Fix a bug in drawBuffers optimization.

gpu/command_buffer/service/framebuffer_manager.cc

Index: gpu/command_buffer/service/framebuffer_manager.cc
diff --git a/gpu/command_buffer/service/framebuffer_manager.cc b/gpu/command_buffer/service/framebuffer_manager.cc
index 0fd59c5fd6ba1dfb618709f97b1a8269481fd6d2..b3320a6139eba6770f80d6abac0d937d142081d9 100644
--- a/gpu/command_buffer/service/framebuffer_manager.cc
+++ b/gpu/command_buffer/service/framebuffer_manager.cc
@@ -526,8 +526,8 @@ bool Framebuffer::ValidateAndAdjustDrawBuffers(
     return false;
 
   if (mask != adjusted_draw_buffer_bound_mask_) {
-    // This won't be reached in every draw/clear call - only when framebuffer
-    // or program has changed.
+    // This won't be reached in every draw call - only when framebuffer or
+    // program has changed.
     for (uint32_t ii = 0; ii < manager_->max_draw_buffers_; ++ii) {
       adjusted_draw_buffers_[ii] = draw_buffers_[ii];
       uint32_t shift_bits = ii * 2;
@@ -542,6 +542,24 @@ bool Framebuffer::ValidateAndAdjustDrawBuffers(
   return true;
 }
 
+void Framebuffer::AdjustDrawBuffers() {
+  if (draw_buffer_bound_mask_ == adjusted_draw_buffer_bound_mask_) {
+    return;
+  }
+  // This won't be reached in every clear call - only when framebuffer has
+  // changed.
+  for (uint32_t ii = 0; ii < manager_->max_draw_buffers_; ++ii) {
+    adjusted_draw_buffers_[ii] = draw_buffers_[ii];
+    uint32_t shift_bits = ii * 2;
+    uint32_t buffer_mask = 0x3 << shift_bits;
+    if ((buffer_mask & draw_buffer_bound_mask_) == 0u) {
+      adjusted_draw_buffers_[ii] = GL_NONE;
+    }
+  }

[Ken Russell (switch to Gerrit), 2017/01/06 22:43:07]

Per offline discussion: this is the same block of code as in
ValidateAndAdjustDrawBuffers. Could you at least refactor it into some sort of
AdjustDrawBuffersImpl and pass in draw_buffer_bound_mask_ as "mask"?

[Zhenyao Mo, 2017/01/07 00:22:24]

Done.

+  adjusted_draw_buffer_bound_mask_ = draw_buffer_bound_mask_;
+  glDrawBuffersARB(manager_->max_draw_buffers_, adjusted_draw_buffers_.get());
+}
+
 bool Framebuffer::ContainsActiveIntegerAttachments() const {
   // 0x55555555 broadcasts SHADER_VARIABLE_FLOAT to all slots.
   uint32_t mask = 0x55555555u * SHADER_VARIABLE_FLOAT;