Fix bug in semispace shrink-to.

src/spaces.cc

Index: src/spaces.cc
diff --git a/src/spaces.cc b/src/spaces.cc
index 1d980cc05d2bd9077d531fabbe1b32420e3ed3ad..ae7542da98f83234b8e994da420e993661fb015d 100644
--- a/src/spaces.cc
+++ b/src/spaces.cc
@@ -1222,18 +1222,18 @@ 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());