Reference

What is it?

Shaders can use Vulkan specialization constants for features that change code without changing the shader inputs or resource layout. Add ShaderConstant() to a StaticCombo declaration, and the compiler can share compiled code between feature combinations, with the selected values supplied when the GPU pipeline is created.

Values of these constants are expected to be used in normal conditions and expressions instead of preprocessor branches. You still need to use regular static combos (without specialization constants) if they change resources or vertex/pixel struct.

Usage

cpp

FEATURES
{
	Feature( F_DETAIL_TEXTURE, 0..4, "Detail Texture" );
}

PS
{
    // ShaderConstant() is an additional argument that turns this static combo
    // into a specialization constant. Remove it and it will become a regular static combo again.
	StaticCombo( S_DETAIL_TEXTURE, F_DETAIL_TEXTURE, Sys( ALL ), ShaderConstant() );

	// Using static combo as a variable, notice if() instead of #if.
    // Still treated by GPU as a static combo, but it isn't required to compile it
	if ( S_DETAIL_TEXTURE == 3 || S_DETAIL_TEXTURE == 4 )
	{
		// detail normal mapping...
	}
}

It essentially turns this static combo into a variable instead of another #define, which would add up to list of shader variants that engine must compile. The benefit is that it is works as a variable while GPU still treats it as a static combo, doing necessary optimizations, but without having to compile it.

Limitations

This can be used only with if, switch and expressions. #if or #elif, texture processing annotations, conditional includes, render states or material attributes will not work with this.
Works only with static combos, dynamic combos are not supported
You can have up to 64 specialization constants in your shader