Vulkan-Docs/chapters/shaders.txt

// Copyright (c) 2015-2018 Khronos Group. This work is licensed under a
// Creative Commons Attribution 4.0 International License; see
// http://creativecommons.org/licenses/by/4.0/

[[shaders]]
= Shaders

A shader specifies programmable operations that execute for each vertex,
control point, tessellated vertex, primitive, fragment, or workgroup in the
corresponding stage(s) of the graphics and compute pipelines.

Graphics pipelines include vertex shader execution as a result of
<<drawing,primitive assembly>>, followed, if enabled, by tessellation
control and evaluation shaders operating on
<<drawing-primitive-topologies-patches,patches>>, geometry shaders, if
enabled, operating on primitives, and fragment shaders, if present,
operating on fragments generated by <<primsrast,Rasterization>>.
In this specification, vertex, tessellation control, tessellation evaluation
and geometry shaders are collectively referred to as vertex processing
stages and occur in the logical pipeline before rasterization.
The fragment shader occurs logically after rasterization.

Only the compute shader stage is included in a compute pipeline.
Compute shaders operate on compute invocations in a workgroup.

Shaders can: read from input variables, and read from and write to output
variables.
Input and output variables can: be used to transfer data between shader
stages, or to allow the shader to interact with values that exist in the
execution environment.
Similarly, the execution environment provides constants that describe
capabilities.

Shader variables are associated with execution environment-provided inputs
and outputs using _built-in_ decorations in the shader.
The available decorations for each stage are documented in the following
subsections.


[[shader-modules]]
== Shader Modules

[open,refpage='VkShaderModule',desc='Opaque handle to a shader module object',type='handles']
--

_Shader modules_ contain _shader code_ and one or more entry points.
Shaders are selected from a shader module by specifying an entry point as
part of <<pipelines,pipeline>> creation.
The stages of a pipeline can: use shaders that come from different modules.
The shader code defining a shader module must: be in the SPIR-V format, as
described by the <<spirvenv,Vulkan Environment for SPIR-V>> appendix.

Shader modules are represented by sname:VkShaderModule handles:

include::../api/handles/VkShaderModule.txt[]

--

[open,refpage='vkCreateShaderModule',desc='Creates a new shader module object',type='protos']
--

To create a shader module, call:

include::../api/protos/vkCreateShaderModule.txt[]

  * pname:device is the logical device that creates the shader module.
  * pname:pCreateInfo is a pointer to an instance of the
    sname:VkShaderModuleCreateInfo structure.
  * pname:pAllocator controls host memory allocation as described in the
    <<memory-allocation, Memory Allocation>> chapter.
  * pname:pShaderModule points to a slink:VkShaderModule handle in which the
    resulting shader module object is returned.

Once a shader module has been created, any entry points it contains can: be
used in pipeline shader stages as described in <<pipelines-compute,Compute
Pipelines>> and <<pipelines-graphics,Graphics Pipelines>>.

ifdef::VK_NV_glsl_shader[]
If the shader stage fails to compile ename:VK_ERROR_INVALID_SHADER_NV will
be generated and the compile log will be reported back to the application by
`<<VK_EXT_debug_report>>` if enabled.
endif::VK_NV_glsl_shader[]

include::../validity/protos/vkCreateShaderModule.txt[]
--

[open,refpage='VkShaderModuleCreateInfo',desc='Structure specifying parameters of a newly created shader module',type='structs']
--

The sname:VkShaderModuleCreateInfo structure is defined as:

include::../api/structs/VkShaderModuleCreateInfo.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:flags is reserved for future use.
  * pname:codeSize is the size, in bytes, of the code pointed to by
    pname:pCode.
  * pname:pCode points to code that is used to create the shader module.
    The type and format of the code is determined from the content of the
    memory addressed by pname:pCode.

.Valid Usage
****
  * [[VUID-VkShaderModuleCreateInfo-codeSize-01085]]
    pname:codeSize must: be greater than 0
ifndef::VK_NV_glsl_shader[]
  * [[VUID-VkShaderModuleCreateInfo-codeSize-01086]]
    pname:codeSize must: be a multiple of 4
  * [[VUID-VkShaderModuleCreateInfo-pCode-01087]]
    pname:pCode must: point to valid SPIR-V code, formatted and packed as
    described by the <<spirv-spec,Khronos SPIR-V Specification>>
  * [[VUID-VkShaderModuleCreateInfo-pCode-01088]]
    pname:pCode must: adhere to the validation rules described by the
    <<spirvenv-module-validation, Validation Rules within a Module>> section
    of the <<spirvenv-capabilities,SPIR-V Environment>> appendix
endif::VK_NV_glsl_shader[]
ifdef::VK_NV_glsl_shader[]
  * [[VUID-VkShaderModuleCreateInfo-pCode-01376]]
    If pname:pCode points to SPIR-V code, pname:codeSize must: be a multiple
    of 4
  * [[VUID-VkShaderModuleCreateInfo-pCode-01377]]
    pname:pCode must: point to either valid SPIR-V code, formatted and
    packed as described by the <<spirv-spec,Khronos SPIR-V Specification>>
    or valid GLSL code which must: be written to the +GL_KHR_vulkan_glsl+
    extension specification
  * [[VUID-VkShaderModuleCreateInfo-pCode-01378]]
    If pname:pCode points to SPIR-V code, that code must: adhere to the
    validation rules described by the <<spirvenv-module-validation,
    Validation Rules within a Module>> section of the
    <<spirvenv-capabilities,SPIR-V Environment>> appendix
  * [[VUID-VkShaderModuleCreateInfo-pCode-01379]]
    If pname:pCode points to GLSL code, it must: be valid GLSL code written
    to the +GL_KHR_vulkan_glsl+ GLSL extension specification
endif::VK_NV_glsl_shader[]
  * [[VUID-VkShaderModuleCreateInfo-pCode-01089]]
    pname:pCode must: declare the code:Shader capability for SPIR-V code
  * [[VUID-VkShaderModuleCreateInfo-pCode-01090]]
    pname:pCode must: not declare any capability that is not supported by
    the API, as described by the <<spirvenv-module-validation,
    Capabilities>> section of the <<spirvenv-capabilities,SPIR-V
    Environment>> appendix
  * [[VUID-VkShaderModuleCreateInfo-pCode-01091]]
    If pname:pCode declares any of the capabilities listed as optional: in
    the <<spirvenv-capabilities-table,SPIR-V Environment>> appendix, the
    corresponding feature(s) must: be enabled.
****

