Fix bug in semispace shrink-to.

src/spaces.cc

 // Copyright 2011 the V8 project authors. All rights reserved.
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@@ 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;
+  Address space_end = start_ + maximum_capacity_;
+  Address old_start = space_end - capacity_;

[Vyacheslav Egorov (Chromium), 2011/06/29 11:16:59]

please add a comment about new space growing backwards from the end of the
reserved space.

[Lasse Reichstein, 2011/06/29 11:31:13]

Done.

   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)) {

[Vyacheslav Egorov (Chromium), 2011/06/29 11:16:59]

I am a bit concerned about uncommiting piece of the page. It seems possible.

[Lasse Reichstein, 2011/06/29 11:31:13]

And you are correct to be worried.
It won't happen that we free part of a page that contains content, but that's
due to the interaction of minimum_capacity and the times-2 multiplier in Shrink,
so instead of trying to codify that, we sohuld just round up before calling
ShrinkTo. I'll change my other CL to do that instead of removing the assertion.

     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