Customizing shader behavior with specialization constants

The process of compiling shader code results in immutability once completed. The compilation procedure carries a substantial time overhead and is generally circumvented during runtime. Even minor adjustments to a shader necessitate recompilation, leading to the creation of a fresh shader module and potentially a new pipeline as well – all entailing significant resource-intensive operations.

In Vulkan, specialization constants allow you to specify constant values for shader parameters at pipeline creation time, instead of having to recompile the shader with new values every time you want to change them. This can be particularly useful when you want to reuse the same shader with different constant values multiple times. In this recipe, we will delve deeper into the practical application of specialization constants in Vulkan to create more efficient and flexible shader programs, allowing you to adjust without the need for resource-intensive recompilations.

Getting ready

Specialization constants are available in the repository through the VulkanCore::Pipeline::GraphicsPipelineDescriptor structure. You need to provide a vector of VkSpecializationMapEntry structures for each shader type you’d like to apply specialization constants to.

How to do it…

Specialization constants are declared in GLSL using the constant_id qualifier along with an integer that specifies the constant’s ID:

layout (constant_id = 0) const bool useShaderDebug = false;

To create a pipeline with specialized constant values, you first need to create a VkSpecializationInfo structure that specifies the constant values and their IDs. You then pass this structure to the VkPipelineShaderStageCreateInfo structure when creating a pipeline:

const bool kUseShaderDebug = false;
const VkSpecializationMapEntry useShaderDebug = {
    .constantID = 0, // matches the constant_id qualifier
    .offset = 0,
    .size = sizeof(bool),
};
const VkSpecializationInfo vertexSpecializationInfo = {
    .mapEntryCount = 1,
    .pMapEntries = &useShaderDebug,
    .dataSize = sizeof(bool),
    .pData = &kUseShaderDebug,
};
const VkPipelineShaderStageCreateInfo shaderStageInfo = {
  ...
  .pSpecializationInfo = &vertexSpecializationInfo,
};

Because specialization constants are real constants, branches that depend on them may be entirely removed during the final compilation of the shader. On the other hand, specialization constants should not be used to control parameters such as uniforms, as they are not as flexible and require to be known during the construction of the pipeline.