLOCATE_IN_NEW_SPACE) ||
+           ((flags & kAllocationFlagsMask) == ALLOCATE_IN_OLD_DATA_SPACE) ||
+           ((flags & kAllocationFlagsMask) == ALLOCATE_IN_OLD_POINTER_SPACE));
     SetOperandAt(0, context);
     SetOperandAt(1, size);
     set_type(type);
     set_representation(Representation::Tagged());
     SetFlag(kTrackSideEffectDominators);
     SetGVNFlag(kChangesNewSpacePromotion);
     SetGVNFlag(kDependsOnNewSpacePromotion);
   }
 
   // Maximum instance size for which allocations will be inlined.
   static const int kMaxInlineSize = 64 * kPointerSize;
 
   static Flags DefaultFlags() {
-    return CAN_ALLOCATE_IN_NEW_SPACE;
+    return ALLOCATE_IN_NEW_SPACE;
   }
 
   static Flags DefaultFlags(ElementsKind kind) {
-    Flags flags = CAN_ALLOCATE_IN_NEW_SPACE;
+    Flags flags = ALLOCATE_IN_NEW_SPACE;
     if (IsFastDoubleElementsKind(kind)) {
       flags = static_cast<HAllocate::Flags>(
           flags | HAllocate::ALLOCATE_DOUBLE_ALIGNED);
     }
     return flags;
   }
 
+  static Flags SetAllocationFlag(Flags flags, Flags allocation_flag) {

[Michael Starzinger, 2013/07/30 12:50:41]

As discussed offline: I don't like this helper very much, it just has minor
advantages over manipulating the flags field by hand. IMHO the DefaultFlags()
helper could instead take a PretenureFlag and already return the correct flags.
This works everywhere expect for FastLiteral, for which we have other plans
anyways.

[Hannes Payer (out of office), 2013/07/30 15:23:05]

I took that idea even further and made the HAllocate flags private and
internalized the allocation decisions.

+    ASSERT(allocation_flag == ALLOCATE_IN_NEW_SPACE ||
+           allocation_flag == ALLOCATE_IN_OLD_DATA_SPACE ||
+           allocation_flag == ALLOCATE_IN_OLD_POINTER_SPACE);
+    flags = static_cast<HAllocate::Flags>(flags & ~kAllocationFlagsMask);
+    return static_cast<HAllocate::Flags>(flags | allocation_flag);
+  }
+
   HValue* context() { return OperandAt(0); }
   HValue* size() { return OperandAt(1); }
 
   virtual Representation RequiredInputRepresentation(int index) {
     if (index == 0) {
       return Representation::Tagged();
     } else {
       return Representation::Integer32();
     }
   }
 
   virtual Handle<Map> GetMonomorphicJSObjectMap() {
     return known_initial_map_;
   }
 
   void set_known_initial_map(Handle<Map> known_initial_map) {
     known_initial_map_ = known_initial_map;
   }
 
-  bool CanAllocateInNewSpace() const {
-    return (flags_ & CAN_ALLOCATE_IN_NEW_SPACE) != 0;
+  bool AllocateInNewSpace() const {
+    return (flags_ & ALLOCATE_IN_NEW_SPACE) != 0;
   }
 
-  bool CanAllocateInOldDataSpace() const {
-    return (flags_ & CAN_ALLOCATE_IN_OLD_DATA_SPACE) != 0;
+  bool AllocateInOldDataSpace() const {
+    return (flags_ & ALLOCATE_IN_OLD_DATA_SPACE) != 0;
   }
 
-  bool CanAllocateInOldPointerSpace() const {
-    return (flags_ & CAN_ALLOCATE_IN_OLD_POINTER_SPACE) != 0;
-  }
-
-  bool CanAllocateInOldSpace() const {
-    return CanAllocateInOldDataSpace() ||
-        CanAllocateInOldPointerSpace();
-  }
-
-  bool GuaranteedInNewSpace() const {
-    return CanAllocateInNewSpace() && !CanAllocateInOldSpace();
+  bool AllocateInOldPointerSpace() const {
+    return (flags_ & ALLOCATE_IN_OLD_POINTER_SPACE) != 0;
   }
 
   bool MustAllocateDoubleAligned() const {
     return (flags_ & ALLOCATE_DOUBLE_ALIGNED) != 0;
   }
 
   bool MustPrefillWithFiller() const {
     return (flags_ & PREFILL_WITH_FILLER) != 0;
   }
 
   void SetFlags(Flags flags) {
     flags_ = static_cast<HAllocate::Flags>(flags_ | flags);
+    ASSERT(((flags_ & kAllocationFlagsMask) == ALLOCATE_IN_NEW_SPACE) ||
+           ((flags_ & kAllocationFlagsMask) == ALLOCATE_IN_OLD_DATA_SPACE) ||
+           ((flags_ & kAllocationFlagsMask) == ALLOCATE_IN_OLD_POINTER_SPACE));
   }
 
   void UpdateSize(HValue* size) {
     SetOperandAt(1, size);
   }
 
   virtual void HandleSideEffectDominator(GVNFlag side_effect,
                                          HValue* dominator);
 
   virtual void PrintDataTo(StringStream* stream);
 
   DECLARE_CONCRETE_INSTRUCTION(Allocate)
 
  private:
+  static const int kAllocationFlagsMask = 0x7;

[Michael Starzinger, 2013/07/30 12:50:41]

Better compute this using the above three enum values, otherwise it might become
out of sync.

[Hannes Payer (out of office), 2013/07/30 15:23:05]

Removed, not needed anymore

   Flags flags_;
   Handle<Map> known_initial_map_;
 };
 
 
 class HInnerAllocatedObject: public HTemplateInstruction<1> {
  public:
   HInnerAllocatedObject(HValue* value, int offset, HType type = HType::Tagged())
       : offset_(offset) {
     ASSERT(value->IsAllocate());
@@ skipping 30 matching lines @@
   if (object->IsInnerAllocatedObject()) {
     return ReceiverObjectNeedsWriteBarrier(
         HInnerAllocatedObject::cast(object)->base_object(),
         new_space_dominator);
   }
   if (object->IsConstant() && HConstant::cast(object)->IsCell()) {
     return false;
   }
   if (object != new_space_dominator) return true;
   if (object->IsAllocate()) {
-    return !HAllocate::cast(object)->GuaranteedInNewSpace();
+    return !HAllocate::cast(object)->AllocateInNewSpace();
   }
   return true;
 }
 
 
 class HStoreGlobalCell: public HUnaryOperation {
  public:
   HStoreGlobalCell(HValue* value,
                    Handle<PropertyCell> cell,
                    PropertyDetails details)
@@ skipping 1603 matching lines @@
   virtual bool IsDeletable() const { return true; }
 };
 
 
 #undef DECLARE_INSTRUCTION
 #undef DECLARE_CONCRETE_INSTRUCTION
 
 } }  // namespace v8::internal
 
 #endif  // V8_HYDROGEN_INSTRUCTIONS_H_