Use more than double buffering in SoftwareOutputDeviceMac

content/browser/compositor/software_output_device_mac.mm

Index: content/browser/compositor/software_output_device_mac.mm
diff --git a/content/browser/compositor/software_output_device_mac.mm b/content/browser/compositor/software_output_device_mac.mm
index 1db33824c81f6d995ece9fc47357560095a79008..88c5956c4042e6831ede5490db5128ad62ef4b5d 100644
--- a/content/browser/compositor/software_output_device_mac.mm
+++ b/content/browser/compositor/software_output_device_mac.mm
@@ -14,8 +14,11 @@
 
 namespace content {
 
+SoftwareOutputDeviceMac::Buffer::Buffer() = default;
+SoftwareOutputDeviceMac::Buffer::~Buffer() = default;
+
 SoftwareOutputDeviceMac::SoftwareOutputDeviceMac(ui::Compositor* compositor)
-    : compositor_(compositor), scale_factor_(1), current_index_(0) {}
+    : compositor_(compositor) {}
 
 SoftwareOutputDeviceMac::~SoftwareOutputDeviceMac() {
 }
@@ -31,21 +34,39 @@
   DiscardBackbuffer();
 }
 
-void SoftwareOutputDeviceMac::CopyPreviousBufferDamage(
+void SoftwareOutputDeviceMac::UpdateAndCopyBufferDamage(
+    Buffer* previous_paint_buffer,
     const SkRegion& new_damage_region) {
   TRACE_EVENT0("browser", "CopyPreviousBufferDamage");
 
-  // The region to copy is the region drawn last frame, minus the region that
-  // is drawn this frame.
-  SkRegion copy_region = previous_buffer_damage_region_;
+  // Expand the |accumulated_damage| of all buffers to include this frame's
+  // damage.
+  for (auto& buffer : buffer_queue_)
+    buffer->accumulated_damage.op(new_damage_region, SkRegion::kUnion_Op);
+
+  // Compute the region to copy from |previous_paint_buffer| to
+  // |current_paint_buffer_| by subtracting |new_damage_region| (which we will
+  // be painting) from |current_paint_buffer_|'s |accumulated_damage|.
+  SkRegion copy_region;
+  current_paint_buffer_->accumulated_damage.swap(copy_region);
   bool copy_region_nonempty =
       copy_region.op(new_damage_region, SkRegion::kDifference_Op);
-  previous_buffer_damage_region_ = new_damage_region;
-
+  last_copy_region_for_testing_ = copy_region;
   if (!copy_region_nonempty)
     return;
 
-  IOSurfaceRef previous_io_surface = io_surfaces_[!current_index_].get();
+  // If we have anything to copy, we had better have a buffer to copy it from.
+  if (!previous_paint_buffer) {
+    DLOG(ERROR) << "No previous paint buffer to copy accumulated damage from.";
+    last_copy_region_for_testing_.setEmpty();
+    return;
+  }
+
+  // It is possible for |previous_paint_buffer| to equal
+  // |current_paint_buffer_|, but if it does, we should not need to do a copy.
+  CHECK_NE(previous_paint_buffer, current_paint_buffer_);
+
+  IOSurfaceRef previous_io_surface = previous_paint_buffer->io_surface.get();
 
   {
     TRACE_EVENT0("browser", "IOSurfaceLock for software copy");
@@ -64,7 +85,7 @@
   size_t bytes_per_element = 4;
   for (SkRegion::Iterator it(copy_region); !it.done(); it.next()) {
     const SkIRect& rect = it.rect();
-    canvas_->writePixels(
+    current_paint_canvas_->writePixels(
         SkImageInfo::MakeN32Premul(rect.width(), rect.height()),
         pixels + bytes_per_element * rect.x() + stride * rect.y(), stride,
         rect.x(), rect.y());
@@ -80,77 +101,104 @@
   }
 }
 
-bool SoftwareOutputDeviceMac::EnsureBuffersExist() {
-  for (int i = 0; i < 2; ++i) {
-    if (!io_surfaces_[i]) {
-      TRACE_EVENT0("browser", "IOSurfaceCreate");
-      io_surfaces_[i].reset(
-          gfx::CreateIOSurface(pixel_size_, gfx::BufferFormat::BGRA_8888));
-    }
-    if (!io_surfaces_[i]) {
-      DLOG(ERROR) << "Failed to allocate IOSurface";
-      return false;
-    }
-  }
-  return true;
-}
-
 SkCanvas* SoftwareOutputDeviceMac::BeginPaint(
     const gfx::Rect& new_damage_rect) {
-  if (!EnsureBuffersExist())
-    return nullptr;
+  // Record the previous paint buffer.
+  Buffer* previous_paint_buffer =
+      buffer_queue_.empty() ? nullptr : buffer_queue_.back().get();
+
+  // Find any buffer in the queue that is not in use by the window server, and
+  // re-use it as the buffer for this paint. Note that this can be any buffer in
+  // any position in the list.
+  for (auto iter = buffer_queue_.begin(); iter != buffer_queue_.end(); ++iter) {
+    Buffer* iter_buffer = iter->get();
+    if (IOSurfaceIsInUse(iter_buffer->io_surface))
+      continue;
+    current_paint_buffer_ = iter_buffer;
+    buffer_queue_.splice(buffer_queue_.end(), buffer_queue_, iter);
+    break;
+  }
 
