OpenGL.GL.ARB.shader_group_vote

OpenGL extension ARB.shader_group_vote

This module customises the behaviour of the OpenGL.raw.GL.ARB.shader_group_vote to provide a more Python-friendly API

Overview (from the spec)

This extension provides new built-in functions to compute the composite of a set of boolean conditions across a group of shader invocations. These composite results may be used to execute shaders more efficiently on a single-instruction multiple-data (SIMD) processor. The set of shader invocations across which boolean conditions are evaluated is implementation-dependent, and this extension provides no guarantee over how individual shader invocations are assigned to such sets. In particular, the set of shader invocations has no necessary relationship with the compute shader local work group -- a pair of shader invocations in a single compute shader work group may end up in different sets used by these built-ins.

Compute shaders operate on an explicitly specified group of threads (a local work group), but many implementations of OpenGL 4.3 will even group non-compute shader invocations and execute them in a SIMD fashion. When executing code like

if (condition) { result = do_fast_path(); } else { result = do_general_path(); }

where <condition> diverges between invocations, a SIMD implementation might first call do_fast_path() for the invocations where <condition> is true and leave the other invocations dormant. Once do_fast_path() returns, it might call do_general_path() for invocations where <condition> is false and leave the other invocations dormant. In this case, the shader executes *both* the fast and the general path and might be better off just using the general path for all invocations.

This extension provides the ability to avoid divergent execution by evaluting a condition across an entire SIMD invocation group using code like:

if (allInvocationsARB(condition)) { result = do_fast_path(); } else { result = do_general_path(); }

The built-in function allInvocationsARB() will return the same value for all invocations in the group, so the group will either execute do_fast_path() or do_general_path(), but never both. For example, shader code might want to evaluate a complex function iteratively by starting with an approximation of the result and then refining the approximation. Some input values may require a small number of iterations to generate an accurate result (do_fast_path) while others require a larger number (do_general_path). In another example, shader code might want to evaluate a complex function (do_general_path) that can be greatly simplified when assuming a specific value for one of its inputs (do_fast_path).

The official definition of this extension is available here: http://www.opengl.org/registry/specs/ARB/shader_group_vote.txt