https://bugs.freedesktop.org/show_bug.cgi?id=65416
Priority: medium Bug ID: 65416 Assignee: dri-devel@lists.freedesktop.org Summary: r300g does not eliminate unread varyings Severity: normal Classification: Unclassified OS: All Reporter: stefandoesinger@gmx.at Hardware: Other Status: NEW Version: git Component: Drivers/Gallium/r300 Product: Mesa
Created attachment 80344 --> https://bugs.freedesktop.org/attachment.cgi?id=80344&action=edit Example shaders
r300g tries to interpolate varyings written by the vertex shader even if the fragment shader does not read them. The attached program code illustrates this with two sample shaders.
The shaders in question were generated by Wine's fixed function pipeline replacement, generated from a fixed function setup set up by 3DMark 2000.
The visible issues caused by this are broken fog because the driver runs out of varyings and reduced performance due to the extra shader instructions.
An argument could be made that this is Wine's bug, and it should not generate such inefficient shaders. Doing this would be a major inconvenience though because the vertex and fragment shader are generated independently. As far as I can see the proprietary drivers optimize this inefficiency away.