include::../validity/structs/VkShaderModuleCreateInfo.txt[]
--

[open,refpage='VkShaderModuleCreateFlags',desc='Reserved for future use',type='enums']
--
include::../api/flags/VkShaderModuleCreateFlags.txt[]

sname:VkShaderModuleCreateFlags is a bitmask type for setting a mask, but is
currently reserved for future use.
--

ifdef::VK_EXT_validation_cache[]
include::VK_EXT_validation_cache/shader-module-validation-cache.txt[]
endif::VK_EXT_validation_cache[]


[open,refpage='vkDestroyShaderModule',desc='Destroy a shader module',type='protos']
--

To destroy a shader module, call:

include::../api/protos/vkDestroyShaderModule.txt[]

  * pname:device is the logical device that destroys the shader module.
  * pname:shaderModule is the handle of the shader module to destroy.
  * pname:pAllocator controls host memory allocation as described in the
    <<memory-allocation, Memory Allocation>> chapter.

A shader module can: be destroyed while pipelines created using its shaders
are still in use.

.Valid Usage
****
  * [[VUID-vkDestroyShaderModule-shaderModule-01092]]
    If sname:VkAllocationCallbacks were provided when pname:shaderModule was
    created, a compatible set of callbacks must: be provided here
  * [[VUID-vkDestroyShaderModule-shaderModule-01093]]
    If no sname:VkAllocationCallbacks were provided when pname:shaderModule
    was created, pname:pAllocator must: be `NULL`
****

include::../validity/protos/vkDestroyShaderModule.txt[]
--


[[shaders-execution]]
== Shader Execution

At each stage of the pipeline, multiple invocations of a shader may: execute
simultaneously.
Further, invocations of a single shader produced as the result of different
commands may: execute simultaneously.
The relative execution order of invocations of the same shader type is
undefined:.
Shader invocations may: complete in a different order than that in which the
primitives they originated from were drawn or dispatched by the application.
However, fragment shader outputs are written to attachments in
<<primrast-order,rasterization order>>.

The relative execution order of invocations of different shader types is
largely undefined:.
However, when invoking a shader whose inputs are generated from a previous
pipeline stage, the shader invocations from the previous stage are
guaranteed to have executed far enough to generate input values for all
required inputs.


[[shaders-execution-memory-ordering]]
== Shader Memory Access Ordering

The order in which image or buffer memory is read or written by shaders is
largely undefined:.
For some shader types (vertex, tessellation evaluation, and in some cases,
fragment), even the number of shader invocations that may: perform loads and
stores is undefined:.

In particular, the following rules apply:

  * <<shaders-vertex-execution,Vertex>> and
    <<shaders-tessellation-evaluation-execution,tessellation evaluation>>
    shaders will be invoked at least once for each unique vertex, as defined
    in those sections.
  * <<shaders-fragment-execution,Fragment>> shaders will be invoked zero or
    more times, as defined in that section.
  * The relative execution order of invocations of the same shader type is
    undefined:.
    A store issued by a shader when working on primitive B might complete
    prior to a store for primitive A, even if primitive A is specified prior
    to primitive B. This applies even to fragment shaders; while fragment
    shader outputs are always written to the framebuffer in
    <<primrast-order, rasterization order>>, stores executed by fragment
    shader invocations are not.
  * The relative execution order of invocations of different shader types is
    largely undefined:.

[NOTE]
.Note
====
The above limitations on shader invocation order make some forms of
synchronization between shader invocations within a single set of primitives
unimplementable.
For example, having one invocation poll memory written by another invocation
assumes that the other invocation has been launched and will complete its
writes in finite time.
====

ifdef::VK_KHR_vulkan_memory_model[]

The <<memory-model,Memory Model>> appendix defines the terminology and rules
for how to correctly communicate between shader invocations, such as when a
write is <<memory-model-visible-to,Visible-To>> a read, and what constitutes
a <<memory-model-access-data-race,Data Race>>.

Applications must: not cause a data race.

endif::VK_KHR_vulkan_memory_model[]

ifndef::VK_KHR_vulkan_memory_model[]

Stores issued to different memory locations within a single shader
invocation may: not be visible to other invocations, or may: not become
visible in the order they were performed.

The code:OpMemoryBarrier instruction can: be used to provide stronger
ordering of reads and writes performed by a single invocation.
code:OpMemoryBarrier guarantees that any memory transactions issued by the
shader invocation prior to the instruction complete prior to the memory
transactions issued after the instruction.
Memory barriers are needed for algorithms that require multiple invocations
to access the same memory and require the operations to be performed in a
partially-defined relative order.
For example, if one shader invocation does a series of writes, followed by
an code:OpMemoryBarrier instruction, followed by another write, then the
results of the series of writes before the barrier become visible to other
shader invocations at a time earlier or equal to when the results of the
final write become visible to those invocations.
In practice it means that another invocation that sees the results of the
final write would also see the previous writes.
Without the memory barrier, the final write may: be visible before the
previous writes.

