Use glDiscardFramebuffer to save memory bandwidth

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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 84 matching lines @@
     }
     bool multisampleSupported = extensionsUtil->supportsExtension("GL_CHROMIUM_framebuffer_multisample")
         && extensionsUtil->supportsExtension("GL_OES_rgb8_rgba8");
     if (multisampleSupported) {
         extensionsUtil->ensureExtensionEnabled("GL_CHROMIUM_framebuffer_multisample");
         extensionsUtil->ensureExtensionEnabled("GL_OES_rgb8_rgba8");
     }
     bool packedDepthStencilSupported = extensionsUtil->supportsExtension("GL_OES_packed_depth_stencil");
     if (packedDepthStencilSupported)
         extensionsUtil->ensureExtensionEnabled("GL_OES_packed_depth_stencil");
+    bool discardFramebufferSupported = extensionsUtil->supportsExtension("GL_EXT_discard_framebuffer");
+    if (discardFramebufferSupported)
+        extensionsUtil->ensureExtensionEnabled("GL_EXT_discard_framebuffer");
 
-    RefPtr<DrawingBuffer> drawingBuffer = adoptRef(new DrawingBuffer(context, extensionsUtil.release(), multisampleSupported, packedDepthStencilSupported, preserve, requestedAttributes, contextEvictionManager));
+    RefPtr<DrawingBuffer> drawingBuffer = adoptRef(new DrawingBuffer(context, extensionsUtil.release(), multisampleSupported, packedDepthStencilSupported, discardFramebufferSupported, preserve, requestedAttributes, contextEvictionManager));
     if (!drawingBuffer->initialize(size)) {
         drawingBuffer->beginDestruction();
         return PassRefPtr<DrawingBuffer>();
     }
     return drawingBuffer.release();
 }
 
 DrawingBuffer::DrawingBuffer(PassOwnPtr<WebGraphicsContext3D> context,
     PassOwnPtr<Extensions3DUtil> extensionsUtil,
     bool multisampleExtensionSupported,
     bool packedDepthStencilExtensionSupported,
+    bool discardFramebufferSupported,
     PreserveDrawingBuffer preserve,
     WebGraphicsContext3D::Attributes requestedAttributes,
     PassRefPtr<ContextEvictionManager> contextEvictionManager)
     : m_preserveDrawingBuffer(preserve)
     , m_scissorEnabled(false)
     , m_texture2DBinding(0)
     , m_framebufferBinding(0)
     , m_activeTextureUnit(GL_TEXTURE0)
     , m_context(context)
     , m_extensionsUtil(extensionsUtil)
     , m_size(-1, -1)
     , m_requestedAttributes(requestedAttributes)
     , m_multisampleExtensionSupported(multisampleExtensionSupported)
     , m_packedDepthStencilExtensionSupported(packedDepthStencilExtensionSupported)
+    , m_discardFramebufferSupported(discardFramebufferSupported)
     , m_fbo(0)
     , m_depthStencilBuffer(0)
     , m_depthBuffer(0)
     , m_stencilBuffer(0)
     , m_multisampleFBO(0)
     , m_multisampleColorBuffer(0)
     , m_contentsChanged(true)
     , m_contentsChangeCommitted(false)
     , m_layerComposited(false)
     , m_multisampleMode(None)
@@ skipping 115 matching lines @@
         std::swap(frontColorBufferMailbox->textureInfo, m_colorBuffer);
         // It appears safe to overwrite the context's framebuffer binding in the Discard case since there will always be a
         // WebGLRenderingContext::clearIfComposited() call made before the next draw call which restores the framebuffer binding.
         // If this stops being true at some point, we should track the current framebuffer binding in the DrawingBuffer and restore
         // it after attaching the new back buffer here.
         m_context->bindFramebuffer(GL_FRAMEBUFFER, m_fbo);
         if (m_multisampleMode == ImplicitResolve)
             m_context->framebufferTexture2DMultisampleEXT(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, m_colorBuffer.textureId, 0, m_sampleCount);
         else
             m_context->framebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, m_colorBuffer.textureId, 0);
+
+        if (m_discardFramebufferSupported) {
+            // Explicitly discard framebuffer to save GPU memory bandwidth for tile-based GPU arch.
+            const WGC3Denum attachments[3] = { GL_COLOR_ATTACHMENT0, GL_DEPTH_ATTACHMENT, GL_STENCIL_ATTACHMENT};
+            m_context->discardFramebufferEXT(GL_FRAMEBUFFER, 3, attachments);
+        }
     } else {
         m_context->copyTextureCHROMIUM(GL_TEXTURE_2D, m_colorBuffer.textureId, frontColorBufferMailbox->textureInfo.textureId, 0, GL_RGBA, GL_UNSIGNED_BYTE);
     }
 
     if (m_multisampleMode != None && !m_framebufferBinding)
         bind();
     else
         restoreFramebufferBinding();
 
     m_contentsChanged = false;
@@ skipping 771 matching lines @@
 void DrawingBuffer::deleteChromiumImageForTexture(TextureInfo* info)
 {
     if (info->imageId) {
         m_context->releaseTexImage2DCHROMIUM(GL_TEXTURE_2D, info->imageId);
         m_context->destroyImageCHROMIUM(info->imageId);
         info->imageId = 0;
     }
 }
 
 } // namespace blink