Add extension `subgroup-size-control` by Jiawei-Shao · Pull Request #5578 · gpuweb/gpuweb

Jiawei-Shao · 2026-02-27T07:02:55Z

This patch adds a new WebGPU and WGSL extension subgroup-size-control
based on subgroup-size-control.md.

This patch adds a new WebGPU and WGSL extension `subgroup-size-control` based on `proposals/subgroup-size-control.md`.

github-actions · 2026-02-27T07:59:25Z

Previews, as seen when this build job started (22e75bf):
WebGPU _{webgpu.idl | Explainer | Correspondence Reference}
WGSL _{grammar.js | wgsl.lalr.txt}

alan-baker · 2026-03-01T19:57:59Z

spec/index.bs

+    readonly attribute unsigned long minExplicitComputeSubgroupSize;
+    readonly attribute unsigned long maxExplicitComputeSubgroupSize;


It's a bit awkward that there are two min/max attributes. Are there devices where subgroupMin/MaxSize do not match min/maxExplicitComputeSubgroupSize?

Separately the naming conventions seem mismatched. If these are needed, maybe explicit[Compute]SubgroupMinSize and explicit[Compute]SubgroupMaxSize. Not sure including compute is helpful.

It's a bit awkward that there are two min/max attributes. Are there devices where subgroupMin/MaxSize do not match min/maxExplicitComputeSubgroupSize?

On D3D12 we should use D3D12_FEATURE_DATA_D3D12_OPTIONS1::waveLaneCountMin and D3D12_FEATURE_DATA_D3D12_OPTIONS1::waveLaneCountMax to verify the range of the [WaveSize] attribute in compute shader, but these two values are not always used as wgpu::AdapterInfo::subgroupMinSize or wgpu::AdapterInfo::subgroupMaxSize.

For example:

On some Intel GPUs, it is possible to run some pixel shaders with wave lane count 8, while on that platform waveLaneCountMin is 16, meaning in compute shaders the wave lane count will always be at least 16 (A toggle "d3d12_relax_min_subgroup_size_to_8" has been added for this issue).

Now Dawn always uses 128 as SubgroupMaxSize because in D3D12 document "the WaveLaneCountMax queried from D3D12 API is not reliable and the meaning is unclear, while waveLaneCountMax is actually the maximum value that can be used as HLSL attribute [WaveSize].

alan-baker · 2026-03-01T20:02:40Z

spec/index.bs

+
+        Note: To preserve privacy, the user agent may choose to not support some features or provide values
+        for the property which do not distinguish different devices, but are still usable
+        (e.g. use the default value of 4 for all devices).


I think this wouldn't work. The user agent can't default to 4 for all devices unless 4 is supported by all devices.

Maybe it would be better to say the user agent may choose a narrower range for min (and max) to limit distinguishing devices.

Now I all use 0 as the default values since they won't be used when subgroup-size-control is not supported.

alan-baker · 2026-03-01T20:10:50Z

spec/index.bs

+
+    1. If {{GPUFeatureName/"subgroup-size-control"}} is supported, set
+        {{GPUAdapterInfo/maxComputeWorkgroupSubgroups}} to the largest supported number of
+        subgroups per compute workgroup with explicit subgroup size. Otherwise, set this value to 32.


Is 32 guaranteed supported on all devices or are implementations expected to filter out devices with less?

vulkan.gpuinfo.org seems to indicate there are devices that only support 16.

Now I all use 0 as the default values since they won't be used when subgroup-size-control is not supported.

wgsl/index.bs

jimblandy · 2026-03-10T18:04:51Z

Should this PR also update the correspondence reference?

Jiawei-Shao · 2026-03-12T03:17:38Z

Should this PR also update the correspondence reference?

Done

Jiawei-Shao · 2026-03-30T08:00:46Z

PTAL, thanks!

jimblandy · 2026-04-08T16:56:54Z

spec/index.bs

+    : <dfn>maxComputeWorkgroupSubgroups</dfn>
+    ::
+        If the {{GPUFeatureName/"subgroup-size-control"}} feature is supported, the maximum
+        number of subgroups per workgroup when [=attribute/subgroup_size=] is used in compute shaders
+        on the [=adapter=].
+


This isn't mentioned in the correspondence. Where does this limit come from?

jimblandy · 2026-04-08T16:59:34Z

spec/index.bs

+        Note: To preserve privacy, the user agent may choose a larger value for the property which do not
+        distinguish different devices, but are still usable.


Suggested change

Note: To preserve privacy, the user agent may choose a larger value for the property which do not

distinguish different devices, but are still usable.

Note: To preserve privacy, the user agent may choose a larger value for the property which does not

distinguish different devices, but is still usable.

jimblandy · 2026-04-08T17:00:03Z

spec/index.bs

+        Note: To preserve privacy, the user agent may choose a smaller value for the property which do not
+        distinguish different devices, but are still usable.


