Avoid to resolve CMAA for read and draw without content changed or with content committed.

third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
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  *     * Redistributions in binary form must reproduce the above
@@ skipping 950 matching lines @@
     m_gl->ClearStencil(0);
     clearMask |= GL_STENCIL_BUFFER_BIT;
     m_gl->StencilMaskSeparate(GL_FRONT, 0xFFFFFFFF);
   }
 
   clearFramebuffersInternal(clearMask);
   return true;
 }
 
 void DrawingBuffer::resolveAndBindForReadAndDraw() {
-  {
+  if (m_contentsChanged) {

[dshwang, 2017/04/08 02:16:32]

I think it's better to change
m_antiAliasingMode != None && !m_contentsChangeResolved
in DrawingBuffer::resolveIfNeeded().
What do you think?

[Zhenyao Mo, 2017/04/12 23:38:08]

I agree !m_contentsChangeResolved is a better condition.  However, this is
already a condition in resolveMultisampleFramebufferInternal() which is called
by resolveIfNeeded().  So why do we want to repeat it here? (Maybe I missed
something?)

[xinghua.cao, 2017/04/13 10:50:24]

It seems a repeating condition here, thank you.

     ScopedStateRestorer scopedStateRestorer(this);
     resolveIfNeeded();
   }
   m_gl->BindFramebuffer(GL_FRAMEBUFFER, m_fbo);
 }
 
 void DrawingBuffer::resolveMultisampleFramebufferInternal() {
   DCHECK(m_stateRestorer);
   m_stateRestorer->setFramebufferBindingDirty();
   if (wantExplicitResolve() && !m_contentsChangeResolved) {
@@ skipping 17 matching lines @@
         contextProvider()
             ->getCapabilities()
             .disable_multisampling_color_mask_usage) {
       m_gl->ClearColor(0, 0, 0, 1);
       m_gl->ColorMask(false, false, false, true);
       m_gl->Clear(GL_COLOR_BUFFER_BIT);
     }
   }
 
   m_gl->BindFramebuffer(GL_FRAMEBUFFER, m_fbo);
-  if (m_antiAliasingMode == ScreenSpaceAntialiasing)
+  if (m_antiAliasingMode == ScreenSpaceAntialiasing &&
+      !m_contentsChangeResolved)
     m_gl->ApplyScreenSpaceAntialiasingCHROMIUM();
 }
 
 void DrawingBuffer::resolveIfNeeded() {
   if (m_antiAliasingMode != None)
     resolveMultisampleFramebufferInternal();
   m_contentsChangeResolved = true;
 }
 
 void DrawingBuffer::restoreFramebufferBindings() {
@@ skipping 276 matching lines @@
   if (m_pixelUnpackBufferBindingDirty)
     client->DrawingBufferClientRestorePixelUnpackBufferBinding();
 }
 
 bool DrawingBuffer::shouldUseChromiumImage() {
   return RuntimeEnabledFeatures::webGLImageChromiumEnabled() &&
          m_chromiumImageUsage == AllowChromiumImage;
 }
 
 }  // namespace blink