[heap] Add page evacuation mode for new->new

src/heap/spaces.cc

Index: src/heap/spaces.cc
diff --git a/src/heap/spaces.cc b/src/heap/spaces.cc
index 6b6725a01eab8977cad4a511d3df146b6e7f399c..497a3c0261375d5e21f0b89baac4373b5229163c 100644
--- a/src/heap/spaces.cc
+++ b/src/heap/spaces.cc
@@ -1891,6 +1891,32 @@ bool SemiSpace::ReplaceWithEmptyPage(Page* old_page) {
   return true;
 }
 
+void SemiSpace::PrependPage(Page* page) {

[ulan, 2016/06/10 12:58:54]

From the name of the function I would expect that it just prepends the page to
the semispace.

But it actually finds a replacement page and swaps it with the given page.

Would it make sense to break this function into two functions?
page2 = semispace2->FindReplacementPage()
new_space->SwapSemispacePages(page1, page2)

[Michael Lippautz, 2016/06/10 15:18:16]

I restructured the page replacement. It's now a specialized one called
SwapFromSpacePageWithToSpacePage that computes the parameters for a generic one
called SwapPages.

+  // Remove page at the end.
+  Page* replacement_page = anchor()->prev_page();
+  replacement_page->next_page()->set_prev_page(replacement_page->prev_page());
+  replacement_page->prev_page()->set_next_page(replacement_page->next_page());
+  DCHECK_NE(current_page(), replacement_page);

[ulan, 2016/06/10 12:58:54]

Why this holds?

[Michael Lippautz, 2016/06/10 15:18:16]

I added a comment (above !OnLastPage() -- because it's essentially the same
check). Basically, the last page should always be a proper replacement page
because in the case where we move a page, this page should still be free at the
end of the space.

+
+  SemiSpace* old_semi_space = reinterpret_cast<SemiSpace*>(page->owner());
+  if (page == old_semi_space->current_page()) {
+    old_semi_space->current_page_ = replacement_page;
+  }
+
+  // Replace it.
+  replacement_page->set_owner(page->owner());
+  replacement_page->SetFlags(page->GetFlags(), Page::kCopyAllFlags);
+  page->next_page()->set_prev_page(replacement_page);
+  page->prev_page()->set_next_page(replacement_page);
+  replacement_page->set_prev_page(page->prev_page());
+  replacement_page->set_next_page(page->next_page());
+  page->set_next_page(nullptr);
+  page->set_prev_page(nullptr);
+  page->SetFlags(current_page()->GetFlags(), Page::kCopyAllFlags);
+  page->set_owner(this);
+  page->InsertAfter(anchor());
+}
+
 void SemiSpace::Swap(SemiSpace* from, SemiSpace* to) {
   // We won't be swapping semispaces without data in them.
   DCHECK_NE(from->anchor_.next_page(), &from->anchor_);