Use glDiscardFramebuffer to save memory bandwidth in PNaCl

gpu/command_buffer/service/gles2_cmd_decoder.cc

Index: gpu/command_buffer/service/gles2_cmd_decoder.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_decoder.cc b/gpu/command_buffer/service/gles2_cmd_decoder.cc
index c55d9e387b11b81eb2f4448a4670fde2cade8a66..8c38e8a0765ac275a321b29005c2d7dffa5583e4 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder.cc
+++ b/gpu/command_buffer/service/gles2_cmd_decoder.cc
@@ -9369,6 +9369,14 @@ void GLES2DecoderImpl::DoSwapBuffers() {
         offscreen_saved_color_texture_.swap(offscreen_target_color_texture_);
         offscreen_target_frame_buffer_->AttachRenderTexture(
             offscreen_target_color_texture_.get());
+
+        // Explicitly discard framebuffer to save GPU memory bandwidth for
+        // tile-based GPU arch.
+        if (feature_info_->feature_flags().ext_discard_framebuffer) {

[Hongbo Min, 2014/10/14 02:27:48]

According to my previous work, an attempt to clear a framebuffer on  tiled GPU
is time-consuming, if glDiscardFramebuffer is used, it gives a performance hint
and allow GPU driver to set the pixel vale to whatever it wishes, and the
glClear could be skipped (I have verified it on ASUS Nexus series, it works well
without glClear if glDiscardFramebuffer is used).

So my question is, where is glClear called to clear
|offscreen_target_frame_buffer_| and prepare a new frame? and can we skip that
glClear if have?

[dshwang, 2014/10/14 12:09:30]

As far as I understand, your CL didn't affect whether glClear is skipped or not.
WebGLRenderingContextBase::clearIfComposited still do glClear every frame.

Let me explain why your CL speed up in tiled-based GPU as I understand. When
binding another fbo, tiled-based GPU must stores tiled cache to gpu memory
(texture, stencil buffer and depth buffer). glDiscardFramebuffer hints GPU to
not read back tiled cache to gpu memory because the software don't need anymore
the changed fbo information. It saves huge gpu bandwidth.

Let's go back to your CL. The fbo lifecycle is as follows:
1. WebGL replaces color texture to front fbo. At that time, you hint
glDiscardFramebuffer because DrawingBuffer doesn't update fbo so you don't want
to copy tiled cache to color texture redundantly. It wastes bandwidth.
2. Something later, WebGLRenderingContextBase::clearIfComposited performs
glClear on the fbo.

My CL is the same to your WebGL CL. This code just bind fbo and replace color
texture, so we don't want to copy back tiled cache to color texture redundantly.
glDiscardFramebuffer can make it.
As I explain above, glClear is not related to glDiscardFramebuffer.

+          const GLenum attachments[3] = {
+              GL_COLOR_ATTACHMENT0, GL_DEPTH_ATTACHMENT, GL_STENCIL_ATTACHMENT};
+          glDiscardFramebufferEXT(GL_FRAMEBUFFER, 3, attachments);
+        }
       }
 
       // Ensure the side effects of the copy are visible to the parent