Promotion is the backup strategy when semi-space copy fails and vice versa.

src/heap.cc

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "src/v8.h"
 
 #include "src/accessors.h"
 #include "src/api.h"
 #include "src/base/once.h"
 #include "src/bootstrapper.h"
@@ skipping 1952 matching lines @@
       heap->OnMoveEvent(target, source, size);
     }
 
     if (marks_handling == TRANSFER_MARKS) {
       if (Marking::TransferColor(source, target)) {
         MemoryChunk::IncrementLiveBytesFromGC(target->address(), size);
       }
     }
   }
 
+  template<int alignment>
+  static inline bool SemiSpaceCopyObject(Map* map,
+                                         HeapObject** slot,
+                                         HeapObject* object,
+                                         int object_size) {
+    Heap* heap = map->GetHeap();
+
+    int allocation_size = object_size;
+    if (alignment != kObjectAlignment) {
+      ASSERT(alignment == kDoubleAlignment);
+      allocation_size += kPointerSize;
+    }
+
+    ASSERT(heap->AllowedToBeMigrated(object, NEW_SPACE));
+    AllocationResult allocation =
+        heap->new_space()->AllocateRaw(allocation_size);
+
+    // Allocation in the other semi-space may fail due to fragmentation.
+    // In that case we allocate in the old generation.

[Igor Sheludko, 2014/06/25 09:06:37]

Probably this comment should not be here since you already added similar comment
in if (!heap->ShouldBePromoted()) below.

[Hannes Payer (out of office), 2014/06/25 09:25:44]

Done.

+    HeapObject* target = NULL;  // Initialization to please compiler.
+    if (allocation.To(&target)) {
+      if (alignment != kObjectAlignment) {
+        target = EnsureDoubleAligned(heap, target, allocation_size);
+      }
+
+      // Order is important: slot might be inside of the target if target
+      // was allocated over a dead object and slot comes from the store
+      // buffer.
+      *slot = target;
+      MigrateObject(heap, object, target, object_size);
+
+      heap->promotion_queue()->SetNewLimit(heap->new_space()->top());
+      heap->IncrementSemiSpaceCopiedObjectSize(object_size);
+      return true;
+    }
+    return false;
+  }
+
 
   template<ObjectContents object_contents, int alignment>
-  static inline void EvacuateObject(Map* map,
-                                    HeapObject** slot,
-                                    HeapObject* object,
-                                    int object_size) {
-    SLOW_ASSERT(object_size <= Page::kMaxRegularHeapObjectSize);
-    SLOW_ASSERT(object->Size() == object_size);
+  static inline bool PromoteObject(Map* map,
+                                   HeapObject** slot,
+                                   HeapObject* object,
+                                   int object_size) {
+    Heap* heap = map->GetHeap();
 
     int allocation_size = object_size;
     if (alignment != kObjectAlignment) {
       ASSERT(alignment == kDoubleAlignment);
       allocation_size += kPointerSize;
     }
 
-    Heap* heap = map->GetHeap();
-    if (heap->ShouldBePromoted(object->address(), object_size)) {
-      AllocationResult allocation;
+    AllocationResult allocation;
+    if (object_contents == DATA_OBJECT) {
+      ASSERT(heap->AllowedToBeMigrated(object, OLD_DATA_SPACE));
+      allocation = heap->old_data_space()->AllocateRaw(allocation_size);
+    } else {
+      ASSERT(heap->AllowedToBeMigrated(object, OLD_POINTER_SPACE));
+      allocation = heap->old_pointer_space()->AllocateRaw(allocation_size);
+    }
 
-      if (object_contents == DATA_OBJECT) {
-        ASSERT(heap->AllowedToBeMigrated(object, OLD_DATA_SPACE));
-        allocation = heap->old_data_space()->AllocateRaw(allocation_size);
-      } else {
-        ASSERT(heap->AllowedToBeMigrated(object, OLD_POINTER_SPACE));
-        allocation = heap->old_pointer_space()->AllocateRaw(allocation_size);
+    HeapObject* target = NULL;  // Initialization to please compiler.
+    if (allocation.To(&target)) {
+      if (alignment != kObjectAlignment) {
+        target = EnsureDoubleAligned(heap, target, allocation_size);
       }
 
