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push_constants.adoc

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Push Constants

The Vulkan spec defines Push Constants as:

Note

A small bank of values writable via the API and accessible in shaders. Push constants allow the application to set values used in shaders without creating buffers or modifying and binding descriptor sets for each update.

How to use

Shader Code

From a shader perspective, push constant are similar to a uniform buffer. The spec provides details for the push constant interface between Vulkan and SPIR-V.

A simple GLSL fragment shader example:

layout(push_constant, std430) uniform pc {
    vec4 data;
};

layout(location = 0) out vec4 outColor;

void main() {
   outColor = data;
}

Which when looking at parts of the disassembled SPIR-V

                  OpMemberDecorate %pc 0 Offset 0
                  OpDecorate %pc Block

         %float = OpTypeFloat 32
       %v4float = OpTypeVector %float 4

%pc             = OpTypeStruct %v4float
%pc_ptr         = OpTypePointer PushConstant %pc
%pc_var         = OpVariable %pc_ptr PushConstant
%pc_v4float_ptr = OpTypePointer PushConstant %v4float

%access_chain   = OpAccessChain %pc_v4float_ptr %pc_var %int_0

it matches the Vulkan spec description of being an OpTypeStruct type with a Block decoration.

Pipeline layout

When calling vkCreatePipelineLayout the push constant ranges needs to be set in VkPipelineLayoutCreateInfo.

An example using the previous shader above:

VkPushConstantRange range = {};
range.stageFlags = VK_SHADER_STAGE_FRAGMENT_BIT;
range.offset = 0;
range.size = 16; // %v4float (vec4) is defined as 16 bytes

VkPipelineLayoutCreateInfo create_info = {};
create_info.sType = VK_STRUCTURE_TYPE_PIPELINE_LAYOUT_CREATE_INFO;
create_info.pNext = NULL;
create_info.flags = 0;
create_info.setLayoutCount = 0;
create_info.pushConstantRangeCount = 1;
create_info.pPushConstantRanges = ⦥

VkPipelineLayout pipeline_layout;
vkCreatePipelineLayout(device, &create_info, NULL, &pipeline_layout);

Updating at record time

Lastly, the value for the push constants needs to be updated to the desired value using vkCmdPushConstants.

float data[4] = {0.0f, 1.0f, 2.0f, 3.0f}; // where sizeof(float) == 4 bytes

// vkBeginCommandBuffer()
uint32_t offset = 0;
uint32_t size = 16;
vkCmdPushConstants(commandBuffer, pipeline_layout, VK_SHADER_STAGE_FRAGMENT_BIT, offset, size, data);
// draw / dispatch / trace rays / etc
// vkEndCommandBuffer()

Offsets

Taking the above shader, a developer can add an offset to the push constant block

layout(push_constant, std430) uniform pc {
-   vec4 data;
+   layout(offset = 32) vec4 data;
};

layout(location = 0) out vec4 outColor;

void main() {
   outColor = data;
}

The difference from the above disassembled SPIR-V is only the member decoration

- OpMemberDecorate %pc 0 Offset 0
+ OpMemberDecorate %pc 0 Offset 32

From here the offset of 32 needs to be also specified in VkPushConstantRange for each shader stage that uses it

VkPushConstantRange range = {};
range.stageFlags = VK_SHADER_STAGE_FRAGMENT_BIT;
-range.offset = 0;
+range.offset = 32;
range.size = 16;

The following diagram provides a visualization of how push constant offsets work.

push_constant_offset

Pipeline layout compatibility

The Vulkan spec defines what Compatibility for push constants as

Note

if they were created with identical push constant ranges

This means before a bound pipeline command is issued (vkCmdDraw, vkCmdDispatch, etc) the VkPipelineLayout used in the last vkCmdPushConstants and vkCmdBindPipeline (for the appropriate VkPipelineBindPoint) must have had identical VkPushConstantRange.

