[ExecuTorch][WebGPU] Request shader-f16 device feature (enablement, fail-open)

backends/webgpu/runtime/WebGPUDevice.cpp

@@ -13,9 +13,7 @@
 #include <cstdlib>
 #include <memory>
 #include <stdexcept>
-#ifdef WGPU_BACKEND_ENABLE_PROFILING
 #include <vector>
-#endif // WGPU_BACKEND_ENABLE_PROFILING
 
 namespace executorch {
 namespace backends {
@@ -143,16 +141,28 @@ WebGPUContext create_webgpu_context() {
     device_desc.requiredLimits = &supported_limits;
   }
 
+  // Request optional features when the adapter advertises them. A feature the
+  // adapter lacks is skipped (its fast path stays disabled). A feature the
+  // adapter advertises becomes required of the device, so if the device were
+  // to reject it, context creation fails below rather than falling back. The
+  // vector must outlive wgpuAdapterRequestDevice below (device_desc points
+  // into it).
+  std::vector<WGPUFeatureName> required_features;
+  if (wgpuAdapterHasFeature(ctx.adapter, WGPUFeatureName_ShaderF16)) {
+    required_features.push_back(WGPUFeatureName_ShaderF16);
+    ctx.shader_f16_supported = true;
+  }
 #ifdef WGPU_BACKEND_ENABLE_PROFILING
   // Bench: enable TimestampQuery if available; fail-open (skip timing if not).
-  std::vector<WGPUFeatureName> required_features;
   if (wgpuAdapterHasFeature(ctx.adapter, WGPUFeatureName_TimestampQuery)) {
     required_features.push_back(WGPUFeatureName_TimestampQuery);
-    device_desc.requiredFeatureCount = required_features.size();
-    device_desc.requiredFeatures = required_features.data();
     ctx.timestamp_supported = true;
   }
 #endif // WGPU_BACKEND_ENABLE_PROFILING
+  if (!required_features.empty()) {
+    device_desc.requiredFeatureCount = required_features.size();
+    device_desc.requiredFeatures = required_features.data();
+  }
 
   device_desc.uncapturedErrorCallbackInfo.callback = on_device_error;
 

backends/webgpu/runtime/WebGPUDevice.h

@@ -25,6 +25,9 @@ struct WebGPUContext {
   WGPUAdapter adapter = nullptr;
   WGPUDevice device = nullptr;
   WGPUQueue queue = nullptr;
+  // True if the device was created with the ShaderF16 feature; reserved for a
+  // future fp16 storage/compute path (fp32 is used when false or unset).
+  bool shader_f16_supported = false;
 #ifdef WGPU_BACKEND_ENABLE_PROFILING
   // True if the device was created with the TimestampQuery feature (bench).
   bool timestamp_supported = false;