Dynamic Branching in Shader

Static branching vs. Dynamic branching:

• Static one to switch on or off, based of a boolean shader constant, for disable or enable a code path。Between draw calls, you can decude which features you want to support, and set the Boolean flags to support this behavior。
• Dynamic one the comparison condition resides in a variable, done for each vertex or each pixel at run time( not at compiling time or between two draw calls)。The performance hit is the cost of branch + the cost of the instructions on the side of the branch token。Implemented in shader model 3.0 or higher。
  

http://msdn.microsoft.com/en-us/library/bb944006%28VS.85%29.aspx

該討論串討論: 當GLSL在Nvidia的GPU時，遇到了雙層迴圈，執行8x8的braching運算，發生效能剩下2fps的問題。 當對於looping做unroll的動作，發現又可以回到30fps。
因此一開始猜想的內容是對於GPU的branching能力，後來發現arry index中使用了constant register index才是影響效能的關鍵，當將array的內容儲存在一個texture上，或是改用uniform array of vec3's都可以獲得改善。
http://www.gamedev.net/community/forums/topic.asp?topic_id=559196


此討論串:討論了使用if condition可能會造成執行了both branching的問題，因為compiler判斷的問題，此耗費的計算比手動切分shader code，讓flow趨向單一branching的計算相對來的多。
(result = cmp( condition, result_a, result_b ); compiler可能會用cmp的asm，而不是if - else)
http://www.gamedev.net/community/forums/topic.asp?topic_id=474592

能用static branch就別用dynamic branching，使用static branch需要將condition內使用boolean ，避免使用variable，否則會造成dyanmic branching。 如果用? : 就可以使用single instruction，而不會如if - else發生較多instruction。
http://www.microsoft.com/downloads/en/confirmation.aspx?familyId=74db343e-e8ff-44e9-a43e-6f1615d9fce0&displayLang=en

當asm中未出現dyanmic branching的支援，反而使用cmp的instruction時，可以嘗試以下方法:

• Texture using branch, have to use tex2Dlod rather than tex2D
• D3DXSHADER_PREFER_FLOW_CONTROL when you call D3DXCompileShader... and of course specify ps_3_0