Revert of Pool implementation for zone segments

src/zone/zone.cc

 // 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.
 
 #include "src/zone/zone.h"
 
 #include <cstring>
 
 #include "src/v8.h"
 
@@ skipping 33 matching lines @@
 Zone::Zone(AccountingAllocator* allocator)
     : allocation_size_(0),
       segment_bytes_allocated_(0),
       position_(0),
       limit_(0),
       allocator_(allocator),
       segment_head_(nullptr) {}
 
 Zone::~Zone() {
   DeleteAll();
+  DeleteKeptSegment();
 
   DCHECK(segment_bytes_allocated_ == 0);
 }
 
 void* Zone::New(size_t size) {
   // Round up the requested size to fit the alignment.
   size = RoundUp(size, kAlignment);
 
   // If the allocation size is divisible by 8 then we return an 8-byte aligned
   // address.
@@ skipping 20 matching lines @@
   DCHECK(redzone_position + kASanRedzoneBytes == position_);
   ASAN_POISON_MEMORY_REGION(redzone_position, kASanRedzoneBytes);
 
   // Check that the result has the proper alignment and return it.
   DCHECK(IsAddressAligned(result, kAlignment, 0));
   allocation_size_ += size;
   return reinterpret_cast<void*>(result);
 }
 
 void Zone::DeleteAll() {
-  // Traverse the chained list of segments and return them all to the allocator.
+  // Find a segment with a suitable size to keep around.
+  Segment* keep = nullptr;
+  // Traverse the chained list of segments, zapping (in debug mode)
+  // and freeing every segment except the one we wish to keep.
   for (Segment* current = segment_head_; current;) {
     Segment* next = current->next();
-    size_t size = current->size();
-
-    // Un-poison the segment content so we can re-use or zap it later.
-    ASAN_UNPOISON_MEMORY_REGION(current->start(), current->capacity());
-
-    segment_bytes_allocated_ -= size;
-    allocator_->ReturnSegment(current);
+    if (!keep && current->size() <= kMaximumKeptSegmentSize) {
+      // Unlink the segment we wish to keep from the list.
+      keep = current;
+      keep->set_next(nullptr);
+    } else {
+      size_t size = current->size();
+#ifdef DEBUG
+      // Un-poison first so the zapping doesn't trigger ASan complaints.
+      ASAN_UNPOISON_MEMORY_REGION(current, size);
+#endif
+      current->ZapContents();
+      segment_bytes_allocated_ -= size;
+      allocator_->FreeSegment(current);
+    }
     current = next;
   }
 
-  position_ = limit_ = 0;
+  // If we have found a segment we want to keep, we must recompute the
+  // variables 'position' and 'limit' to prepare for future allocate
+  // attempts. Otherwise, we must clear the position and limit to
+  // force a new segment to be allocated on demand.
+  if (keep) {
+    Address start = keep->start();
+    position_ = RoundUp(start, kAlignment);
+    limit_ = keep->end();
+    // Un-poison so we can re-use the segment later.
+    ASAN_UNPOISON_MEMORY_REGION(start, keep->capacity());
+    keep->ZapContents();
+  } else {
+    position_ = limit_ = 0;
+  }
 
   allocation_size_ = 0;
   // Update the head segment to be the kept segment (if any).
-  segment_head_ = nullptr;
+  segment_head_ = keep;
+}
+
+void Zone::DeleteKeptSegment() {
+  DCHECK(segment_head_ == nullptr || segment_head_->next() == nullptr);
+  if (segment_head_ != nullptr) {
+    size_t size = segment_head_->size();
+#ifdef DEBUG
+    // Un-poison first so the zapping doesn't trigger ASan complaints.
+    ASAN_UNPOISON_MEMORY_REGION(segment_head_, size);
+#endif
+    segment_head_->ZapContents();
+    segment_bytes_allocated_ -= size;
+    allocator_->FreeSegment(segment_head_);
+    segment_head_ = nullptr;
+  }
+
+  DCHECK(segment_bytes_allocated_ == 0);
 }
 
 // Creates a new segment, sets it size, and pushes it to the front
 // of the segment chain. Returns the new segment.
-Segment* Zone::NewSegment(size_t requested_size) {
-  Segment* result = allocator_->GetSegment(requested_size);
-  DCHECK_GE(result->size(), requested_size);
-  segment_bytes_allocated_ += result->size();
+Segment* Zone::NewSegment(size_t size) {
+  Segment* result = allocator_->AllocateSegment(size);
+  segment_bytes_allocated_ += size;
   if (result != nullptr) {
-    result->set_zone(this);
-    result->set_next(segment_head_);
+    result->Initialize(segment_head_, size, this);
     segment_head_ = result;
   }
   return result;
 }
 
 Address Zone::NewExpand(size_t size) {
   // Make sure the requested size is already properly aligned and that
   // there isn't enough room in the Zone to satisfy the request.
   DCHECK_EQ(size, RoundDown(size, kAlignment));
   DCHECK(limit_ < position_ ||
@@ skipping 43 matching lines @@
   // size bytes + header and alignment padding)
   DCHECK(reinterpret_cast<uintptr_t>(position_) >=
          reinterpret_cast<uintptr_t>(result));
   limit_ = segment->end();
   DCHECK(position_ <= limit_);
   return result;
 }
 
 }  // namespace internal
 }  // namespace v8