Vulkan-Docs/doc/specs/vulkan/chapters/shaders.txt

// Copyright (c) 2015-2016 The Khronos Group Inc.
// Copyright notice at https://www.khronos.org/registry/speccopyright.html

[[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

_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,{apiname}
Environment for SPIR-V>> appendix.

A shader module is created by calling:

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

  * pname:device is the logical device that creates the shader module.
  * pname:pCreateInfo parameter 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 sname:VkShaderModule handle in which the
    resulting render pass object is returned.

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

The sname:VkShaderModuleCreateInfo structure is defined as:

include::../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.

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

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>>.

To destroy a shader module, call:

include::../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.

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
<<fundamentals-queueoperation-apiorder,API order>>.

The relative 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 order of invocations of the same shader type are 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
    <<fundamentals-queueoperation-apiorder,in primitive order>>, stores
    executed by fragment shader invocations are not.
  * The relative 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.
====

Stores issued to different memory locations within a single shader
invocation may: not be visible to other invocations 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.

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.

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


[[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.


[[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. Graphics pipelines must: include a vertex shader, and
the vertex shader stage is always the first shader stage in the graphics
pipeline.


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

A vertex shader must: be executed at least once for each vertex specified by
a draw command. 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 a vertex is a part of more than one input primitive, for example
by including the same index value multiple times in an index buffer, the
vertex shader may: be invoked only once and the results shared amongst the
resulting primitives. This is known as _vertex reuse_.

ifdef::implementation-guide[]
.Implementor's Note
****
If a vertex is repeated in a draw command (i.e. the same index is repeated
in an indexed draw), the shader may: be executed anywhere from one to the
number of repetitions times for that vertex, depending on the
implementation's ability to reuse shader results.
****
endif::implementation-guide[]


[[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.

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.


[[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. The number
of geometry shader invocations per input primitive is determined from the
invocation count of the geometry shader specified by the
code:OpExecutionMode code:Invocations in the geometry shader. If the
invocation count is not specified, then a default of one invocation is
executed.


[[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 mustnot: be invoked if the <<fragops-early,Early
Per-Fragment Tests>> cause it to have no coverage.

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.

The number of fragment shader invocations produced per-pixel is determined
as follows:

  - If per-sample shading is enabled, the fragment shader is invoked once
    per covered sample.
  - Otherwise, the fragment shader is invoked at least once per fragment but
    no more than once per covered sample.

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 mustnot: 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.


[[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
either in the shader code or via
<<pipelines-specialization-constants,Specialization Constants>>. 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_noperspective_interpolation,lines>> and
    <<triangle_noperspective_interpolation,polygons>>).

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 pixel and a single value may: be assigned to each sample
within the pixel.

code:Centroid and code:Sample can: be used to control the location and
frequency of the sampling of the decorated fragment shader input. If a
fragment shader input is decorated with code:Centroid, a single value may:
be assigned to that variable for all samples in the pixel, but that value
must: be interpolated to a location that lies in both the pixel and in the
primitive being rendered, including any of the pixel's samples covered by
the primitive. Because the location at which the variable is interpolated
may be different in neighboring pixels, and derivatives may be computed by
computing differences between neighboring pixels, derivatives of
centroid-sampled inputs may: be less accurate than those for non-centroid
interpolated variables. 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 pixel, and that value must: be sampled at the location
of the individual sample. When pname:rasterizationSamples is
ename:VK_SAMPLE_COUNT_1_BIT, the pixel 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.


[[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. Derivatives are
undefined for a sampled image instruction if the instruction is in flow
control that is not uniform across the derivative group.