GL_IMG_shader_group_vote

Supported Hardware

Series5, Series5XT, Series6, Series6XE, Series6XT

Valid APIs

OpenGL ES 2.0, 3.0

Description

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 ES 3.0 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 evaluating a condition across an entire SIMD invocation group using code like:

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

The built-in function allInvocationsIMG() 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).

Note

If the GL_ARB_shader_group_vote is present, the ARB variant of this extension will be recommended to use.

Example

#extension GL_IMG_shader_group_vote : require 
if (allInvocationsIMG(condition)) 
{ 
    result = do_fast_path();
} 
else 
{ 
    result = do_general_path();
}