With the Mesa Git code since yesterday, MESA_GLSL_CACHE_ENABLE=1 must be set as the environment variable for controlling the shader cache behavior for R600g and RadeonSI. But a patch sent out today by Timothy Arceri would enable it by default. He commented, "Piglit and all games tested so far seem to be working without issue. This change will allow wide user testing and we can decided before the next release if we need to turn it off again."
There's also another patch as well. This patch would implement the RadeonSI on-disk shader cache for the GPU-generated shader code where as right now in Mesa it's just caching to disk the TGSI (Gallium3D IR) that still needs to be compiled via the AMDGPU compiler back-end back to the native GPU instructions. This additional 69 lines of code building upon the rest of the Mesa shader cache work would cache the hardware binary, providing the greatest benefit.
I'm still running some tests of Mesa Git from yesterday and hope to have those results to share shortly.