Avalanche Studios head of research Emil Persson recently spoke about DirectX 11 low-level shader optimization for both the Xbox One and PS4, especially with concerns to the render output unit or ROP, which explains how many pixels can be output per clock. He talks about how the PS4’s GPU can handle a 64 bit path until bandwidth issues arise while explaining why the Xbox One will need to stick to a 32bit path. Both consoles’ GPUs were compared to the ATI Radeon HD7970, which can handle a 128bit path.
“As hardware has gotten increasingly more powerful over the years, some parts of it has lagged behind. The number of ROPs (i.e. how many pixels we can output per clock) remains very low. While this reflects typical use cases where the shader is reasonably long, it may limit the performance of short shaders.
“Unless the output format is wide, we are not even theoretically capable of using the full bandwidth available. For the HD7970 we need a 128bit format to become bandwidth bound. For the PS4 64bit would suffice.”
“On the XB1, if we are rendering to ESRAM, 64bit just about hits the crossover point between ROP and bandwidth-bound. But even if we render to the relatively slow DDR3 memory, we will still be ROP-bound if the render-target is a typical 32bit texture.”
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