The tessellator ignores all writes to gl_TessLevelInner and gl_TessLevelOuter by the tessellation control shader except except those done by the last TCS invocation (gl_InvocationID == patch output vertices - 1).
This results in no primitives being generated unless all entries of gl_TessLevelOuter are written in that invocation (the 'initial' values seem to be 0 for gl_TessLevelOuter and 1 for gl_TessLevelInner).
Additionally, barrier() does not synchronize changes to the gl_TessLevel* between TCS invocations.
Neither of these problems can be reproduced with user-defined 'patch out' variables - writes by any TCS invocation are visible in the evaluation shader and barrier() makes them visible to the other TCS invocations.
GPU: Radeon HD 7950 (TAHITI)
Mesa: 10.7.0-devel (git-9deb614)
Created attachment 117373 [details]
Tessellation test case
I have attached a test case for this. It can be compiled and run using:
$ sh tessellation.cpp
Use the arrow keys to change which TCS invocation writes to gl_TessLevel* (or use space/enter to toggle between the first and the last).
It should display a yellow grid no matter what TCS invocation is selected but instead displays a green grid for invocation 3 and nothing for all other invocations.
I can confirm that the patch "radeonsi: before storing tess levels, load them from LDS instead of temporary" on the mailing list fixes the issue.
Fixed by 60159bcfc66a067b50da06f5cabfa20d72e89. Closing.
on Mar 27, 2017 at 08:35:05.
(provided by the Example extension).