+  // If we failed to find a suitable buffer, allocate a new one, and initialize
+  // it with complete damage.
+  if (!current_paint_buffer_) {
+    std::unique_ptr<Buffer> new_buffer(new Buffer);
+    new_buffer->io_surface.reset(
+        gfx::CreateIOSurface(pixel_size_, gfx::BufferFormat::BGRA_8888));
+    if (!new_buffer->io_surface)
+      return nullptr;
+    // Set the initial damage to be the full buffer.
+    new_buffer->accumulated_damage.setRect(
+        gfx::RectToSkIRect(gfx::Rect(pixel_size_)));
+    current_paint_buffer_ = new_buffer.get();
+    buffer_queue_.push_back(std::move(new_buffer));
+  }
+  DCHECK(current_paint_buffer_);
+
+  // Animating in steady-state should require no more than 4 buffers. If we have
+  // more than that, then purge the older buffers (the window server will
+  // continue to hold on to the IOSurfaces as long as is needed, so the
+  // consequence will be extra allocate-free cycles).
+  size_t kMaxBuffers = 4;
+  while (buffer_queue_.size() > kMaxBuffers)
+    buffer_queue_.pop_front();
+
+  // Lock the |current_paint_buffer_|'s IOSurface and wrap it in
+  // |current_paint_canvas_|.
   {
     TRACE_EVENT0("browser", "IOSurfaceLock for software paint");
-    IOReturn io_result = IOSurfaceLock(io_surfaces_[current_index_],
+    IOReturn io_result = IOSurfaceLock(current_paint_buffer_->io_surface,
                                        kIOSurfaceLockAvoidSync, nullptr);
     if (io_result) {
       DLOG(ERROR) << "Failed to lock IOSurface " << io_result;
+      current_paint_buffer_ = nullptr;
       return nullptr;
     }
   }
+  {
+    SkPMColor* pixels = static_cast<SkPMColor*>(
+        IOSurfaceGetBaseAddress(current_paint_buffer_->io_surface));
+    size_t stride = IOSurfaceGetBytesPerRow(current_paint_buffer_->io_surface);
+    current_paint_canvas_ = SkCanvas::MakeRasterDirectN32(
+        pixel_size_.width(), pixel_size_.height(), pixels, stride);
+  }
 
-  SkPMColor* pixels = static_cast<SkPMColor*>(
-      IOSurfaceGetBaseAddress(io_surfaces_[current_index_]));
-  size_t stride = IOSurfaceGetBytesPerRow(io_surfaces_[current_index_]);
-
-  canvas_ = SkCanvas::MakeRasterDirectN32(pixel_size_.width(),
-                                          pixel_size_.height(), pixels, stride);
-
-  CopyPreviousBufferDamage(SkRegion(gfx::RectToSkIRect(new_damage_rect)));
+  UpdateAndCopyBufferDamage(previous_paint_buffer,
+                            SkRegion(gfx::RectToSkIRect(new_damage_rect)));
 
-  return canvas_.get();
+  return current_paint_canvas_.get();
 }
 
 void SoftwareOutputDeviceMac::EndPaint() {
   SoftwareOutputDevice::EndPaint();
   {
     TRACE_EVENT0("browser", "IOSurfaceUnlock");
-    IOReturn io_result = IOSurfaceUnlock(io_surfaces_[current_index_],
+    IOReturn io_result = IOSurfaceUnlock(current_paint_buffer_->io_surface,
                                          kIOSurfaceLockAvoidSync, nullptr);
     if (io_result)
       DLOG(ERROR) << "Failed to unlock IOSurface " << io_result;
   }
-  canvas_.reset();
-
-  ui::AcceleratedWidgetMac* widget =
-      ui::AcceleratedWidgetMac::Get(compositor_->widget());
-  if (widget) {
-    widget->GotIOSurfaceFrame(io_surfaces_[current_index_], pixel_size_,
-                              scale_factor_);
-    base::TimeTicks vsync_timebase;
-    base::TimeDelta vsync_interval;
-    widget->GetVSyncParameters(&vsync_timebase, &vsync_interval);
-    if (!update_vsync_callback_.is_null())
-      update_vsync_callback_.Run(vsync_timebase, vsync_interval);
+  current_paint_canvas_.reset();
+
+  if (compositor_) {
+    ui::AcceleratedWidgetMac* widget =
+        ui::AcceleratedWidgetMac::Get(compositor_->widget());
+    if (widget) {
+      widget->GotIOSurfaceFrame(current_paint_buffer_->io_surface, pixel_size_,
+                                scale_factor_);
+      base::TimeTicks vsync_timebase;
+      base::TimeDelta vsync_interval;
+      widget->GetVSyncParameters(&vsync_timebase, &vsync_interval);
+      if (!update_vsync_callback_.is_null())
+        update_vsync_callback_.Run(vsync_timebase, vsync_interval);
+    }
   }
 
-  current_index_ = !current_index_;
+  current_paint_buffer_ = nullptr;
 }
 
 void SoftwareOutputDeviceMac::DiscardBackbuffer() {
-  for (int i = 0; i < 2; ++i)
-    io_surfaces_[i].reset();
+  buffer_queue_.clear();
 }
 
 void SoftwareOutputDeviceMac::EnsureBackbuffer() {}