On modern GPUs, what hardware do vertex/pixel shaders have access to that compute shaders don't?

Question

Suppose that someone implemented rendering for a video game using entirely GPGPU apis, like compute shaders or CUDA.

Would it be possible to achieve the same performance as standard graphics pipelines on common modern GPUs? If not, then what hardware do standard graphics pipelines have access to which general purpose GPU programming APIs dont give access to?

Answer 1

what hardware do standard graphics pipelines have access to which general purpose GPU programming APIs dont give access to?

Every part of the graphics pipeline and architecture that isn't a shader. This includes but is not limited to:

• The rasterizer
• The post T&L cache
• Any vertex buffer fetching/decompressing logic
• ROPs, which do read/modify&test/write operation with the framebuffer, such as:
  • blending
  • depth/stencil testing
  • write masking
• All of the hardware buffers and pipelines between various stages

None of these are accessible by shader logic. They're hard-wired parts of the graphics pipeline.