Suggested change

Note: To preserve privacy, the user agent may choose a smaller value for the property which do not

distinguish different devices, but are still usable.

Note: To preserve privacy, the user agent may choose a smaller value for the property which does not

distinguish different devices, but is still usable.

jimblandy · 2026-04-08T17:01:02Z

spec/index.bs


+    1. If {{GPUFeatureName/"subgroup-size-control"}} is supported, set
+        {{GPUAdapterInfo/explicitComputeSubgroupMinSize}} to the smallest supported explicit compute
+        subgroup size. Otherwise, set this value to 0.


This text should explicitly refer to the @subgroup_size attribute in WGSL. Similarly for the Max limit.

jimblandy · 2026-04-08T17:02:20Z

spec/index.bs

+        Note: To preserve privacy, the user agent may choose a smaller value for the property which do not
+        distinguish different devices, but are still usable.


Suggested change

Note: To preserve privacy, the user agent may choose a smaller value for the property which do not

distinguish different devices, but are still usable.

Note: To preserve privacy, the user agent may choose a smaller value for the property which does not

distinguish different devices, but is still usable.

jimblandy · 2026-04-08T17:07:00Z

spec/index.bs

+                            1. |entryPoint| |must| use &le;
+                                <code>|subgroupSize| &times; |this|.{{GPUDevice/adapterInfo}}.{{GPUAdapterInfo/maxComputeWorkgroupSubgroups}}</code>
+                                per workgroup.


I think there's a word missing from this sentence. What must be less than or equal to the given expression?

jimblandy · 2026-04-08T17:07:44Z

spec/index.bs

+<h3 id=dom-gpufeaturename-subgroup-size-control data-dfn-type=enum-value data-dfn-for=GPUFeatureName>`"subgroup-size-control"`
+</h3>
+
+Allows the use of [=attribute/subgroup_size=] attribute in WGSL to control over compute pipeline subgroup size.


Suggested change

Allows the use of [=attribute/subgroup_size=] attribute in WGSL to control over compute pipeline subgroup size.

Allows the use of the [=attribute/subgroup_size=] attribute in WGSL to control compute pipeline subgroup size.

jimblandy

My comments above were mostly editorial, but there was one spot that required a fix.

mwyrzykowski

In general the concept seems fine as an optional extension, as noted on Metal this property is read only and can not be configured as far as I know.

Jiawei-Shao added 7 commits February 27, 2026 14:38

Add extension subgroup-size-control

f17c4e8

This patch adds a new WebGPU and WGSL extension `subgroup-size-control` based on `proposals/subgroup-size-control.md`.

Move subgroup-size-control and subgroup_size to the last

a6d6e7a

Fix a typo

5f91fdd

Simply some statements

7837e6d

Fix build error

d38045f

More fixes

502846a

More fix

7aaed3a

Jiawei-Shao marked this pull request as draft February 27, 2026 07:19

Jiawei-Shao added 2 commits February 27, 2026 15:24

More fix

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More fix

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Jiawei-Shao marked this pull request as ready for review February 27, 2026 07:58

dneto0 requested review from dneto0 February 27, 2026 14:40

alan-baker requested changes Mar 1, 2026

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Jiawei-Shao marked this pull request as draft March 4, 2026 07:59

Address reviewer's comments

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Jiawei-Shao marked this pull request as ready for review March 4, 2026 08:02

jimblandy mentioned this pull request Mar 10, 2026

Subgroup Size Control #5545

Open

Jiawei-Shao added 2 commits March 12, 2026 09:44

Merge branch 'main' into add-subgroup-size-control

0d7ecc5

Update correspondence reference

a36b76f

Jiawei-Shao marked this pull request as draft March 12, 2026 03:06

Jiawei-Shao added 2 commits March 12, 2026 11:11

Fix build error

a78a7e3

More fix

a02d357

Jiawei-Shao marked this pull request as ready for review March 12, 2026 03:17

Jiawei-Shao added 2 commits March 17, 2026 10:06

Merge branch 'main' into add-subgroup-size-control

07a258e

Merge branch 'main' into add-subgroup-size-control

22e75bf

Jiawei-Shao mentioned this pull request Apr 7, 2026

[subgroup-size-control] New Device attributes explicitComputeSubgroupMinSize and explicitComputeSubgroupMaxSize #6241

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jimblandy reviewed Apr 8, 2026

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jimblandy requested changes Apr 8, 2026

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mwyrzykowski reviewed Apr 8, 2026

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		readonly attribute unsigned long minExplicitComputeSubgroupSize;
		readonly attribute unsigned long maxExplicitComputeSubgroupSize;

		Note: To preserve privacy, the user agent may choose a larger value for the property which do not
		distinguish different devices, but are still usable.

	Allows the use of [=attribute/subgroup_size=] attribute in WGSL to control over compute pipeline subgroup size.
	Allows the use of the [=attribute/subgroup_size=] attribute in WGSL to control compute pipeline subgroup size.

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jimblandy commented Mar 10, 2026

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