Writes that are the result of shader stores through a variable decorated
with code:Coherent automatically have available writes to the same buffer,
buffer view, or image view made visible to them, and are themselves
automatically made available to access by the same buffer, buffer view, or
image view.
Reads that are the result of shader loads through a variable decorated with
code:Coherent automatically have available writes to the same buffer, buffer
view, or image view made visible to them.
The order that coherent writes to different locations become available is
undefined:, unless enforced by a memory barrier instruction or other memory
dependency.

[NOTE]
.Note
====
Explicit memory dependencies must: still be used to guarantee availability
and visibility for access via other buffers, buffer views, or image views.
====

The built-in atomic memory transaction instructions can: be used to read and
write a given memory address atomically.
While built-in atomic functions issued by multiple shader invocations are
executed in undefined: order relative to each other, these functions perform
both a read and a write of a memory address and guarantee that no other
memory transaction will write to the underlying memory between the read and
write.
Atomic operations ensure automatic availability and visibility for writes
and reads in the same way as those to code:Coherent variables.

[NOTE]
.Note
====
Memory accesses performed on different resource descriptors with the same
memory backing may: not be well-defined even with the code:Coherent
decoration or via atomics, due to things such as image layouts or ownership
of the resource - as described in the <<synchronization, Synchronization and
Cache Control>> chapter.
====

[NOTE]
.Note
====
Atomics allow shaders to use shared global addresses for mutual exclusion or
as counters, among other uses.
====

endif::VK_KHR_vulkan_memory_model[]

[[shaders-inputs]]
== Shader Inputs and Outputs

Data is passed into and out of shaders using variables with input or output
storage class, respectively.
User-defined inputs and outputs are connected between stages by matching
their code:Location decorations.
Additionally, data can: be provided by or communicated to special functions
provided by the execution environment using code:BuiltIn decorations.

In many cases, the same code:BuiltIn decoration can: be used in multiple
shader stages with similar meaning.
The specific behavior of variables decorated as code:BuiltIn is documented
in the following sections.

ifdef::VK_NV_mesh_shader[]
[[shaders-task]]
== Task Shaders

Task shaders operate in conjunction with the mesh shaders to produce a
collection of primitives that will be processed by subsequent stages of the
graphics pipeline.
Its primary purpose is to create a variable amount of subsequent mesh shader
invocations.

Task shaders are invoked via the execution of the
<<drawing-mesh-shading,programmable mesh shading>> pipeline.

The task shader has no fixed-function inputs other than variables
identifying the specific workgroup and invocation.
The only fixed output of the task shader is a task count, identifying the
number of mesh shader workgroups to create.
The task shader can write additional outputs to task memory, which can be
read by all of the mesh shader workgroups it created.

=== Task Shader Execution

Task workloads are formed from groups of work items called workgroups and
processed by the task shader in the current graphics pipeline.
A workgroup is a collection of shader invocations that execute the same
shader, potentially in parallel.
Task shaders execute in _global workgroups_ which are divided into a number
of _local workgroups_ with a size that can: be set by assigning a value to
the code:LocalSize execution mode or via an object decorated by the
code:WorkgroupSize decoration.
An invocation within a local workgroup can: share data with other members of
the local workgroup through shared variables and issue memory and control
flow barriers to synchronize with other members of the local workgroup.

[[shaders-mesh]]
== Mesh Shaders

Mesh shaders operate in workgroups to produce a collection of primitives
that will be processed by subsequent stages of the graphics pipeline.
Each workgroup emits zero or more output primitives and the group of
vertices and their associated data required for each output primitive.

Mesh shaders are invoked via the execution of the
<<drawing-mesh-shading,programmable mesh shading>> pipeline.

The only inputs available to the mesh shader are variables identifying the
specific workgroup and invocation and, if applicable, any outputs written to
task memory by the task shader that spawned the mesh shader's workgroup.
The mesh shader can operate without a task shader as well.

The invocations of the mesh shader workgroup write an output mesh,
comprising a set of primitives with per-primitive attributes, a set of
vertices with per-vertex attributes, and an array of indices identifying the
mesh vertices that belong to each primitive.
The primitives of this mesh are then processed by subsequent graphics
pipeline stages, where the outputs of the mesh shader form an interface with
the fragment shader.

=== Mesh Shader Execution

Mesh workloads are formed from groups of work items called workgroups and
processed by the mesh shader in the current graphics pipeline.
A workgroup is a collection of shader invocations that execute the same
shader, potentially in parallel.
Mesh shaders execute in _global workgroups_ which are divided into a number
of _local workgroups_ with a size that can: be set by assigning a value to
the code:LocalSize execution mode or via an object decorated by the
code:WorkgroupSize decoration.
An invocation within a local workgroup can: share data with other members of
the local workgroup through shared variables and issue memory and control
flow barriers to synchronize with other members of the local workgroup.

The _global workgroups_ may be generated explcitly via the API, or
implicitly through the task shader's work creation mechanism.
endif::VK_NV_mesh_shader[]

[[shaders-vertex]]
== Vertex Shaders

Each vertex shader invocation operates on one vertex and its associated
<<fxvertex-attrib,vertex attribute>> data, and outputs one vertex and
associated data.
ifndef::VK_NV_mesh_shader[]
Graphics pipelines must: include a vertex shader, and the vertex shader
stage is always the first shader stage in the graphics pipeline.
endif::VK_NV_mesh_shader[]
ifdef::VK_NV_mesh_shader[]
Graphics pipelines using primitive shading must: include a vertex shader,
and the vertex shader stage is always the first shader stage in the graphics
pipeline.
endif::VK_NV_mesh_shader[]

[[shaders-vertex-execution]]
=== Vertex Shader Execution

A vertex shader must: be executed at least once for each vertex specified by
a draw command.
ifdef::VK_VERSION_1_1,VK_KHR_multiview[]
If the subpass includes multiple views in its view mask, the shader may: be
invoked separately for each view.
endif::VK_VERSION_1_1,VK_KHR_multiview[]
During execution, the shader is presented with the index of the vertex and
instance for which it has been invoked.
Input variables declared in the vertex shader are filled by the
implementation with the values of vertex attributes associated with the
invocation being executed.

