cmake: add consolidated shared library and pkg-config support (EXECUTORCH_BUILD_SHARED=ON)

When EXECUTORCH_BUILD_SHARED=ON is passed, all static libraries are built
with PIC and a new executorch_shared target bundles executorch_core plus
commonly-used extensions (data_loader, flat_tensor, named_data_map,
module, tensor) into a single libexecutorch.so via whole-archive linking.

A pkg-config file is also generated and installed for consumer
discovery.

When EXECUTORCH_BUILD_SHARED is OFF (the default), behavior is unchanged.

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

Author: tomeuv

CMakeLists.txt

@@ -46,7 +46,6 @@
 #
 
 cmake_minimum_required(VERSION 3.24)
-project(executorch)
 
 set(EXECUTORCH_ROOT ${CMAKE_CURRENT_SOURCE_DIR})
 
@@ -87,6 +86,10 @@ if("${ET_VERSION_MAJOR}" STREQUAL ""
   )
 endif()
 
+project(executorch
+        VERSION "${ET_VERSION_MAJOR}.${ET_VERSION_MINOR}.${ET_VERSION_PATCH}"
+)
+
 message(
   STATUS
     "ExecuTorch version: ${ET_VERSION_MAJOR}.${ET_VERSION_MINOR}.${ET_VERSION_PATCH}"
@@ -160,6 +163,10 @@ announce_configured_options(BUILD_TESTING)
 load_build_preset()
 include(${PROJECT_SOURCE_DIR}/tools/cmake/preset/default.cmake)
 
+if(EXECUTORCH_BUILD_SHARED)
+  set(CMAKE_POSITION_INDEPENDENT_CODE ON)
+endif()
+
 # Enable ccache if available
 find_program(CCACHE_PROGRAM ccache)
 if(CCACHE_PROGRAM)
@@ -1158,6 +1165,65 @@ if(EXECUTORCH_BUILD_VGF)
   list(APPEND _executorch_backends vgf_backend)
 endif()
 
+# Consolidated shared library: bundles executorch_core plus commonly used
+# extensions into a single libexecutorch.so.
+if(EXECUTORCH_BUILD_SHARED)
+  # CMake requires at least one source for shared library targets.
+  file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/executorch_shared_empty.cpp
+       "// intentionally empty"
+  )
+  add_library(
+    executorch_shared SHARED
+    ${CMAKE_CURRENT_BINARY_DIR}/executorch_shared_empty.cpp
+  )
+  set_target_properties(
+    executorch_shared
+    PROPERTIES OUTPUT_NAME executorch
+               EXPORT_NAME executorch
+               VERSION "${PROJECT_VERSION}"
+               SOVERSION "${PROJECT_VERSION_MAJOR}"
+               LINKER_LANGUAGE CXX
+  )
+  target_include_directories(
+    executorch_shared PUBLIC ${_common_include_directories}
+  )
+  target_compile_definitions(
+    executorch_shared PUBLIC C10_USING_CUSTOM_GENERATED_MACROS
+  )
+  # Link executorch without WHOLE_ARCHIVE because its INTERFACE link options
+  # (from executorch_target_link_options_shared_lib) already force
+  # whole-archive. Link executorch_core explicitly since executorch only has a
+  # PRIVATE dep on it (symbols wouldn't propagate otherwise).
+  target_link_libraries(
+    executorch_shared PRIVATE executorch
+                              $<LINK_LIBRARY:WHOLE_ARCHIVE,executorch_core>
+  )
+  foreach(_ext_target
+          extension_data_loader extension_flat_tensor extension_named_data_map
+          extension_module_static extension_tensor
+  )
+    if(TARGET ${_ext_target})
+      target_link_libraries(
+        executorch_shared PRIVATE $<LINK_LIBRARY:WHOLE_ARCHIVE,${_ext_target}>
+      )
+    endif()
+  endforeach()
+  install(
+    TARGETS executorch_shared
+    EXPORT ExecuTorchTargets
+    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
+    RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
+    ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}
+  )
+  configure_file(
+    tools/cmake/executorch.pc.in ${CMAKE_CURRENT_BINARY_DIR}/executorch.pc
+    @ONLY
+  )
+  install(FILES ${CMAKE_CURRENT_BINARY_DIR}/executorch.pc
+          DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig
+  )
+endif()
+
 # Top-level interface targets.
 
 # A target containing all configured backends.

tools/cmake/executorch.pc.in

@@ -0,0 +1,10 @@
+prefix=@CMAKE_INSTALL_PREFIX@
+exec_prefix=${prefix}
+libdir=${prefix}/@CMAKE_INSTALL_LIBDIR@
+includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
+
+Name: ExecuTorch
+Description: On-device AI framework for PyTorch models
+Version: @PROJECT_VERSION@
+Cflags: -I${includedir} -I${includedir}/executorch/runtime/core/portable_type -DC10_USING_CUSTOM_GENERATED_MACROS
+Libs: -L${libdir} -lexecutorch

tools/cmake/preset/default.cmake

@@ -218,6 +218,10 @@ define_overridable_option(
   EXECUTORCH_BUILD_CPUINFO "Build cpuinfo library." BOOL
   ${_default_executorch_build_cpuinfo}
 )
+define_overridable_option(
+  EXECUTORCH_BUILD_SHARED
+  "Build a consolidated ExecuTorch shared library" BOOL OFF
+)
 
 # Threadpool size options. At most one can be specified. Note that the default
 # is managed in threadpool.cpp to allow the user to specify an alternate mode