-      HeapObject* target = NULL;  // Initialization to please compiler.
-      if (allocation.To(&target)) {
-        if (alignment != kObjectAlignment) {
-          target = EnsureDoubleAligned(heap, target, allocation_size);
+      // Order is important: slot might be inside of the target if target
+      // was allocated over a dead object and slot comes from the store
+      // buffer.
+      *slot = target;
+      MigrateObject(heap, object, target, object_size);
+
+      if (object_contents == POINTER_OBJECT) {
+        if (map->instance_type() == JS_FUNCTION_TYPE) {
+          heap->promotion_queue()->insert(
+              target, JSFunction::kNonWeakFieldsEndOffset);
+        } else {
+          heap->promotion_queue()->insert(target, object_size);
         }
+      }
+      heap->IncrementPromotedObjectsSize(object_size);
+      return true;
+    }
+    return false;
+  }
 
-        // Order is important: slot might be inside of the target if target
-        // was allocated over a dead object and slot comes from the store
-        // buffer.
-        *slot = target;
-        MigrateObject(heap, object, target, object_size);
 
-        if (object_contents == POINTER_OBJECT) {
-          if (map->instance_type() == JS_FUNCTION_TYPE) {
-            heap->promotion_queue()->insert(
-                target, JSFunction::kNonWeakFieldsEndOffset);
-          } else {
-            heap->promotion_queue()->insert(target, object_size);
-          }
-        }
+  template<ObjectContents object_contents, int alignment>
+  static inline void EvacuateObject(Map* map,
+                                    HeapObject** slot,
+                                    HeapObject* object,
+                                    int object_size) {
+    SLOW_ASSERT(object_size <= Page::kMaxRegularHeapObjectSize);
+    SLOW_ASSERT(object->Size() == object_size);
+    Heap* heap = map->GetHeap();
 
-        heap->IncrementPromotedObjectsSize(object_size);
+    if (!heap->ShouldBePromoted(object->address(), object_size)) {
+      // A semi-space copy may fail due to fragmentation. In that case, we
+      // try to promote the object.
+      if (SemiSpaceCopyObject<alignment>(map, slot, object, object_size)) {
         return;
       }
     }
-    ASSERT(heap->AllowedToBeMigrated(object, NEW_SPACE));
-    AllocationResult allocation =
-        heap->new_space()->AllocateRaw(allocation_size);
-    heap->promotion_queue()->SetNewLimit(heap->new_space()->top());
 
-    // Allocation in the other semi-space may fail due to fragmentation.
-    // In that case we allocate in the old generation.
-    if (allocation.IsRetry()) {
-      if (object_contents == DATA_OBJECT) {
-        ASSERT(heap->AllowedToBeMigrated(object, OLD_DATA_SPACE));
-        allocation = heap->old_data_space()->AllocateRaw(allocation_size);
-      } else {
-        ASSERT(heap->AllowedToBeMigrated(object, OLD_POINTER_SPACE));
-        allocation = heap->old_pointer_space()->AllocateRaw(allocation_size);
-      }
+    if (PromoteObject<object_contents, alignment>(
+        map, slot, object, object_size)) {
+      return;
     }
 
-    HeapObject* target = HeapObject::cast(allocation.ToObjectChecked());
+    // If promotion failed, we try to copy the object to the other semi-space
+    if (SemiSpaceCopyObject<alignment>(map, slot, object, object_size)) return;
 
-    if (alignment != kObjectAlignment) {
-      target = EnsureDoubleAligned(heap, target, allocation_size);
-    }
-
-    // Order is important: slot might be inside of the target if target
-    // was allocated over a dead object and slot comes from the store
-    // buffer.
-    *slot = target;
-    MigrateObject(heap, object, target, object_size);
-    heap->IncrementSemiSpaceCopiedObjectSize(object_size);
-    return;
+    UNREACHABLE();
   }
 
 
   static inline void EvacuateJSFunction(Map* map,
                                         HeapObject** slot,
                                         HeapObject* object) {
     ObjectEvacuationStrategy<POINTER_OBJECT>::
         template VisitSpecialized<JSFunction::kSize>(map, slot, object);
 
     HeapObject* target = *slot;
@@ skipping 4305 matching lines @@
       static_cast<int>(object_sizes_last_time_[index]));
   CODE_AGE_LIST_COMPLETE(ADJUST_LAST_TIME_OBJECT_COUNT)
 #undef ADJUST_LAST_TIME_OBJECT_COUNT
 
   MemCopy(object_counts_last_time_, object_counts_, sizeof(object_counts_));
   MemCopy(object_sizes_last_time_, object_sizes_, sizeof(object_sizes_));
   ClearObjectStats();
 }
 
 } }  // namespace v8::internal