If the same vertex is specified multiple times in a draw command (e.g. by
including the same index value multiple times in an index buffer) the
implementation may: reuse the results of vertex shading if it can statically
determine that the vertex shader invocations will produce identical results.

[NOTE]
.Note
====
It is implementation-dependent when and if results of vertex shading are
reused, and thus how many times the vertex shader will be executed.
This is true also if the vertex shader contains stores or atomic operations
(see <<features-features-vertexPipelineStoresAndAtomics,
pname:vertexPipelineStoresAndAtomics>>).
====


[[shaders-tessellation-control]]
== Tessellation Control Shaders

The tessellation control shader is used to read an input patch provided by
the application and to produce an output patch.
Each tessellation control shader invocation operates on an input patch
(after all control points in the patch are processed by a vertex shader) and
its associated data, and outputs a single control point of the output patch
and its associated data, and can: also output additional per-patch data.
The input patch is sized according to the pname:patchControlPoints member of
slink:VkPipelineTessellationStateCreateInfo, as part of input assembly.
The size of the output patch is controlled by the code:OpExecutionMode
code:OutputVertices specified in the tessellation control or tessellation
evaluation shaders, which must: be specified in at least one of the shaders.
The size of the input and output patches must: each be greater than zero and
less than or equal to
sname:VkPhysicalDeviceLimits::pname:maxTessellationPatchSize.


[[shaders-tessellation-control-execution]]
=== Tessellation Control Shader Execution

A tessellation control shader is invoked at least once for each _output_
vertex in a patch.
ifdef::VK_VERSION_1_1,VK_KHR_multiview[]
If the subpass includes multiple views in its view mask, the shader may: be
invoked separately for each view.
endif::VK_VERSION_1_1,VK_KHR_multiview[]

Inputs to the tessellation control shader are generated by the vertex
shader.
Each invocation of the tessellation control shader can: read the attributes
of any incoming vertices and their associated data.
The invocations corresponding to a given patch execute logically in
parallel, with undefined: relative execution order.
However, the code:OpControlBarrier instruction can: be used to provide
limited control of the execution order by synchronizing invocations within a
patch, effectively dividing tessellation control shader execution into a set
of phases.
Tessellation control shaders will read undefined: values if one invocation
reads a per-vertex or per-patch attribute written by another invocation at
any point during the same phase, or if two invocations attempt to write
different values to the same per-patch output in a single phase.


[[shaders-tessellation-evaluation]]
== Tessellation Evaluation Shaders

The Tessellation Evaluation Shader operates on an input patch of control
points and their associated data, and a single input barycentric coordinate
indicating the invocation's relative position within the subdivided patch,
and outputs a single vertex and its associated data.


[[shaders-tessellation-evaluation-execution]]
=== Tessellation Evaluation Shader Execution

A tessellation evaluation shader is invoked at least once for each unique
vertex generated by the tessellator.
ifdef::VK_VERSION_1_1,VK_KHR_multiview[]
If the subpass includes multiple views in its view mask, the shader may: be
invoked separately for each view.
endif::VK_VERSION_1_1,VK_KHR_multiview[]


[[shaders-geometry]]
== Geometry Shaders

The geometry shader operates on a group of vertices and their associated
data assembled from a single input primitive, and emits zero or more output
primitives and the group of vertices and their associated data required for
each output primitive.


[[shaders-geometry-execution]]
=== Geometry Shader Execution

A geometry shader is invoked at least once for each primitive produced by
the tessellation stages, or at least once for each primitive generated by
<<drawing,primitive assembly>> when tessellation is not in use.
A shader can request that the geometry shader runs multiple
<<geometry-invocations, instances>>.
A geometry shader is invoked at least once for each instance.
ifdef::VK_VERSION_1_1,VK_KHR_multiview[]
If the subpass includes multiple views in its view mask, the shader may: be
invoked separately for each view.
endif::VK_VERSION_1_1,VK_KHR_multiview[]


[[shaders-fragment]]
== Fragment Shaders

Fragment shaders are invoked as the result of rasterization in a graphics
pipeline.
Each fragment shader invocation operates on a single fragment and its
associated data.
With few exceptions, fragment shaders do not have access to any data
associated with other fragments and are considered to execute in isolation
of fragment shader invocations associated with other fragments.


[[shaders-fragment-execution]]
=== Fragment Shader Execution

For each fragment generated by rasterization, a fragment shader may: be
invoked.
A fragment shader must: not be invoked if the <<fragops-early,Early
Per-Fragment Tests>> cause it to have no coverage.
ifdef::VK_VERSION_1_1,VK_KHR_multiview[]
If the subpass includes multiple views in its view mask, the shader may: be
invoked separately for each view.
endif::VK_VERSION_1_1,VK_KHR_multiview[]

Furthermore, if it is determined that a fragment generated as the result of
rasterizing a first primitive will have its outputs entirely overwritten by
a fragment generated as the result of rasterizing a second primitive in the
same subpass, and the fragment shader used for the fragment has no other
side effects, then the fragment shader may: not be executed for the fragment
from the first primitive.

Relative ordering of execution of different fragment shader invocations is
not defined.

For each fragment generated by a primitive, the number of times the fragment
shader is invoked is implementation-dependent, but must: obey the following
constraints:

  * Each covered sample is included in a single fragment shader invocation.
  * When sample shading is not enabled, there is at least one fragment
    shader invocation.
  * When sample shading is enabled, the minimum number of fragment shader
    invocations is as defined in
ifdef::VK_NV_shading_rate_image[]
    <<primsrast-shading-rate-image,Shading Rate Image>> and
endif::VK_NV_shading_rate_image[]
    <<primsrast-sampleshading,Sample Shading>>.

When there is more than one fragment shader invocation per fragment, the
association of samples to invocations is implementation-dependent.

