The Vivante family of 3D GPUs are available as IP cores, developed by the same named company[1].
Vivante GPUs are used in some ARM SoCs like the NXP i.MX6 and i.MX8.
Etnaviv is the name of the reverse engineered FLOSS Mesa driver. It's support for OpenGL/ES features is sadly lagging quite a bit behind[2].
This article discusses shader compilation and ISA documentation in the Mesa driver using auto-generated code from a high-level, formal specification of the instruction set (in XML), rather than a tons of hand-written C code. A merge request for auto-generated ISA documentation is currently pending [3].
[1] https://en.wikipedia.org/wiki/Vivante_Corporation
[3] https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23...