You can look at MIAOW which is an open source implementation of the AMD Southern Islands GPU ISA.[0]
The RTL can be found here [1]
The best part is the white paper [2], PDF
I had been looking around for information similar to ECS[3], building a GPU from first principles and MIAOW is one step closer that.
[0] http://miaowgpu.org/index.html
[1] https://github.com/VerticalResearchGroup/miaow
[2] https://raw.githubusercontent.com/wiki/VerticalResearchGroup...