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)
LLVM: r243203