Lifetime of push constants

The lifetime of push constants can open room for some edge cases and the following is designed to give some common examples of what is and is not valid with push constants.

Note

There are some CTS under dEQP-VK.pipeline.push_constant.lifetime.*

Binding descriptor sets has no effect

Because push constants are not tied to descriptors, the use of vkCmdBindDescriptorSets has no effect on the lifetime or pipeline layout compatibility of push constants.

Mixing bind points

It is possible to use two different VkPipelineBindPoint that each have different uses of push constants in their shader

// different ranges and therefore not compatible layouts
VkPipelineLayout layout_graphics; // VK_SHADER_STAGE_FRAGMENT_BIT
VkPipelineLayout layout_compute;  // VK_SHADER_STAGE_COMPUTE_BIT

// vkBeginCommandBuffer()
vkCmdBindPipeline(pipeline_graphics); // layout_graphics
vkCmdBindPipeline(pipeline_compute);  // layout_compute

vkCmdPushConstants(layout_graphics); // VK_SHADER_STAGE_FRAGMENT_BIT
// Still valid as the last pipeline and push constant for graphics are compatible
vkCmdDraw();

vkCmdPushConstants(layout_compute); // VK_SHADER_STAGE_COMPUTE_BIT
vkCmdDispatch(); // valid
// vkEndCommandBuffer()

Binding non-compatible pipelines

The spec say:

Note

Binding a pipeline with a layout that is not compatible with the push constant layout does not disturb the push constant values.

The following examples helps illustrate this:

// vkBeginCommandBuffer()
vkCmdPushConstants(layout_0);
vkCmdBindPipeline(pipeline_b); // non-compatible with layout_0
vkCmdBindPipeline(pipeline_a); // compatible with layout_0
vkCmdDraw(); // valid
// vkEndCommandBuffer()

// vkBeginCommandBuffer()
vkCmdBindPipeline(pipeline_b); // non-compatible with layout_0
vkCmdPushConstants(layout_0);
vkCmdBindPipeline(pipeline_a); // compatible with layout_0
vkCmdDraw(); // valid
// vkEndCommandBuffer()

// vkBeginCommandBuffer()
vkCmdPushConstants(layout_0);
vkCmdBindPipeline(pipeline_a); // compatible with layout_0
vkCmdBindPipeline(pipeline_b); // non-compatible with layout_0
vkCmdDraw(); // INVALID
// vkEndCommandBuffer()

Layouts with no static push constants

It is also valid to have a VkPushConstantRange in the pipeline layout but no push constants in the shader, for example:

VkPushConstantRange range = {VK_SHADER_STAGE_VERTEX_BIT, 0, 4};
VkPipelineLayoutCreateInfo pipeline_layout_info = {VK_SHADER_STAGE_VERTEX_BIT. 1, &range};
void main() {
   gl_Position = vec4(1.0);
}

If a VkPipeline is created with the above shader and pipeline layout, it is still valid to call vkCmdPushConstants on it.

The mental model can be thought of as vkCmdPushConstants is tied to the VkPipelineLayout usage and therefore why they must match before a call to a command such as vkCmdDraw().

The same way it is possible to bind descriptor sets that are never used by the shader, the same is true for push constants.

Updated incrementally

Push constants can be incrementally updated over the course of a command buffer.

The following shows an example of the values of a vec4 push constant

// vkBeginCommandBuffer()
vkCmdBindPipeline();
vkCmdPushConstants(offset: 0, size: 16, value = [0, 0, 0, 0]);
vkCmdDraw(); // values = [0, 0, 0, 0]

vkCmdPushConstants(offset: 4, size: 8, value = [1 ,1]);
vkCmdDraw(); // values = [0, 1, 1, 0]

vkCmdPushConstants(offset: 8, size: 8, value = [2, 2]);
vkCmdDraw(); // values = [0, 1, 2, 2]
// vkEndCommandBuffer()