Add discardable memory emulation for non-android/mac platforms

base/memory/discardable_memory_emulated.cc

+// Copyright 2013 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 "base/memory/discardable_memory.h"
+
+#include "base/memory/discardable_memory_provider.h"
+
+using base::internal::DiscardableMemoryProvider;
+
+namespace base {
+
+// static
+bool DiscardableMemory::SupportedNatively() {
+  return false;
+}
+
+DiscardableMemory::~DiscardableMemory() {
+  if (is_locked())
+    Unlock();
+  if (memory_)
+    Deallocate();
+  DiscardableMemoryProvider::GetInstance()->Unregister(this);
+}
+
+bool DiscardableMemory::InitializeAndLock(size_t size) {
+  DiscardableMemoryProvider::GetInstance()->Register(this);
+  size_ = size;
+  return Lock() == DISCARDABLE_MEMORY_PURGED;
+}
+
+LockDiscardableMemoryStatus DiscardableMemory::Lock() {
+  DCHECK(!is_locked_);
+
+  // This can fail if we have not been registered. That is, it's an error to
+  // |Lock| before calling |InitializeAndLock|.
+  if (!DiscardableMemoryProvider::GetInstance()->DidAccess(this)) {
+    NOTREACHED();
+    return DISCARDABLE_MEMORY_FAILED;
+  }
+
+  is_locked_ = true;
+  if (memory_)
+    return DISCARDABLE_MEMORY_SUCCESS;
+
+  if (Allocate())
+    return DISCARDABLE_MEMORY_PURGED;
+
+  // Attempt to purge to get Allocate to succeed.
+  DiscardableMemoryProvider::GetInstance()->PurgeAll();

[willchan no longer on Chromium, 2013/10/01 18:12:05]

This is kind of a big hammer. No need to prematurely optimize, but perhaps leave
a note to say this might be worth optimizing later on rather than purging all
instances?

[reveman, 2013/10/09 22:40:24]

My latest patch is purging the minimal amount of LRU memory in these cases.

+  if (Allocate())
+    return DISCARDABLE_MEMORY_PURGED;
+
+  is_locked_ = false;
+  return DISCARDABLE_MEMORY_FAILED;
+}
+
+void DiscardableMemory::Unlock() {
+  DCHECK(is_locked_);
+  DiscardableMemoryProvider::GetInstance()->DidAccess(this);
+  is_locked_ = false;
+}
+
+bool DiscardableMemory::Allocate() {
+  DCHECK(!memory_);
+  DiscardableMemoryProvider* provider =
+      DiscardableMemoryProvider::GetInstance();
+
+  if (size_ == 0)

[willchan no longer on Chromium, 2013/10/01 18:12:05]

Interesting. I hadn't considered this case before. Is it a bug to try to
initialize a DiscardableMemory with size 0?

The interesting implication of this here is that Allocate() will return false,
which then can lead to purging all of the DiscardableMemory instances. I see
that you've added a test for this, but I sorta wonder if we should treat
allocating zero bytes as a bug instead. Is there a good reason to allow this
behavior?

+    return false;
+
+  size_t limit = provider->discardable_memory_limit();
+  if (limit > 0 && size_ > limit)
+    return false;
+
+  memory_ = malloc(size_);
+  if (memory_)
+    provider->DidAllocate(size_);
+
+  return !!memory_;
+}
+
+void DiscardableMemory::Deallocate() {
+  DCHECK(memory_);
+  free(memory_);
+  memory_ = NULL;
+  DiscardableMemoryProvider::GetInstance()->DidDeallocate(size_);
+}
+
+// static
+bool DiscardableMemory::PurgeForTestingSupported() {
+  return true;
+}
+
+// static
+void DiscardableMemory::PurgeForTesting() {
+  DiscardableMemoryProvider::GetInstance()->PurgeAll();
+}
+
+}  // namespace base