[ExecuTorch][WebGPU] Select emdawnwebgpu for Emscripten builds

backends/webgpu/CMakeLists.txt

@@ -61,22 +61,27 @@ target_include_directories(
 
 target_link_libraries(webgpu_backend PRIVATE vulkan_schema executorch_core)
 
-# Native WebGPU backend: Dawn (Tint) + SwiftShader; deps script sets Dawn_DIR.
-# Native-only: browser/Emscripten builds use the system webgpu.h and never reach
-# this find_package (root CMake gates it via EXECUTORCH_BUILD_WEBGPU).
-# dawn::webgpu_dawn's link interface references Threads::Threads.
-find_package(Threads REQUIRED)
-find_package(Dawn REQUIRED)
-set(WEBGPU_GPU_LIB dawn::webgpu_dawn)
-target_link_libraries(webgpu_backend PUBLIC ${WEBGPU_GPU_LIB})
-
-if(APPLE)
-  target_link_libraries(
-    webgpu_backend PRIVATE "-framework Metal" "-framework QuartzCore"
-                           "-framework CoreGraphics" "-framework Foundation"
-  )
+# WASM gets its WebGPU implementation from emdawnwebgpu at executable link time.
+# Native builds link Dawn (Tint) and the platform GPU libraries.
+if(EMSCRIPTEN)
+  target_compile_options(webgpu_backend PUBLIC "--use-port=emdawnwebgpu")
+  # --use-port is also required at link time: the link step pulls in the port's
+  # headers, JS glue, and libraries for consumers that link webgpu_backend.
+  target_link_options(webgpu_backend PUBLIC "--use-port=emdawnwebgpu")
 else()
-  target_link_libraries(webgpu_backend PRIVATE dl m pthread)
+  find_package(Threads REQUIRED)
+  find_package(Dawn REQUIRED)
+  set(WEBGPU_GPU_LIB dawn::webgpu_dawn)
+  target_link_libraries(webgpu_backend PUBLIC ${WEBGPU_GPU_LIB})
+
+  if(APPLE)
+    target_link_libraries(
+      webgpu_backend PRIVATE "-framework Metal" "-framework QuartzCore"
+                             "-framework CoreGraphics" "-framework Foundation"
+    )
+  else()
+    target_link_libraries(webgpu_backend PRIVATE dl m pthread)
+  endif()
 endif()
 
 target_compile_options(webgpu_backend PRIVATE -fexceptions)

backends/webgpu/test/test_cmake_configuration.py

@@ -0,0 +1,25 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+import pathlib
+import unittest
+
+
+class TestCMakeConfiguration(unittest.TestCase):
+    def test_emscripten_uses_port_instead_of_native_dawn(self) -> None:
+        cmake = pathlib.Path(__file__).parents[1] / "CMakeLists.txt"
+        source = cmake.read_text()
+        wasm_branch = """if(EMSCRIPTEN)
+  target_compile_options(webgpu_backend PUBLIC \"--use-port=emdawnwebgpu\")
+  target_link_options(webgpu_backend PUBLIC \"--use-port=emdawnwebgpu\")
+else()"""
+
+        self.assertIn(wasm_branch, source)
+        # Dawn is linked only in the native (else) branch, never under EMSCRIPTEN.
+        # Split on the branch head so the assertion does not depend on which
+        # endif() (the nested APPLE one vs the outer one) appears first.
+        native_branch = source.split(wasm_branch, 1)[1]
+        self.assertIn("find_package(Dawn REQUIRED)", native_branch)