[heap] Add length_ field to FreeListCategory

Having O(1) access to the length of the free lists on a given Page
will be useful to compute fragmentation and to implement efficient
heuristics to select evacuation candidates.

Bug: v8:9329
Change-Id: I92c7fcc38c89dcb18fef4ce7cc9ebf0b83d03dc5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1664065Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Darius Mercadier <dmercadier@google.com>
Cr-Commit-Position: refs/heads/master@{#62251}

src/heap/spaces.cc

@@ -2926,6 +2926,7 @@ void FreeListCategory::Reset() {
   set_prev(nullptr);
   set_next(nullptr);
   available_ = 0;
+  length_ = 0;
 }
 
 FreeSpace FreeListCategory::PickNodeFromList(size_t minimum_size,
@@ -2940,6 +2941,7 @@ FreeSpace FreeListCategory::PickNodeFromList(size_t minimum_size,
   set_top(node.next());
   *node_size = node.Size();
   available_ -= *node_size;
+  length_--;
   return node;
 }
 
@@ -2953,6 +2955,7 @@ FreeSpace FreeListCategory::SearchForNodeInList(size_t minimum_size,
     if (size >= minimum_size) {
       DCHECK_GE(available_, size);
       available_ -= size;
+      length_--;
       if (cur_node == top()) {
         set_top(cur_node.next());
       }
@@ -2978,6 +2981,7 @@ void FreeListCategory::Free(Address start, size_t size_in_bytes,
   free_space.set_next(top());
   set_top(free_space);
   available_ += size_in_bytes;
+  length_++;
   if ((mode == kLinkCategory) && (prev() == nullptr) && (next() == nullptr)) {
     owner()->AddCategory(this);
   }
@@ -3191,6 +3195,16 @@ void FreeList::PrintCategories(FreeListCategoryType type) {
   PrintF("null\n");
 }
 
+int MemoryChunk::FreeListsLength() {
+  int length = 0;
+  for (int cat = kFirstCategory; cat <= kLastCategory; cat++) {
+    if (categories_[cat] != nullptr) {
+      length += categories_[cat]->FreeListLength();
+    }
+  }
+  return length;
+}
+
 size_t FreeListCategory::SumFreeList() {
   size_t sum = 0;
   FreeSpace cur = top();
@@ -3205,17 +3219,6 @@ size_t FreeListCategory::SumFreeList() {
   return sum;
 }
 
-int FreeListCategory::FreeListLength() {
-  int length = 0;
-  FreeSpace cur = top();
-  while (!cur.is_null()) {
-    length++;
-    cur = cur.next();
-    if (length == kVeryLongFreeList) return length;
-  }
-  return length;
-}
-
 #ifdef DEBUG
 bool FreeList::IsVeryLong() {
   int len = 0;

src/heap/spaces.h

@@ -151,12 +151,14 @@ class FreeListCategory {
         page_(page),
         type_(kInvalidCategory),
         available_(0),
+        length_(0),
         prev_(nullptr),
         next_(nullptr) {}
 
   void Initialize(FreeListCategoryType type) {
     type_ = type;
     available_ = 0;
+    length_ = 0;
     prev_ = nullptr;
     next_ = nullptr;
   }
@@ -189,7 +191,7 @@ class FreeListCategory {
   void set_free_list(FreeList* free_list) { free_list_ = free_list; }
 
   size_t SumFreeList();
-  int FreeListLength();
+  int FreeListLength() { return length_; }
 
  private:
   // For debug builds we accurately compute free lists lengths up until
@@ -216,6 +218,9 @@ class FreeListCategory {
   // category.
   size_t available_;
 
+  // |length_|: Total blocks in this free list category.
+  int length_;
+
   // |top_|: Points to the top FreeSpace in the free list category.
   FreeSpace top_;
 
@@ -562,6 +567,8 @@ class MemoryChunk {
   Address area_end() { return area_end_; }
   size_t area_size() { return static_cast<size_t>(area_end() - area_start()); }
 
+  int FreeListsLength();
+
   // Approximate amount of physical memory committed for this chunk.
   V8_EXPORT_PRIVATE size_t CommittedPhysicalMemory();