Fix bug in semispace shrink-to.

src/spaces.cc

 // Copyright 2011 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 1204 matching lines @@
     last_page = new_page;
   }
   return true;
 }
 
 
 bool SemiSpace::ShrinkTo(int new_capacity) {
   ASSERT((new_capacity & Page::kPageAlignmentMask) == 0);
   ASSERT(new_capacity >= initial_capacity_);
   ASSERT(new_capacity < capacity_);
-  Address end = start_ + maximum_capacity_;
-  Address start = end - new_capacity;
+  // Semispaces grow backwards from the end of their allocated capacity,
+  // so we find the before and after start addresses relative to the
+  // end of the space.
+  Address space_end = start_ + maximum_capacity_;
+  Address old_start = space_end - capacity_;
   size_t delta = capacity_ - new_capacity;
   ASSERT(IsAligned(delta, OS::AllocateAlignment()));
-  if (!heap()->isolate()->memory_allocator()->UncommitBlock(start, delta)) {
+  if (!heap()->isolate()->memory_allocator()->UncommitBlock(old_start, delta)) {
     return false;
   }
   capacity_ = new_capacity;
 
   int pages_after = capacity_ / Page::kPageSize;
   NewSpacePage* new_last_page =
-      NewSpacePage::FromAddress(end - pages_after * Page::kPageSize);
+      NewSpacePage::FromAddress(space_end - pages_after * Page::kPageSize);
   new_last_page->set_next_page(anchor());
   anchor()->set_prev_page(new_last_page);
   ASSERT(current_page_ == first_page());
 
   return true;
 }
 
 
 void SemiSpace::FlipPages(intptr_t flags, intptr_t mask) {
   anchor_.set_owner(this);
@@ skipping 1259 matching lines @@
   for (HeapObject* obj = obj_it.Next(); obj != NULL; obj = obj_it.Next()) {
     if (obj->IsCode()) {
       Code* code = Code::cast(obj);
       isolate->code_kind_statistics()[code->kind()] += code->Size();
     }
   }
 }
 #endif  // DEBUG
 
 } }  // namespace v8::internal