Add some support for promptly purging pages.

third_party/WebKit/Source/platform/heap/HeapPage.cpp

Index: third_party/WebKit/Source/platform/heap/HeapPage.cpp
diff --git a/third_party/WebKit/Source/platform/heap/HeapPage.cpp b/third_party/WebKit/Source/platform/heap/HeapPage.cpp
index d632988f0987021f7a9111030195c8b57365c5c3..27fa07ec3d6e4fcc7e8c1038e2aa3704abcd4774 100644
--- a/third_party/WebKit/Source/platform/heap/HeapPage.cpp
+++ b/third_party/WebKit/Source/platform/heap/HeapPage.cpp
@@ -446,6 +446,27 @@ size_t NormalPageArena::FreeListSize() {
   return free_size;
 }
 
+void NormalPage::CompactionContext::AddAvailable(BasePage* page) {
+  if (!available_pages_) {
+    DCHECK(!last_available_);
+    available_pages_ = last_available_ = page;
+    return;
+  }
+  DCHECK(last_available_);
+  BasePage* next_available_page = page;
+  last_available_->Link(&next_available_page);
+  last_available_ = page;
+}
+
+BasePage* NormalPage::CompactionContext::TakeAvailable() {
+  DCHECK(available_pages_);
+  BasePage* page = available_pages_;
+  page->Unlink(&available_pages_);
+  if (page == last_available_)
+    last_available_ = nullptr;
+  return page;
+}
+
 void NormalPageArena::SweepAndCompact() {
   ThreadHeap& heap = GetThreadState()->Heap();
   if (!heap.Compaction()->IsCompactingArena(ArenaIndex()))
@@ -500,7 +521,7 @@ void NormalPageArena::SweepAndCompact() {
     if (!context.current_page_)
       context.current_page_ = normal_page;
     else
-      normal_page->Link(&context.available_pages_);
+      context.AddAvailable(normal_page);
     normal_page->SweepAndCompact(context);
   }
 
@@ -559,7 +580,9 @@ void NormalPageArena::SweepAndCompact() {
     NormalPage* unused_page = reinterpret_cast<NormalPage*>(available_pages);
     memset(unused_page->Payload(), 0, unused_page->PayloadSize());
 #endif
-    available_pages->RemoveFromHeap();
+    // Try to cycle out the pages right away; done to try get
+    // immediately observable memory reduction across platforms.
+    available_pages->RemoveFromHeap(DecommitMemoryTiming::DecommitPromptly);
     available_pages = static_cast<NormalPage*>(next_page);
   }
   if (freed_page_count)
@@ -658,12 +681,14 @@ void NormalPageArena::AllocatePage() {
   AddToFreeList(page->Payload(), page->PayloadSize());
 }
 
-void NormalPageArena::FreePage(NormalPage* page) {
+void NormalPageArena::FreePage(NormalPage* page,
+                               DecommitMemoryTiming decommit_hint) {
   GetThreadState()->Heap().HeapStats().DecreaseAllocatedSpace(page->size());
 
   PageMemory* memory = page->Storage();
   page->~NormalPage();
-  GetThreadState()->Heap().GetFreePagePool()->Add(ArenaIndex(), memory);
+  GetThreadState()->Heap().GetFreePagePool()->Add(ArenaIndex(), memory,
+                                                  decommit_hint);
 }
 
 bool NormalPageArena::Coalesce() {
@@ -1291,8 +1316,8 @@ bool NormalPage::IsEmpty() {
   return header->IsFree() && header->size() == PayloadSize();
 }
 
-void NormalPage::RemoveFromHeap() {
-  ArenaForNormalPage()->FreePage(this);
+void NormalPage::RemoveFromHeap(DecommitMemoryTiming decommit_hint) {
+  ArenaForNormalPage()->FreePage(this, decommit_hint);
 }
 
 #if !DCHECK_IS_ON() && !defined(LEAK_SANITIZER) && !defined(ADDRESS_SANITIZER)
@@ -1441,10 +1466,7 @@ void NormalPage::SweepAndCompact(CompactionContext& context) {
                                                           free_size);
       }
 
-      BasePage* next_available_page;
-      context.available_pages_->Unlink(&next_available_page);
-      current_page = reinterpret_cast<NormalPage*>(context.available_pages_);
-      context.available_pages_ = next_available_page;
+      current_page = static_cast<NormalPage*>(context.TakeAvailable());
       allocation_point = 0;
       compact_frontier = current_page->Payload();
     }
@@ -1729,7 +1751,7 @@ bool LargeObjectPage::IsEmpty() {
   return !GetHeapObjectHeader()->IsMarked();
 }
 
-void LargeObjectPage::RemoveFromHeap() {
+void LargeObjectPage::RemoveFromHeap(DecommitMemoryTiming) {
   static_cast<LargeObjectArena*>(Arena())->FreeLargeObjectPage(this);
 }