[ExecuTorch][WebGPU] Use requiredFeatures instance API on native + emscripten Dawn

[JulianCloudNTH]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][WebGPU] Convert remaining native tests to GTest #20706
• -> [ExecuTorch][WebGPU] Use requiredFeatures instance API on native + emscripten Dawn #20652
• [ExecuTorch][WebGPU] 2D-fold mul + permute dispatch (lift 65535 1D cap) #20651
• [ExecuTorch][WebGPU] 2D compute dispatch tests — prefill golden + fold unit test #20584
• [ExecuTorch][WebGPU] 2D compute dispatch — lift the 65535 per-dim cap (prefill path) #20583
• [ExecuTorch][WebGPU] Dynamic-shape integration test (allocate-at-max + per-op resize) #20582
• [ExecuTorch][WebGPU] Dynamic resize hook for slice_copy (dynamic start/end) #20581
• [ExecuTorch][WebGPU] Dynamic resize hook for SDPA (live seq-len S) #20580
• [ExecuTorch][WebGPU] Dynamic resize hook for view_copy #20579
• [ExecuTorch][WebGPU] Dynamic resize hooks for sigmoid and select_copy #20578
• [ExecuTorch][WebGPU] Dynamic resize hooks for add and mul #20577
• [ExecuTorch][WebGPU] Dynamic resize hook for linear_q4gsw #20576
• [ExecuTorch][WebGPU] Dynamic resize hooks for rms_norm, embedding, rope #20575
• [ExecuTorch][WebGPU] Dynamic tensor-shape resize engine core #20574
• [ExecuTorch][WebGPU] SymInt arithmetic ops (add/sub/mul/floordiv) for dynamic shapes #20573

Key the timedWaitAny instance setup to the actual Dawn API instead of __EMSCRIPTEN__, so native-rig Dawn and emscripten/emdawnwebgpu use the modern requiredFeatures path and only the vendored Dawn uses the legacy capabilities.* path.

The instance-descriptor setup was guarded by #if defined(__EMSCRIPTEN__), which routed emscripten (emdawnwebgpu, emcc 4.0.19+) through the legacy capabilities.* API that no longer exists there. The guard now keys off the API actually present.

Key changes:

• WebGPUDevice.cpp — #if defined(__EMSCRIPTEN__) → #if defined(WEBGPU_DAWN_INSTANCE_CAPABILITIES). The legacy instance_desc.capabilities.* path is taken only by the buck-vendored Dawn (which defines the macro); native cmake Dawn and emscripten leave it undefined and take the requiredFeatures / WGPUInstanceFeatureName_TimedWaitAny path.

Co-authored-with: Claude Code.
@exported-using-ghexport

Differential Revision: D110149678

Differential Revision: D110149678

[pytorch-bot]

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❌ 1 New Failure

As of commit c8dad50 with merge base 124b677 ():

NEW FAILURE - The following job has failed:

• pull / test-binary-size-linux-gcc / linux-job (gh)
  RuntimeError: Command docker exec -t 800d539295355c49853020872f98ddf40bb105b173bb3a45131640f325708004 /exec failed with exit code 1

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Review automatically exported from Phabricator review in Meta.

[JulianCloudNTH]

@claude Review thoroughly. Check for stale code / bugs.

[claude]

Claude finished @JulianCloudNTH's task in 1m 54s —— View job

---

Review: WebGPU requiredFeatures instance API

• Read PR diff
• Analyze WebGPUDevice.cpp for stale code / bugs
• Check where WEBGPU_DAWN_INSTANCE_CAPABILITIES is defined
• Post review

Verdict: the change is correct and does exactly what the description claims. No bugs or stale code in the diff. A couple of notes below.

---

The change is sound

The old guard #if defined(__EMSCRIPTEN__) conflated "is this emscripten?" with "does this Dawn expose the legacy capabilities.* API?". Since emcc 4.0.19+ / emdawnwebgpu switched to requiredFeatures, emscripten was being routed through an API that no longer exists there. Keying off WEBGPU_DAWN_INSTANCE_CAPABILITIES (defined only by the buck-vendored Dawn) is the right fix — native cmake Dawn and emscripten both fall through to the requiredFeatures / WGPUInstanceFeatureName_TimedWaitAny path.

No lifetime bug in the #else branch

WGPUInstanceFeatureName features[1] = {WGPUInstanceFeatureName_TimedWaitAny};
instance_desc.requiredFeatureCount = 1;
instance_desc.requiredFeatures = features;
...
ctx.instance = wgpuCreateInstance(&instance_desc);   // features[] still in scope

instance_desc.requiredFeatures is a borrowed pointer, but wgpuCreateInstance consumes it synchronously on the very next line while features is still alive. No dangling pointer. ✅

Notes / things to confirm (not blocking)

1. WEBGPU_DAWN_INSTANCE_CAPABILITIES is not defined anywhere in the OSS tree. I grepped the whole repo — the macro only appears at the #if site. That is expected for OSS (native cmake + emscripten want the requiredFeatures path, so the macro should stay undefined). The definition lives in the internal buck build. Worth double-checking that the buck-vendored Dawn target still -D's this macro; if that define is ever dropped, buck-vendored Dawn would silently take the requiredFeatures branch and fail to compile against its older capabilities.* header. Since the OSS side can't verify it, a one-line comment near the macro's buck definition pointing back here would help future readers.

2. The other __EMSCRIPTEN__ guard (line 188) is correctly left alone. get_default_webgpu_context()'s #if !defined(__EMSCRIPTEN__) gates the native-only lazy process-wide context — that's genuinely about platform (browser has no eager device bring-up), not about the Dawn API version, so it should not be converted. Good that it wasn't touched.

3. Comment accuracy — the inline comment ("Vendored (buck) Dawn uses the older capabilities.* API…") is accurate and matches the guard. Nice to have the rationale in-tree.

Nothing to change. LGTM. 👍
· branch gh/JulianCloudNTH/84/head