In addition to the conditions outlined above for the invocation of a
fragment shader, a fragment shader invocation may: be produced as a _helper
invocation_.
A helper invocation is a fragment shader invocation that is created solely
for the purposes of evaluating derivatives for use in non-helper fragment
shader invocations.
Stores and atomics performed by helper invocations must: not have any effect
on memory, and values returned by atomic instructions in helper invocations
are undefined:.


[[shaders-fragment-earlytest]]
=== Early Fragment Tests

An explicit control is provided to allow fragment shaders to enable early
fragment tests.
If the fragment shader specifies the code:EarlyFragmentTests
code:OpExecutionMode, the per-fragment tests described in
<<fragops-early-mode,Early Fragment Test Mode>> are performed prior to
fragment shader execution.
Otherwise, they are performed after fragment shader execution.

ifdef::VK_EXT_post_depth_coverage[]
[[shaders-fragment-earlytest-postdepthcoverage]]
If the fragment shader additionally specifies the code:PostDepthCoverage
code:OpExecutionMode, the value of a variable decorated with the
<<interfaces-builtin-variables-samplemask,code:SampleMask>> built-in
reflects the coverage after the early fragment tests.
Otherwise, it reflects the coverage before the early fragment tests.
endif::VK_EXT_post_depth_coverage[]

[[shaders-compute]]
== Compute Shaders

Compute shaders are invoked via flink:vkCmdDispatch and
flink:vkCmdDispatchIndirect commands.
In general, they have access to similar resources as shader stages executing
as part of a graphics pipeline.

Compute workloads are formed from groups of work items called workgroups and
processed by the compute shader in the current compute pipeline.
A workgroup is a collection of shader invocations that execute the same
shader, potentially in parallel.
Compute shaders execute in _global workgroups_ which are divided into a
number of _local workgroups_ with a size that can: be set by assigning a
value to the code:LocalSize execution mode or via an object decorated by the
code:WorkgroupSize decoration.
An invocation within a local workgroup can: share data with other members of
the local workgroup through shared variables and issue memory and control
flow barriers to synchronize with other members of the local workgroup.


[[shaders-interpolation-decorations]]
== Interpolation Decorations

Interpolation decorations control the behavior of attribute interpolation in
the fragment shader stage.
Interpolation decorations can: be applied to code:Input storage class
variables in the fragment shader stage's interface, and control the
interpolation behavior of those variables.

Inputs that could be interpolated can: be decorated by at most one of the
following decorations:

  * code:Flat: no interpolation
  * code:NoPerspective: linear interpolation (for
    <<line_linear_interpolation,lines>> and
    <<triangle_linear_interpolation,polygons>>)
ifdef::NV_VK_fragment_shader_barycentric[]
  * code:PerVertexNV: values fetched from shader-specified primitive vertex
endif::NV_VK_fragment_shader_barycentric[]

Fragment input variables decorated with neither code:Flat nor
code:NoPerspective use perspective-correct interpolation (for
<<line_perspective_interpolation,lines>> and
<<triangle_perspective_interpolation,polygons>>).

The presence of and type of interpolation is controlled by the above
interpolation decorations as well as the auxiliary decorations code:Centroid
and code:Sample.

A variable decorated with code:Flat will not be interpolated.
Instead, it will have the same value for every fragment within a triangle.
This value will come from a single <<vertexpostproc-flatshading,provoking
vertex>>.
A variable decorated with code:Flat can: also be decorated with
code:Centroid or code:Sample, which will mean the same thing as decorating
it only as code:Flat.

For fragment shader input variables decorated with neither code:Centroid nor
code:Sample, the assigned variable may: be interpolated anywhere within the
fragment and a single value may: be assigned to each sample within the
fragment.

If a fragment shader input is decorated with code:Centroid, a single value
may: be assigned to that variable for all samples in the fragment, but that
value must: be interpolated to a location that lies in both the fragment and
in the primitive being rendered, including any of the fragment's samples
covered by the primitive.
Because the location at which the variable is interpolated may: be different
in neighboring fragments, and derivatives may: be computed by computing
differences between neighboring fragments, derivatives of centroid-sampled
inputs may: be less accurate than those for non-centroid interpolated
variables.
ifdef::VK_NV_shading_rate_image[]
If
slink:VkPipelineViewportShadingRateImageStateCreateInfoNV::pname:shadingRateImageEnable
is enabled, implementations may: estimate derivatives using differencing
without dividing by the distance between adjacent sample locations when the
fragment size is larger than one pixel.
endif::VK_NV_shading_rate_image[]
ifdef::VK_EXT_post_depth_coverage[]
The <<shaders-fragment-earlytest-postdepthcoverage,code:PostDepthCoverage>>
execution mode does not affect the determination of the centroid location.
endif::VK_EXT_post_depth_coverage[]

If a fragment shader input is decorated with code:Sample, a separate value
must: be assigned to that variable for each covered sample in the fragment,
and that value must: be sampled at the location of the individual sample.
When pname:rasterizationSamples is ename:VK_SAMPLE_COUNT_1_BIT, the fragment
center must: be used for code:Centroid, code:Sample, and undecorated
attribute interpolation.

