* <a for=language_extension lt=pointer_composite_access>`'pointer_composite_access'`</a>

* <a for=language_extension lt=uniform_buffer_standard_layout>`'uniform_buffer_standard_layout'`</a>

* <a for=language_extension lt=subgroup_id>`'subgroup_id'`</a>

* <a for=language_extension lt=subgroup_uniformity>`'subgroup_uniformity'`</a>

### Interpolation Type Names ### {#interpolation-type-names}

Allows the use of the [=built-in values/subgroup_id=] and [=built-in

values/num_subgroups=] built-in values when the [=extension/subgroups=]

extension is enabled.

<tr><td><dfn for="language_extension">subgroup_uniformity</dfn>

<td>

Adds an additional scope, subgroup, for [=uniform control flow=]

[[#subgroup-builtin-functions|subgroup]] and

[[#quad-builtin-functions|quad]] built-in functions to be all invocations

in the same [=subgroup=].

Note: The use of subgroup and quad built-in functions requires the

[=extension/subgroups=] extension to be enabled.

</table>

Note: The intent is that, over time, WGSL will define language extensions embodying all functionality in language extensions commonly supported at that time.

For a given group of invocations:

- If all invocations in a given scope execute as if they are executing in lockstep at a given

point in the program, that point is said to have <dfn noexport>uniform control flow</dfn>.

- For a [=compute shader stage=], the scope of uniform control flow is all invocations

in the same [=compute shader stage/workgroup=].

- For other shader stages, the scope of uniform control flow is all invocations for that

[=entry point=] in the same [=draw command=].

point in the program, that point is said to have <dfn noexport>uniform control flow</dfn>

for the given <dfn noexport>uniformity scope</dfn>.

- <dfn noexport>Workgroup uniformity scope</dfn>: For a [=compute shader

stage=], the uniformity scope is all invocations in the same [=compute

shader stage/workgroup=].

- <dfn noexport>Draw uniformity scope</dfn>: For other shader stages, the

uniformity scope is all invocations for that [=entry point=] in the same

[=draw command=].

- <dfn noexport>Subgroup uniformity scope</dfn>: If the

[=language_extension/subgroup_uniformity=] feature is supported, the

uniformity scope for [[#subgroup-builtin-functions|subgroup]] and

[[#quad-builtin-functions|quad]] built-in functions instead is all

invocations in the same [=subgroup=].

- If an expression is executed in uniform control flow, and all invocations compute the

same value, it is said to be a <dfn noexport>uniform value</dfn>.

- If invocations hold the same value for a local variable at every point where it is live,

The remaining subsections specify a static analysis that verifies that

[[#collective-operations|collective operations]] are only executed in [=uniform

control flow=].

If the [=language_extension/subgroup_uniformity=] feature is supported then

there are multiple [=uniformity scopes=].

This analysis is performed once for each scope.

Note: The analysis is described as being executed once per scope, but

implementations may perform a single analysis that includes each scope.

[=Workgroup uniformity scope|Workgroup=] and [=draw uniformity scope|draw=]

uniformity scopes are effectively equivalent since they operate on different

shader stages and represent the largest uniformity scope in a shader stage.

The analysis assumes [=dynamic errors=] do not occur.

A shader stage with a [=dynamic error=] is already non-portable, no matter the outcome

Here is the list of exceptions:

- A call to a function in [[#sync-builtin-functions]]:

- Has a [=function tag=] of [=NoRestriction=].

- Has a [=call site tag=] of [=CallSiteRequiredToBeUniform.S|CallSiteRequiredToBeUniform.error=], with [=potential-trigger-set=]

consisting of an unnamed [=diagnostic/triggering rule=].

- Note: The triggering rule has no name, and so it cannot be filtered.

- Additionally for the case of a call to [[#workgroupUniformLoad-builtin|workgroupUniformLoad]],

- Has a [=call site tag=] of:

- [=CallSiteRequiredToBeUniform.S|CallSiteRequiredToBeUniform.error=], with [=potential-trigger-set=]

consisting of an unnamed [=diagnostic/triggering rule=]

if [=language_extension/subgroup_uniformity=] is not supported or

the [=uniformity scope=] is not subgroup.

- Note: The triggering rule has no name, and so it cannot be filtered.

- [=CallSiteNoRestriction=] otherwise.

