cmake: build shared libexecutorch_portable_ops when EXECUTORCH_BUILD_SHARED=ON

Create a single libexecutorch_portable_ops.so that bundles
portable_ops_lib and portable_kernels, allowing consumers to link
portable op implementations via a shared library.

portable_ops_lib already has --whole-archive as an INTERFACE link
option from Codegen.cmake, so we do not wrap it in
$<LINK_LIBRARY:WHOLE_ARCHIVE,...> to avoid pulling the registration
constructor in twice. Link against executorch_shared (not
executorch_core) to share a single operator registry.

Co-authored-by: Claude <noreply@anthropic.com>

Author: tomeuv

kernels/portable/CMakeLists.txt

@@ -106,6 +106,12 @@ install(
     DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/executorch/kernels/portable/
 )
 
+if(EXECUTORCH_BUILD_SHARED)
+  executorch_add_shared_library(
+    executorch_portable_ops portable_ops_lib portable_kernels executorch_shared
+  )
+endif()
+
 # Build the portable custom ops AOT library for registering custom ops into
 # PyTorch. Requires find_package(Torch), which must be called at root scope
 # before this subdirectory is processed. Not targeting ARM_BAREMETAL as aot_lib

tools/cmake/Utils.cmake

@@ -211,3 +211,27 @@ function(executorch_target_copy_mlx_metallib target)
     endif()
   endif()
 endfunction()
+
+# Create and install a shared library composed from dependency libraries. The
+# target links the provided dependencies and carries VERSION/SOVERSION.
+function(executorch_add_shared_library target_name)
+  set(_empty_source_name "${target_name}_empty.cpp")
+  file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/${_empty_source_name}
+       "// intentionally empty"
+  )
+  add_library(
+    ${target_name} SHARED ${CMAKE_CURRENT_BINARY_DIR}/${_empty_source_name}
+  )
+  target_link_libraries(${target_name} PRIVATE ${ARGN})
+  set_target_properties(
+    ${target_name}
+    PROPERTIES VERSION "${PROJECT_VERSION}"
+               SOVERSION "${PROJECT_VERSION_MAJOR}"
+               LINKER_LANGUAGE CXX
+  )
+  install(
+    TARGETS ${target_name}
+    EXPORT ExecuTorchTargets
+    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+  )
+endfunction()