Fragment shader inputs that are signed or unsigned integers, integer
vectors, or any double-precision floating-point type must: be decorated with
code:Flat.

ifdef::VK_AMD_shader_explicit_vertex_parameter[]
When the `<<VK_AMD_shader_explicit_vertex_parameter>>` device extension is
enabled inputs can: be also decorated with the code:CustomInterpAMD
interpolation decoration, including fragment shader inputs that are signed
or unsigned integers, integer vectors, or any double-precision
floating-point type.
Inputs decorated with code:CustomInterpAMD can: only be accessed by the
extended instruction code:InterpolateAtVertexAMD and allows accessing the
value of the input for individual vertices of the primitive.
endif::VK_AMD_shader_explicit_vertex_parameter[]

ifdef::VK_NV_fragment_shader_barycentric[]
[[shaders-interpolation-decorations-pervertexnv]]
When the pname:fragmentShaderBarycentric feature is enabled, inputs can: be
also decorated with the code:PerVertexNV interpolation decoration, including
fragment shader inputs that are signed or unsigned integers, integer
vectors, or any double-precision floating-point type.
Inputs decorated with code:PerVertexNV can: only be accessed using an extra
array dimension, where the extra index identifies one of the vertices of the
primitive that produced the fragment.
endif::VK_NV_fragment_shader_barycentric[]

ifdef::VK_NV_ray_tracing[]
include::VK_NV_ray_tracing/raytracing-shaders.txt[]
endif::VK_NV_ray_tracing[]

[[shaders-staticuse]]
== Static Use

A SPIR-V module declares a global object in memory using the code:OpVariable
instruction, which results in a pointer code:x to that object.
A specific entry point in a SPIR-V module is said to _statically use_ that
object if that entry point's call tree contains a function that contains a
memory instruction or image instruction with code:x as an code:id operand.
See the "`Memory Instructions`" and "`Image Instructions`" subsections of
section 3 "`Binary Form`" of the SPIR-V specification for the complete list
of SPIR-V memory instructions.

Static use is not used to control the behavior of variables with code:Input
and code:Output storage.
The effects of those variables are applied based only on whether they are
present in a shader entry point's interface.

[[shaders-invocationgroups]]
== Invocation and Derivative Groups

An _invocation group_ (see the subsection "`Control Flow`" of section 2 of
the SPIR-V specification) for a compute shader is the set of invocations in
a single local workgroup.
For graphics shaders, an invocation group is an implementation-dependent
subset of the set of shader invocations of a given shader stage which are
produced by a single drawing command.
For indirect drawing commands with pname:drawCount greater than one,
invocations from separate draws are in distinct invocation groups.

[NOTE]
.Note
====
Because the partitioning of invocations into invocation groups is
implementation-dependent and not observable, applications generally need to
assume the worst case of all invocations in a draw belonging to a single
invocation group.
====

A _derivative group_ (see the subsection "`Control Flow`" of section 2 of
the SPIR-V 1.00 Revision 4 specification) for a fragment shader is the set
of invocations generated by a single primitive (point, line, or triangle),
including any helper invocations generated by that primitive.
ifdef::VK_NV_compute_shader_derivatives[]
A derivative group for a compute shader is a single local workgroup.
endif::VK_NV_compute_shader_derivatives[]
Derivatives are undefined: for a sampled image instruction if the
instruction is in flow control that is not uniform across the derivative
group.

ifdef::VK_VERSION_1_1[]
[[shaders-subgroup]]
== Subgroups

A _subgroup_ (see the subsection "`Control Flow`" of section 2 of the SPIR-V
1.3 Revision 1 specification) is a set of invocations that can synchronize
and share data with each other efficiently.
An invocation group is partitioned into one or more subgroups.

Subgroup operations are divided into various categories as described in
elink:VkSubgroupFeatureFlagBits.

[[shaders-subgroup-basic]]
=== Basic Subgroup Operations

The basic subgroup operations allow two classes of functionality within
shaders
- elect and barrier.
Invocations within a subgroup can: choose a single invocation to perform
some task for the subgroup as a whole using elect.
Invocations within a subgroup can: perform a subgroup barrier to ensure the
ordering of execution or memory accesses within a subgroup.
Barriers can: be performed on buffer memory accesses, code:WorkgroupLocal
memory accesses, and image memory accesses to ensure that any results
written are visible by other invocations within the subgroup.
An code:OpControlBarrier can: also be used to perform a full execution
control barrier.
A full execution control barrier will ensure that each active invocation
within the subgroup reaches a point of execution before any are allowed to
continue.

[[shaders-subgroup-vote]]
=== Vote Subgroup Operations

The vote subgroup operations allow invocations within a subgroup to compare
values across a subgroup.
The types of votes enabled are:

  * Do all active subgroup invocations agree that an expression is true?
  * Do any active subgroup invocations evaluate an expression to true?
  * Do all active subgroup invocations have the same value of an expression?

[NOTE]
.Note
====
These operations are useful in combination with control flow in that they
allow for developers to check whether conditions match across the subgroup
and choose potentially faster code-paths in these cases.
====

[[shaders-subgroup-arithmetic]]
=== Arithmetic Subgroup Operations

The arithmetic subgroup operations allow invocations to perform scan and
reduction operations across a subgroup.
For reduction operations, each invocation in a subgroup will obtain the same
result of these arithmetic operations applied across the subgroup.
For scan operations, each invocation in the subgroup will perform an
inclusive or exclusive scan, cumulatively applying the operation across the
invocations in a subgroup in linear order.
The operations supported are add, mul, min, max, and, or, xor.

[[shaders-subgroup-ballot]]
=== Ballot Subgroup Operations

The ballot subgroup operations allow invocations to perform more complex
votes across the subgroup.
The ballot functionality allows all invocations within a subgroup to provide
a boolean value and get as a result what each invocation provided as their
boolean value.
The broadcast functionality allows values to be broadcast from an invocation
to all other invocations within the subgroup, given that the invocation to
be broadcast from is known at pipeline creation time.

[[shaders-subgroup-shuffle]]
=== Shuffle Subgroup Operations

The shuffle subgroup operations allow invocations to read values from other
invocations within a subgroup.

[[shaders-subgroup-shuffle-relative]]
=== Shuffle Relative Subgroup Operations

The shuffle relative subgroup operations allow invocations to read values
from other invocations within the subgroup relative to the current
invocation in the group.
The relative operations supported allow data to be shifted up and down
through the invocations within a subgroup.