- Additionally if [=language_extension/subgroup_uniformity=] is not

supported or the [=uniformity scope=] is not subgroup, for a

call to [[#workgroupUniformLoad-builtin|workgroupUniformLoad]],

the parameter `p` has a [=parameter tag=] of [=ParameterRequiredToBeUniform.S|ParameterRequiredToBeUniform.error=],

with [=potential-trigger-set=] consisting of an unnamed [=diagnostic/triggering rule=].

- A call to a function in

[[#derivative-builtin-functions]], [[#texturesample]], [[#texturesamplebias]], and [[#texturesamplecompare]]:

- Has a [=function tag=] of [=ReturnValueMayBeNonUniform=].

- Has a [=call site tag=] as follows:

- Let *DF* be the [=nearest enclosing diagnostic filter=] for the call site location and triggering rule [=trigger/derivative_uniformity=]

- If *DF* exists, then let *S* be the *DF*'s new severity parameter.

- If *S* is the severity [=severity/off=], the call site tag is [=CallSiteNoRestriction=].

- Otherwise the call site tag is [=CallSiteRequiredToBeUniform.S=], with [=potential-trigger-set=]

- If [=language_extension/subgroup_uniformity=] is not supported or the [=uniformity scope=] is not subgroup

- Let *DF* be the [=nearest enclosing diagnostic filter=] for the call site location and triggering rule [=trigger/derivative_uniformity=]

- If *DF* exists, then let *S* be the *DF*'s new severity parameter.

- If *S* is the severity [=severity/off=], the call site tag is [=CallSiteNoRestriction=].

- Otherwise the call site tag is [=CallSiteRequiredToBeUniform.S=], with [=potential-trigger-set=]

consisting of a [=trigger/derivative_uniformity=] element.

- If there is no such *DF*,

the call site tag is [=CallSiteRequiredToBeUniform.S|CallSiteRequiredToBeUniform.error=], with [=potential-trigger-set=]

consisting of a [=trigger/derivative_uniformity=] element.

- If there is no such *DF*,

the call site tag is [=CallSiteRequiredToBeUniform.S|CallSiteRequiredToBeUniform.error=], with [=potential-trigger-set=]

consisting of a [=trigger/derivative_uniformity=] element.

- [=CallSiteNoRestriction=] otherwise.

- A call to [[#textureload]]:

- Has a [=call site tag=] of [=CallSiteNoRestriction=]

- Has a [=function tag=] as follows:

- [=NoRestriction=] otherwise

- A call to a function in

[[#subgroup-builtin-functions]] or [[#quad-builtin-functions]]:

- Has a [=function tag=] of [=ReturnValueMayBeNonUniform=].

- Has a [=function tag=] of:

- [=NoRestriction=] if the [=uniformity scope=] is subgroup and the function is one of the following:

- [[#subgroupadd-builtin|subgroupAdd]]

- [[#subgroupall-builtin|subgroupAll]]

- [[#subgroupand-builtin|subgroupAnd]]

- [[#subgroupany-builtin|subgroupAny]]

- [[#subgroupballot-builtin|subgroupBallot]]

- [[#subgroupbroadcast-builtin|subgroupBroadcast]]

- [[#subgroupbroadcastfirst-builtin|subgroupBroadcastFirst]]

- [[#subgroupmax-builtin|subgroupMax]]

- [[#subgroupmin-builtin|subgroupMin]]

- [[#subgroupmul-builtin|subgroupMul]]

- [[#subgroupor-builtin|subgroupOr]]

- [[#subgroupxor-builtin|subgroupXor]]

- [=ReturnValueMayBeNonUniform=] otherwise

- Let *DF* be the [=nearest enclosing diagnostic filter=] for the call site location and triggering rule [=trigger/subgroup_uniformity=]

- Has a [=call site tag=] as follows:

- If *DF* exists, then let *S* be the *DF*'s new severity parameter.

- If *S* is the severity [=severity/off=], the call site tag is [=CallSiteNoRestriction=].

- Otherwise, the call site tag is [=CallSiteRequiredToBeUniform.S=], with [=potential-trigger-set=]

- If [=language_extension/subgroup_uniformity=] is not supported or the [=uniformity scope=] is subgroup then:

- If *DF* exists, then let *S* be the *DF*'s new severity parameter.

- If *S* is the severity [=severity/off=], the call site tag is [=CallSiteNoRestriction=].

