Revert of Pool implementation for zone segments

src/zone/zone.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_ZONE_ZONE_H_
 #define V8_ZONE_ZONE_H_
 
 #include <limits>
 
 #include "src/base/hashmap.h"
 #include "src/base/logging.h"
 #include "src/globals.h"
 #include "src/list.h"
 #include "src/splay-tree.h"
 #include "src/zone/accounting-allocator.h"
 
 namespace v8 {
 namespace internal {
 
 // The Zone supports very fast allocation of small chunks of
 // memory. The chunks cannot be deallocated individually, but instead
 // the Zone supports deallocating all chunks in one fast
 // operation. The Zone is used to hold temporary data structures like
 // the abstract syntax tree, which is deallocated after compilation.
 //
 // Note: There is no need to initialize the Zone; the first time an
 // allocation is attempted, a segment of memory will be requested
-// through the allocator.
+// through a call to malloc().
 //
 // Note: The implementation is inherently not thread safe. Do not use
 // from multi-threaded code.
 class V8_EXPORT_PRIVATE Zone final {
  public:
   explicit Zone(AccountingAllocator* allocator);
   ~Zone();
 
   // Allocate 'size' bytes of memory in the Zone; expands the Zone by
   // allocating new segments of memory on demand using malloc().
   void* New(size_t size);
 
   template <typename T>
   T* NewArray(size_t length) {
     DCHECK_LT(length, std::numeric_limits<size_t>::max() / sizeof(T));
     return static_cast<T*>(New(length * sizeof(T)));
   }
 
-  // Deletes all objects and free all memory allocated in the Zone.
+  // Deletes all objects and free all memory allocated in the Zone. Keeps one
+  // small (size <= kMaximumKeptSegmentSize) segment around if it finds one.
   void DeleteAll();
 
+  // Deletes the last small segment kept around by DeleteAll(). You
+  // may no longer allocate in the Zone after a call to this method.
+  void DeleteKeptSegment();
+
   // Returns true if more memory has been allocated in zones than
   // the limit allows.
   bool excess_allocation() const {
     return segment_bytes_allocated_ > kExcessLimit;
   }
 
   size_t allocation_size() const { return allocation_size_; }
 
   AccountingAllocator* allocator() const { return allocator_; }
 
  private:
 // All pointers returned from New() have this alignment.  In addition, if the
 // object being allocated has a size that is divisible by 8 then its alignment
 // will be 8. ASan requires 8-byte alignment.
 #ifdef V8_USE_ADDRESS_SANITIZER
   static const size_t kAlignment = 8;
   STATIC_ASSERT(kPointerSize <= 8);
 #else
   static const size_t kAlignment = kPointerSize;
 #endif
 
   // Never allocate segments smaller than this size in bytes.
   static const size_t kMinimumSegmentSize = 8 * KB;
 
   // Never allocate segments larger than this size in bytes.
   static const size_t kMaximumSegmentSize = 1 * MB;
 
+  // Never keep segments larger than this size in bytes around.
+  static const size_t kMaximumKeptSegmentSize = 64 * KB;
+
   // Report zone excess when allocation exceeds this limit.
   static const size_t kExcessLimit = 256 * MB;
 
   // The number of bytes allocated in this zone so far.
   size_t allocation_size_;
 
   // The number of bytes allocated in segments.  Note that this number
   // includes memory allocated from the OS but not yet allocated from
   // the zone.
   size_t segment_bytes_allocated_;
 
   // Expand the Zone to hold at least 'size' more bytes and allocate
   // the bytes. Returns the address of the newly allocated chunk of
   // memory in the Zone. Should only be called if there isn't enough
   // room in the Zone already.
   Address NewExpand(size_t size);
 
   // Creates a new segment, sets it size, and pushes it to the front
   // of the segment chain. Returns the new segment.
-  inline Segment* NewSegment(size_t requested_size);
+  inline Segment* NewSegment(size_t size);
 
   // The free region in the current (front) segment is represented as
   // the half-open interval [position, limit). The 'position' variable
   // is guaranteed to be aligned as dictated by kAlignment.
   Address position_;
   Address limit_;
 
   AccountingAllocator* allocator_;
 
   Segment* segment_head_;
@@ skipping 119 matching lines @@
 
 typedef base::PointerTemplateHashMapImpl<ZoneAllocationPolicy> ZoneHashMap;
 
 typedef base::CustomMatcherTemplateHashMapImpl<ZoneAllocationPolicy>
     CustomMatcherZoneHashMap;
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_ZONE_ZONE_H_