[[shaders-subgroup-clustered]]
=== Clustered Subgroup Operations

The clustered subgroup operations allow invocations to perform an operation
among partitions of a subgroup, such that the operation is only performed
within the subgroup invocations within a partition.
The partitions for clustered subgroup operations are consecutive
power-of-two size groups of invocations and the cluster size must: be known
at pipeline creation time.
The operations supported are add, mul, min, max, and, or, xor.

[[shaders-subgroup-quad]]
=== Quad Subgroup Operations

The quad subgroup operations allow clusters of 4 invocations (a quad), to
share data efficiently with each other.
ifdef::VK_NV_compute_shader_derivatives[]
For compute shaders using the code:DerivativeGroupQuadsNV or
code:DerivativeGroupLinearNV execution modes, each quad corresponds to one
of the groups of four shader invocations used for
<<texture-derivatives-compute,derivatives>>.
The invocations in each quad are ordered to have attribute values of
P~i0,j0~, P~i1,j0~, P~i0,j1~, and P~i1,j1~, respectively.
endif::VK_NV_compute_shader_derivatives[]

ifdef::VK_NV_shader_subgroup_partitioned[]

[[shaders-subgroup-partitioned]]
=== Partitioned Subgroup Operations

The partitioned subgroup operations allow invocations to perform an
operation among partitions of a subgroup, such that the operation is only
performed within the subgroup invocations within a partition.
The partitions for partitioned subgroup operations can: group the
invocations into arbitrary subsets and can: be computed at runtime.
The operations supported are add, mul, min, max, and, or, xor.

endif::VK_NV_shader_subgroup_partitioned[]

endif::VK_VERSION_1_1[]

ifdef::VK_EXT_validation_cache[]
[[shaders-validation-cache]]
== Validation Cache

[open,refpage='VkValidationCacheEXT',desc='Opaque handle to a validation cache object',type='handles']
--

Validation cache objects allow the result of internal validation to be
reused, both within a single application run and between multiple runs.
Reuse within a single run is achieved by passing the same validation cache
object when creating supported Vulkan objects.
Reuse across runs of an application is achieved by retrieving validation
cache contents in one run of an application, saving the contents, and using
them to preinitialize a validation cache on a subsequent run.
The contents of the validation cache objects are managed by the validation
layers.
Applications can: manage the host memory consumed by a validation cache
object and control the amount of data retrieved from a validation cache
object.

Validation cache objects are represented by sname:VkValidationCacheEXT
handles:

include::../api/handles/VkValidationCacheEXT.txt[]

--

[open,refpage='vkCreateValidationCacheEXT',desc='Creates a new validation cache',type='protos']
--

To create validation cache objects, call:

include::../api/protos/vkCreateValidationCacheEXT.txt[]

  * pname:device is the logical device that creates the validation cache
    object.
  * pname:pCreateInfo is a pointer to a slink:VkValidationCacheCreateInfoEXT
    structure that contains the initial parameters for the validation cache
    object.
  * pname:pAllocator controls host memory allocation as described in the
    <<memory-allocation, Memory Allocation>> chapter.
  * pname:pValidationCache is a pointer to a slink:VkValidationCacheEXT
    handle in which the resulting validation cache object is returned.

[NOTE]
.Note
====
Applications can: track and manage the total host memory size of a
validation cache object using the pname:pAllocator.
Applications can: limit the amount of data retrieved from a validation cache
object in fname:vkGetValidationCacheDataEXT.
Implementations should: not internally limit the total number of entries
added to a validation cache object or the total host memory consumed.
====

Once created, a validation cache can: be passed to the
fname:vkCreateShaderModule command as part of the
sname:VkShaderModuleCreateInfo pname:pNext chain.
If a sname:VkShaderModuleValidationCacheCreateInfoEXT object is part of the
sname:VkShaderModuleCreateInfo::pname:pNext chain, and its
pname:validationCache field is not dlink:VK_NULL_HANDLE, the implementation
will query it for possible reuse opportunities and update it with new
content.
The use of the validation cache object in these commands is internally
synchronized, and the same validation cache object can: be used in multiple
threads simultaneously.

[NOTE]
.Note
====
Implementations should: make every effort to limit any critical sections to
the actual accesses to the cache, which is expected to be significantly
shorter than the duration of the fname:vkCreateShaderModule command.
====

include::../validity/protos/vkCreateValidationCacheEXT.txt[]
--

[open,refpage='VkValidationCacheCreateInfoEXT',desc='Structure specifying parameters of a newly created validation cache',type='structs']
--

The sname:VkValidationCacheCreateInfoEXT structure is defined as:

include::../api/structs/VkValidationCacheCreateInfoEXT.txt[]

  * pname:sType is the type of this structure.
  * pname:pNext is `NULL` or a pointer to an extension-specific structure.
  * pname:flags is reserved for future use.
  * pname:initialDataSize is the number of bytes in pname:pInitialData.
    If pname:initialDataSize is zero, the validation cache will initially be
    empty.
  * pname:pInitialData is a pointer to previously retrieved validation cache
    data.
    If the validation cache data is incompatible (as defined below) with the
    device, the validation cache will be initially empty.
    If pname:initialDataSize is zero, pname:pInitialData is ignored.