- Otherwise, the call site tag is [=CallSiteRequiredToBeUniform.S=], with [=potential-trigger-set=]

consisting of a [=trigger/subgroup_uniformity=] element.

- If there is no such *DF*,

the call site tag is [=CallSiteRequiredToBeUniform.S|CallSiteRequiredToBeUniform.error=], with [=potential-trigger-set=]

consisting of a [=trigger/subgroup_uniformity=] element.

- If there is no such *DF*,

the call site tag is [=CallSiteRequiredToBeUniform.S|CallSiteRequiredToBeUniform.error=], with [=potential-trigger-set=]

consisting of a [=trigger/subgroup_uniformity=] element.

- [=CallSiteNoRestriction=] otherwise.

- Additionally for the case of a call to [[#subgroupshuffleup-builtin|subgroupShuffleUp]] or [[#subgroupshuffledown-builtin|subgroupShuffleDown]],

the parameter `delta` has a [=parameter tag=] of:

- If *DF* exists, then let *S* be *DF*'s new severity parameter.

- If *S* is the severity [=severity/off=], the parameter tag is [=NoRestriction=].

- Otherwise, the parameter tag is [=ParameterRequiredToBeUniform.S=] with [=potential-trigger-set=]

consisting of a [=trigger/subgroup_uniformity=] element.

- If there is no such *DF*,

the parameter tag is [=ParameterRequiredToBeUniform.S|ParameterRequiredToBeUniform.error=], with [=potential-trigger-set=]

consisting of a [=trigger/subgroup_uniformity=] element.

- If [=language_extension/subgroup_uniformity=] is not supported or the [=uniformity scope=] is subgroup then:

- If *DF* exists, then let *S* be *DF*'s new severity parameter.

- If *S* is the severity [=severity/off=], the parameter tag is [=NoRestriction=].

- Otherwise, the parameter tag is [=ParameterRequiredToBeUniform.S=] with [=potential-trigger-set=]

consisting of a [=trigger/subgroup_uniformity=] element.

- If there is no such *DF*,

the parameter tag is [=ParameterRequiredToBeUniform.S|ParameterRequiredToBeUniform.error=], with [=potential-trigger-set=]

consisting of a [=trigger/subgroup_uniformity=] element.

- [=NoRestriction=] otherwise.

- Additionally for the case of a call to [[#subgroupshufflexor-builtin|subgroupShuffleXor]],

the parameter `mask` has a [=parameter tag=] of:

- If *DF* exists, then let *S* be *DF*'s new severity parameter.

- If *S* is the severity [=severity/off=], the parameter tag is [=NoRestriction=].

- Otherwise, the parameter tag is [=ParameterRequiredToBeUniform.S=] with [=potential-trigger-set=]

consisting of a [=trigger/subgroup_uniformity=] element.

- If there is no such *DF*,

the parameter tag is [=ParameterRequiredToBeUniform.S|ParameterRequiredToBeUniform.error=], with [=potential-trigger-set=]

consisting of a [=trigger/subgroup_uniformity=] element.

- If [=language_extension/subgroup_uniformity=] is not supported or the [=uniformity scope=] is subgroup then:

- If *DF* exists, then let *S* be *DF*'s new severity parameter.

- If *S* is the severity [=severity/off=], the parameter tag is [=NoRestriction=].

- Otherwise, the parameter tag is [=ParameterRequiredToBeUniform.S=] with [=potential-trigger-set=]

consisting of a [=trigger/subgroup_uniformity=] element.

- If there is no such *DF*,

the parameter tag is [=ParameterRequiredToBeUniform.S|ParameterRequiredToBeUniform.error=], with [=potential-trigger-set=]

consisting of a [=trigger/subgroup_uniformity=] element.

- [=NoRestriction=] otherwise.

Note: A WGSL implementation will ensure that if control flow prior to a

function call is [=uniform control flow|uniform=], it will also be uniform

after the function call.

function call is [=uniform control flow|uniform=] for a particular scope, it

will also be uniformafter the function call.

### Uniformity Rules for Expressions ### {#uniformity-expressions}

- [=built-in values/subgroup_size=] when used in a [=compute shader stage=]

- [=built-in values/num_subgroups=]

At [=subgroup uniformity scope=], the following built-in

input variables are also considered uniform:

- [=built-in values/subgroup_id=]

All other ones (see [=built-in values=]) are considered non-uniform.

Note: An author should avoid grouping the uniform built-in values together with