[heap] Move semantics for LocalAllocationBuffer

Implement move semantics for the LocalAllocationBuffer as noted in
the comments. Also moved Close() invocation out of the assignment
operator and renamed it to CloseWithFiller().

Bug: v8:10315
Change-Id: Idc36fb7923e1f8857c05ec5d5110fa16087b07e7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2170087Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67428}

src/heap/concurrent-allocator-inl.h

@@ -10,6 +10,7 @@
 
 #include "src/heap/heap.h"
 #include "src/heap/spaces-inl.h"
+#include "src/heap/spaces.h"
 #include "src/objects/heap-object.h"
 
 namespace v8 {
@@ -54,21 +55,20 @@ AllocationResult ConcurrentAllocator::AllocateInLab(
 }
 
 bool ConcurrentAllocator::EnsureLab(AllocationOrigin origin) {
-  LocalAllocationBuffer saved_lab = lab_;
   auto result = space_->SlowGetLinearAllocationAreaBackground(
       kLabSize, kMaxLabSize, kWordAligned, origin);
 
   if (!result) return false;
 
   HeapObject object = HeapObject::FromAddress(result->first);
+  LocalAllocationBuffer saved_lab = std::move(lab_);
   lab_ = LocalAllocationBuffer::FromResult(
       local_heap_->heap(), AllocationResult(object), result->second);
-  if (lab_.IsValid()) {
-    lab_.TryMerge(&saved_lab);
-    return true;
+  DCHECK(lab_.IsValid());
+  if (!lab_.TryMerge(&saved_lab)) {
+    saved_lab.CloseWithFiller();
   }
-  lab_ = saved_lab;
-  return false;
+  return true;
 }
 
 }  // namespace internal

src/heap/local-allocator-inl.h

@@ -83,17 +83,19 @@ AllocationResult EvacuationAllocator::AllocateInLAB(
 
 bool EvacuationAllocator::NewLocalAllocationBuffer() {
   if (lab_allocation_will_fail_) return false;
-  LocalAllocationBuffer saved_lab_ = new_space_lab_;
   AllocationResult result =
       new_space_->AllocateRawSynchronized(kLabSize, kWordAligned);
-  new_space_lab_ = LocalAllocationBuffer::FromResult(heap_, result, kLabSize);
-  if (new_space_lab_.IsValid()) {
-    new_space_lab_.TryMerge(&saved_lab_);
-    return true;
-  }
-  new_space_lab_ = saved_lab_;
+  if (result.IsRetry()) {
     lab_allocation_will_fail_ = true;
     return false;
+  }
+  LocalAllocationBuffer saved_lab = std::move(new_space_lab_);
+  new_space_lab_ = LocalAllocationBuffer::FromResult(heap_, result, kLabSize);
+  DCHECK(new_space_lab_.IsValid());
+  if (!new_space_lab_.TryMerge(&saved_lab)) {
+    saved_lab.CloseWithFiller();
+  }
+  return true;
 }
 
 AllocationResult EvacuationAllocator::AllocateInNewSpace(

src/heap/local-allocator.h

@@ -33,7 +33,7 @@ class EvacuationAllocator {
     heap_->code_space()->MergeLocalSpace(compaction_spaces_.Get(CODE_SPACE));
     // Give back remaining LAB space if this EvacuationAllocator's new space LAB
     // sits right next to new space allocation top.
-    const LinearAllocationArea info = new_space_lab_.Close();
+    const LinearAllocationArea info = new_space_lab_.CloseWithFiller();
     const Address top = new_space_->top();
     if (info.limit() != kNullAddress && info.limit() == top) {
       DCHECK_NE(info.top(), kNullAddress);

src/heap/spaces.cc

@@ -2510,7 +2510,7 @@ bool SemiSpace::EnsureCurrentCapacity() {
   return true;
 }
 
-LinearAllocationArea LocalAllocationBuffer::Close() {
+LinearAllocationArea LocalAllocationBuffer::CloseWithFiller() {
   if (IsValid()) {
     heap_->CreateFillerObjectAt(
         allocation_info_.top(),
@@ -2535,22 +2535,17 @@ LocalAllocationBuffer::LocalAllocationBuffer(
   }
 }
 
-LocalAllocationBuffer::LocalAllocationBuffer(const LocalAllocationBuffer& other)
+LocalAllocationBuffer::LocalAllocationBuffer(LocalAllocationBuffer&& other)
     V8_NOEXCEPT {
-  *this = other;
+  *this = std::move(other);
 }
 
 LocalAllocationBuffer& LocalAllocationBuffer::operator=(
-    const LocalAllocationBuffer& other) V8_NOEXCEPT {
-  Close();
+    LocalAllocationBuffer&& other) V8_NOEXCEPT {
   heap_ = other.heap_;
   allocation_info_ = other.allocation_info_;
 
-  // This is needed since we (a) cannot yet use move-semantics, and (b) want
-  // to make the use of the class easy by it as value and (c) implicitly call
-  // {Close} upon copy.
-  const_cast<LocalAllocationBuffer&>(other).allocation_info_.Reset(
-      kNullAddress, kNullAddress);
+  other.allocation_info_.Reset(kNullAddress, kNullAddress);
   return *this;
 }
 

src/heap/spaces.h

@@ -2186,13 +2186,15 @@ class LocalAllocationBuffer {
                                                  AllocationResult result,
                                                  intptr_t size);
 
-  ~LocalAllocationBuffer() { Close(); }
+  ~LocalAllocationBuffer() { CloseWithFiller(); }
 
-  // Convert to C++11 move-semantics once allowed by the style guide.
-  V8_EXPORT_PRIVATE LocalAllocationBuffer(const LocalAllocationBuffer& other)
+  LocalAllocationBuffer(const LocalAllocationBuffer& other) = delete;
+  V8_EXPORT_PRIVATE LocalAllocationBuffer(LocalAllocationBuffer&& other)
       V8_NOEXCEPT;
+
+  LocalAllocationBuffer& operator=(const LocalAllocationBuffer& other) = delete;
   V8_EXPORT_PRIVATE LocalAllocationBuffer& operator=(
-      const LocalAllocationBuffer& other) V8_NOEXCEPT;
+      LocalAllocationBuffer&& other) V8_NOEXCEPT;
 
   V8_WARN_UNUSED_RESULT inline AllocationResult AllocateRawAligned(
       int size_in_bytes, AllocationAlignment alignment);
@@ -2206,7 +2208,7 @@ class LocalAllocationBuffer {
   inline bool TryFreeLast(HeapObject object, int object_size);
 
   // Close a LAB, effectively invalidating it. Returns the unused area.
-  V8_EXPORT_PRIVATE LinearAllocationArea Close();
+  V8_EXPORT_PRIVATE LinearAllocationArea CloseWithFiller();
 
  private:
   V8_EXPORT_PRIVATE LocalAllocationBuffer(