The NIR-To-TGSI translation layer has been in the works for most of the year with hopes of using that to eventually kill the Mesa state tracker GLSL-to-TGSI code that is quite large and crusty. While RadeonSI, Iris, and the other larger Gallium3D drivers are making use of NIR for a while now, this NIR-to-TGSI path can help other Gallium3D drivers like Softpipe that still rely on TGSI. If getting rid of the GLSL-to-TGSI path, GLSL shaders would ultimately go through NIR and then translated to TGSI.
This can allow for better optimizations and ultimately better performance . This translation layer allows these other Gallium3D drivers to make use of NIR optimizations and all the attention it receives these days where as the TGSI-specific work is largely untouched.
Eric Anholt of Google who has been working on this support commented of the immediate benefits for TGSI-based Gallium3D drivers, " I've seen it reduce instruction count massively in compute, geometry, and UBO-loading shaders, but even a bit in trivial texturing shaders like glmark2! "
This patch also goes on to explain, " The goal is to replace glsl_to_tgsi.cpp and its supporting code (~10k LOC). This code ends up being smaller because NIR has many lowering passes that help it map better to TGSI than GLSL IR does. As a benefit, this brings NIR optimizations to TGSI-only drivers. Many of the softpipe shaders I've looked at end up being significantly shorter. "
Mesa 20.3 remains under active development and should be out as stable in early December.