base: Add free list implementation to browser-wide discardable memory system.

content/common/discardable_shared_memory_heap.cc

+// Copyright 2014 The Chromium 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 "content/common/discardable_shared_memory_heap.h"
+
+#include "base/memory/discardable_shared_memory.h"
+
+namespace content {
+namespace {
+
+bool IsPowerOfTwo(size_t x) {
+  return (x & (x - 1)) == 0;
+}
+
+bool IsInFreeList(DiscardableSharedMemoryHeap::Span* span) {
+  return span->previous() || span->next();
+}
+
+}  // namespace
+
+DiscardableSharedMemoryHeap::Span::Span(
+    linked_ptr<base::DiscardableSharedMemory> shared_memory,
+    size_t start,
+    size_t length)
+    : shared_memory_(shared_memory), start_(start), length_(length) {
+}
+
+DiscardableSharedMemoryHeap::Span::~Span() {
+}
+
+DiscardableSharedMemoryHeap::DiscardableSharedMemoryHeap(size_t block_size)
+    : block_size_(block_size) {
+  DCHECK_NE(block_size_, 0u);
+  DCHECK(IsPowerOfTwo(block_size_));
+}
+
+DiscardableSharedMemoryHeap::~DiscardableSharedMemoryHeap() {
+  while (!free_spans_.empty())
+    RemoveFromFreeList(free_spans_.tail()->value());
+}
+
+scoped_ptr<DiscardableSharedMemoryHeap::Span> DiscardableSharedMemoryHeap::Grow(
+    scoped_ptr<base::DiscardableSharedMemory> memory,
+    size_t size) {
+  linked_ptr<base::DiscardableSharedMemory> shared_memory(memory.release());
+
+  // Memory must be aligned to block size.
+  DCHECK_EQ(
+      reinterpret_cast<size_t>(shared_memory->memory()) & (block_size_ - 1),
+      0u);
+  DCHECK_EQ(size & (block_size_ - 1), 0u);
+
+  scoped_ptr<Span> span(
+      new Span(shared_memory,
+               reinterpret_cast<size_t>(shared_memory->memory()) / block_size_,
+               size / block_size_));
+  RegisterSpan(span.get());
+  return span.Pass();
+}
+
+void DiscardableSharedMemoryHeap::MergeIntoFreeList(scoped_ptr<Span> span) {
+  // Merge with previous span if possible.
+  SpanMap::iterator prev_it = spans_.find(span->start_ - 1);
+  if (prev_it != spans_.end() && IsInFreeList(prev_it->second)) {
+    scoped_ptr<Span> prev = RemoveFromFreeList(prev_it->second);
+    DCHECK_EQ(prev->start_ + prev->length_, span->start_);
+    span->start_ -= prev->length_;
+    span->length_ += prev->length_;
+    spans_[span->start_] = span.get();
+  }
+
+  // Merge with next span if possible.
+  SpanMap::iterator next_it = spans_.find(span->start_ + span->length_);
+  if (next_it != spans_.end() && IsInFreeList(next_it->second)) {
+    scoped_ptr<Span> next = RemoveFromFreeList(next_it->second);
+    DCHECK_EQ(next->start_, span->start_ + span->length_);
+    span->length_ += next->length_;
+    spans_[span->start_ + span->length_ - 1] = span.get();
+  }
+
+  free_spans_.Append(span.release());
+}
+
+scoped_ptr<DiscardableSharedMemoryHeap::Span>
+DiscardableSharedMemoryHeap::Split(Span* span, size_t blocks) {
+  DCHECK(blocks);
+  DCHECK_LT(blocks, span->length_);
+
+  scoped_ptr<Span> leftover(new Span(
+      span->shared_memory_, span->start_ + blocks, span->length_ - blocks));
+  RegisterSpan(leftover.get());
+  spans_[span->start_ + blocks - 1] = span;
+  span->length_ = blocks;
+  return leftover.Pass();
+}
+
+scoped_ptr<DiscardableSharedMemoryHeap::Span>
+DiscardableSharedMemoryHeap::SearchFreeList(size_t blocks) {
+  DCHECK(blocks);
+
+  // Search through list to find best span.
+  Span* best = nullptr;
+
+  // This implements address-ordered best-fit.
+  for (Span* span = free_spans_.head()->value(); span != free_spans_.end();
+       span = span->next()->value()) {
+    // Skip span if it's not large enough.
+    if (span->length_ < blocks)
+      continue;
+
+    if (best) {
+      // Skip span if |best| is a better fit.
+      if (span->length_ > best->length_)
+        continue;
+
+      // Skip span if |best| starts at a lower address.
+      if ((span->length_ == best->length_) && (span->start_ > best->start_))
+        continue;
+    }
+
+    best = span;
+  }
+
+  return best ? Carve(best, blocks) : nullptr;
+}
+
+void DiscardableSharedMemoryHeap::DeleteSpan(scoped_ptr<Span> span) {
+  DCHECK(!IsInFreeList(span.get()));
+  spans_.erase(span->start_);
+  if (span->length_ > 1)
+    spans_.erase(span->start_ + span->length_ - 1);
+}
+
+scoped_ptr<DiscardableSharedMemoryHeap::Span>
+DiscardableSharedMemoryHeap::RemoveFromFreeList(Span* span) {
+  span->RemoveFromList();
+  return make_scoped_ptr(span);
+}
+
+scoped_ptr<DiscardableSharedMemoryHeap::Span>
+DiscardableSharedMemoryHeap::Carve(Span* span, size_t blocks) {
+  scoped_ptr<Span> serving = RemoveFromFreeList(span);
+
+  const int extra = serving->length_ - blocks;
+  if (extra) {
+    scoped_ptr<Span> leftover(
+        new Span(serving->shared_memory_, serving->start_ + blocks, extra));
+    RegisterSpan(leftover.get());
+
+    // No need to coalesce as the previous span of |leftover| was just split
+    // and the next span of |leftover| was not previously coalesced with
+    // |span|.
+    free_spans_.Append(leftover.release());
+
+    serving->length_ = blocks;
+    spans_[serving->start_ + blocks - 1] = serving.get();
+  }
+
+  return serving.Pass();
+}
+
+void DiscardableSharedMemoryHeap::RegisterSpan(Span* span) {
+  spans_[span->start_] = span;
+  if (span->length_ > 1)
+    spans_[span->start_ + span->length_ - 1] = span;
+}
+
+}  // namespace content