OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Shader Invocation Control Functions

The shader invocation control function is available only in tessellation control shaders and compute shaders. It is used to control the relative execution order of multiple shader invocations used to process a patch (in the case of tessellation control shaders) or a local work group (in the case of compute shaders), which are otherwise executed with an undefined relative order.

void barrier();

For any given static instance of barrier, all tessellation control shader invocations for a single input patch must enter it before any will be allowed to continue beyond it, or all invocations for a single work group must enter it before any will continue beyond it.

The function barrier provides a partially defined ...