.Valid Usage
****
  * [[VUID-VkValidationCacheCreateInfoEXT-initialDataSize-01534]]
    If pname:initialDataSize is not `0`, it must: be equal to the size of
    pname:pInitialData, as returned by fname:vkGetValidationCacheDataEXT
    when pname:pInitialData was originally retrieved
  * [[VUID-VkValidationCacheCreateInfoEXT-initialDataSize-01535]]
    If pname:initialDataSize is not `0`, pname:pInitialData must: have been
    retrieved from a previous call to fname:vkGetValidationCacheDataEXT
****

include::../validity/structs/VkValidationCacheCreateInfoEXT.txt[]
--

[open,refpage='VkValidationCacheCreateFlagsEXT',desc='Reserved for future use',type='enums']
--
include::../api/flags/VkValidationCacheCreateFlagsEXT.txt[]

sname:VkValidationCacheCreateFlagsEXT is a bitmask type for setting a mask,
but is currently reserved for future use.
--

[open,refpage='vkMergeValidationCachesEXT',desc='Combine the data stores of validation caches',type='protos']
--

Validation cache objects can: be merged using the command:

include::../api/protos/vkMergeValidationCachesEXT.txt[]

  * pname:device is the logical device that owns the validation cache
    objects.
  * pname:dstCache is the handle of the validation cache to merge results
    into.
  * pname:srcCacheCount is the length of the pname:pSrcCaches array.
  * pname:pSrcCaches is an array of validation cache handles, which will be
    merged into pname:dstCache.
    The previous contents of pname:dstCache are included after the merge.

[NOTE]
.Note
====
The details of the merge operation are implementation dependent, but
implementations should: merge the contents of the specified validation
caches and prune duplicate entries.
====

.Valid Usage
****
  * [[VUID-vkMergeValidationCachesEXT-dstCache-01536]]
    pname:dstCache must: not appear in the list of source caches
****

include::../validity/protos/vkMergeValidationCachesEXT.txt[]
--

[open,refpage='vkGetValidationCacheDataEXT',desc='Get the data store from a validation cache',type='protos']
--

Data can: be retrieved from a validation cache object using the command:

include::../api/protos/vkGetValidationCacheDataEXT.txt[]

  * pname:device is the logical device that owns the validation cache.
  * pname:validationCache is the validation cache to retrieve data from.
  * pname:pDataSize is a pointer to a value related to the amount of data in
    the validation cache, as described below.
  * pname:pData is either `NULL` or a pointer to a buffer.

If pname:pData is `NULL`, then the maximum size of the data that can: be
retrieved from the validation cache, in bytes, is returned in
pname:pDataSize.
Otherwise, pname:pDataSize must: point to a variable set by the user to the
size of the buffer, in bytes, pointed to by pname:pData, and on return the
variable is overwritten with the amount of data actually written to
pname:pData.

If pname:pDataSize is less than the maximum size that can: be retrieved by
the validation cache, at most pname:pDataSize bytes will be written to
pname:pData, and fname:vkGetValidationCacheDataEXT will return
ename:VK_INCOMPLETE.
Any data written to pname:pData is valid and can: be provided as the
pname:pInitialData member of the sname:VkValidationCacheCreateInfoEXT
structure passed to fname:vkCreateValidationCacheEXT.

Two calls to fname:vkGetValidationCacheDataEXT with the same parameters
must: retrieve the same data unless a command that modifies the contents of
the cache is called between them.

[[validation-cache-header]]
Applications can: store the data retrieved from the validation cache, and
use these data, possibly in a future run of the application, to populate new
validation cache objects.
The results of validation, however, may: depend on the vendor ID, device ID,
driver version, and other details of the device.
To enable applications to detect when previously retrieved data is
incompatible with the device, the initial bytes written to pname:pData must:
be a header consisting of the following members:

.Layout for validation cache header version ename:VK_VALIDATION_CACHE_HEADER_VERSION_ONE_EXT
[width="85%",cols="8%,21%,71%",options="header"]
|====
| Offset | Size | Meaning
| 0 | 4                    | length in bytes of the entire validation cache header
                             written as a stream of bytes, with the least
                             significant byte first
| 4 | 4                    | a elink:VkValidationCacheHeaderVersionEXT value
                             written as a stream of bytes, with the least
                             significant byte first
| 8 | ename:VK_UUID_SIZE   | a layer commit ID expressed as a UUID, which uniquely
                             identifies the version of the validation layers used
                             to generate these validation results
|====

The first four bytes encode the length of the entire validation cache
header, in bytes.
This value includes all fields in the header including the validation cache
version field and the size of the length field.

The next four bytes encode the validation cache version, as described for
elink:VkValidationCacheHeaderVersionEXT.
A consumer of the validation cache should: use the cache version to
interpret the remainder of the cache header.

If pname:pDataSize is less than what is necessary to store this header,
nothing will be written to pname:pData and zero will be written to
pname:pDataSize.

include::../validity/protos/vkGetValidationCacheDataEXT.txt[]
--

[open,refpage='VkValidationCacheHeaderVersionEXT',desc='Encode validation cache version',type='enums',xrefs='vkCreateValdiationCacheEXT vkGetValidationCacheDataEXT']
--
Possible values of the second group of four bytes in the header returned by
flink:vkGetValidationCacheDataEXT, encoding the validation cache version,
are:

include::../api/enums/VkValidationCacheHeaderVersionEXT.txt[]

  * ename:VK_VALIDATION_CACHE_HEADER_VERSION_ONE_EXT specifies version one
    of the validation cache.
--

[open,refpage='vkDestroyValidationCacheEXT',desc='Destroy a validation cache object',type='protos']
--

To destroy a validation cache, call:

include::../api/protos/vkDestroyValidationCacheEXT.txt[]

  * pname:device is the logical device that destroys the validation cache
    object.
  * pname:validationCache is the handle of the validation cache to destroy.
  * pname:pAllocator controls host memory allocation as described in the
    <<memory-allocation, Memory Allocation>> chapter.

.Valid Usage
****
  * [[VUID-vkDestroyValidationCacheEXT-validationCache-01537]]
    If sname:VkAllocationCallbacks were provided when pname:validationCache
    was created, a compatible set of callbacks must: be provided here
  * [[VUID-vkDestroyValidationCacheEXT-validationCache-01538]]
    If no sname:VkAllocationCallbacks were provided when
    pname:validationCache was created, pname:pAllocator must: be `NULL`
****

include::../validity/protos/vkDestroyValidationCacheEXT.txt[]
--
endif::VK_EXT_validation_cache[]