Mesa Git Lands RadeonSI 32-bit Pointers Support
Written by Michael Larabel in Mesa on 17 February 2018 at 11:53 AM EST. 4 Comments
At the start of the new year Marek Olšák of AMD posted a set of patches for 32-bit GPU pointers in RadeonSI. That work has now landed in mainline Mesa Git.

The function of the RadeonSI Gallium3D driver adding this 32-bit pointers support would be freeing up some SGPR registers. In the comments to that article Marek noted, "32-bit pointers will only be used internally by the driver for passing pointers to shaders. Older GCN has 64 bytes (and Vega has 128 bytes) of shader input storage. 32-bit pointers will help use that storage more effectively. For example, in Vulkan, the pointers typically point to descriptor pools. In OpenGL, the pointers point to texture/UBO/TBO/SSBO/etc. descriptor lists...32-bit pointers are necessary for decreasing the number of used user data SGPRs on AMD GCN. It's an internal driver micro-optimization that can help all applications."

Overnight landing in Mesa Git was the 32-bit pointers for user data SGPRs and the related code for 32-bit VM allocations in the Radeon winsys and other necessary changes.

It will be interesting to see if this has any real-world impact on the performance for freeing up some scalar general purpose registers (SGPRs). This change is in Git for Mesa 18.1.
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