Build flatc for the host instead of the target platform

Author: jathu

CMakeLists.txt

@@ -482,34 +482,29 @@ if(EXECUTORCH_BUILD_FLATC)
       FATAL_ERROR "May not set both EXECUTORCH_BUILD_FLATC and FLATC_EXECUTABLE"
     )
   endif()
-  set(FLATC_EXECUTABLE flatc)
-  set(FLATBUFFERS_BUILD_FLATC
-      ON
-      CACHE BOOL ""
-  )
-  set(FLATBUFFERS_BUILD_FLATHASH
-      OFF
-      CACHE BOOL ""
-  )
-  set(FLATBUFFERS_BUILD_FLATLIB
-      OFF
-      CACHE BOOL ""
-  )
-  set(FLATBUFFERS_BUILD_TESTS
-      OFF
-      CACHE BOOL ""
-  )
-  set(FLATBUFFERS_INSTALL
-      OFF
-      CACHE BOOL ""
+
+  include(ExternalProject)
+
+  ExternalProject_Add(
+    host_flatc
+    PREFIX ${CMAKE_CURRENT_BINARY_DIR}/third-party/flatbuffers
+    SOURCE_DIR ${CMAKE_SOURCE_DIR}/third-party/flatbuffers
+    CMAKE_ARGS -DFLATBUFFERS_BUILD_FLATC=ON
+               -DFLATBUFFERS_BUILD_FLATHASH=OFF
+               -DFLATBUFFERS_BUILD_FLATLIB=OFF
+               -DFLATBUFFERS_BUILD_TESTS=OFF
+               -DFLATBUFFERS_INSTALL=OFF
+               # exir lets users set the alignment of tensor data embedded in
+               # the flatbuffer, and some users need an alignment larger than
+               # the default, which is typically 32.
+               -DCMAKE_CXX_FLAGS=-DFLATBUFFERS_MAX_ALIGNMENT=1024
+    INSTALL_COMMAND ""
   )
-  add_subdirectory(third-party/flatbuffers)
+  ExternalProject_Get_Property(host_flatc BINARY_DIR)
+  set(FLATC_EXECUTABLE ${BINARY_DIR}/flatc)
 
-  # exir lets users set the alignment of tensor data embedded in the flatbuffer,
-  # and some users need an alignment larger than the default, which is typically
-  # 32.
-  target_compile_definitions(flatc PRIVATE FLATBUFFERS_MAX_ALIGNMENT=1024)
 endif()
+
 if(NOT FLATC_EXECUTABLE)
   message(
     FATAL_ERROR

backends/apple/mps/CMakeLists.txt

@@ -50,6 +50,7 @@ add_custom_command(
     "${_mps_schema__include_dir}/executorch/backends/apple/mps"
     ${_mps_schema__srcs}
   WORKING_DIRECTORY ${EXECUTORCH_ROOT}
+  DEPENDS host_flatc
   COMMENT "Generating mps_schema headers"
   VERBATIM
 )

backends/qualcomm/CMakeLists.txt

@@ -49,6 +49,7 @@ add_custom_command(
   OUTPUT qcir_schema_output
   COMMAND ${FLATC_EXECUTABLE} --cpp --cpp-std c++11 --scoped-enums -o
           ${qcir_schema_include_dir} ${qcir_schema_include_dir}/qcir.fbs
+  DEPENDS host_flatc
   COMMENT "Generating qualcomm ir schema headers"
   VERBATIM
 )

backends/vulkan/CMakeLists.txt

@@ -92,6 +92,7 @@ add_custom_command(
     ${FLATC_EXECUTABLE} --cpp --cpp-std c++11 --scoped-enums -o
     "${SCHEMA_INCLUDE_DIR}/executorch/backends/vulkan/serialization/" ${_vulkan_schema__srcs}
   WORKING_DIRECTORY ${EXECUTORCH_ROOT}
+  DEPENDS host_flatc
   COMMENT "Generating vulkan_schema headers"
   VERBATIM
 )

backends/xnnpack/CMakeLists.txt

@@ -82,6 +82,7 @@ add_custom_command(
     ${_xnnpack_schema__srcs}
   COMMAND mv ${_xnnpack_flatbuffer__outputs} ${_xnnpack_schema__outputs}
   WORKING_DIRECTORY ${EXECUTORCH_ROOT}
+  DEPENDS host_flatc
   COMMENT "Generating xnnpack_schema headers"
   VERBATIM
 )

devtools/CMakeLists.txt

@@ -205,7 +205,7 @@ add_custom_command(
     "${_bundled_schema__include_dir}/executorch/devtools/bundled_program/schema"
     ${_bundled_program_schema__srcs}
   WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/devtools
-  DEPENDS ${FLATC_EXECUTABLE} ${_bundled_program_schema__srcs}
+  DEPENDS host_flatc ${_bundled_program_schema__srcs}
   COMMENT "Generating bundled_program headers"
   VERBATIM
 )

extension/flat_tensor/serialize/CMakeLists.txt

@@ -37,7 +37,7 @@ function(generate_flat_tensor_schema _schema_srcs _schema_name)
       ${FLATC_EXECUTABLE} --cpp --cpp-std c++11 --gen-mutable --scoped-enums -o
       "${_flat_tensor_schema__output_dir}" ${_schema_srcs}
     WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
-    DEPENDS ${FLATC_EXECUTABLE} ${_schema_srcs}
+    DEPENDS host_flatc ${_schema_srcs}
     COMMENT "Generating ${_schema_name} headers"
     VERBATIM
   )

schema/CMakeLists.txt

@@ -37,7 +37,7 @@ function(generate_program_schema _schema_srcs _schema_name)
       ${FLATC_EXECUTABLE} --cpp --cpp-std c++11 --gen-mutable --scoped-enums -o
       "${_program_schema__output_dir}" ${_schema_srcs}
     WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
-    DEPENDS ${FLATC_EXECUTABLE} ${_schema_srcs}
+    DEPENDS host_flatc ${_schema_srcs}
     COMMENT "Generating ${_schema_name} headers"
     VERBATIM
   )