Build flatc for the host instead of the target platform

CMakeLists.txt

@@ -474,6 +474,17 @@ cmake_dependent_option(
   "NOT FLATC_EXECUTABLE;EXECUTORCH_BUILD_HOST_TARGETS" OFF
 )
 
+
+set(FLATBUFFERS_BUILD_FLATC OFF 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 "")
+# 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.
+set(FLATBUFFERS_MAX_ALIGNMENT 1024)
+
 if(EXECUTORCH_BUILD_FLATC)
   if(FLATC_EXECUTABLE)
     # We could ignore this, but it could lead to confusion about which `flatc`
@@ -482,41 +493,49 @@ 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 ""
-  )
-  add_subdirectory(third-party/flatbuffers)
 
-  # 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)
+  # Build flatc for the *host* to generate files as part of the build step.
+  include(ExternalProject)
+  ExternalProject_Add(
+    flatbuffers
+    PREFIX ${CMAKE_CURRENT_BINARY_DIR}/third-party/flatbuffers
+    BINARY_DIR ${CMAKE_CURRENT_BINARY_DIR}/third-party/flatbuffers
+    SOURCE_DIR ${CMAKE_CURRENT_SOURCE_DIR}/third-party/flatbuffers
+    CMAKE_ARGS -DFLATBUFFERS_BUILD_FLATC=ON
+               -DFLATBUFFERS_BUILD_FLATHASH=${FLATBUFFERS_BUILD_FLATHASH}
+               -DFLATBUFFERS_BUILD_FLATLIB=${FLATBUFFERS_BUILD_FLATLIB}
+               -DFLATBUFFERS_BUILD_TESTS=${FLATBUFFERS_BUILD_TESTS}
+               -DFLATBUFFERS_INSTALL=${FLATBUFFERS_INSTALL}
+               -DCMAKE_BUILD_TYPE=Release
+               -DCMAKE_CXX_FLAGS="-DFLATBUFFERS_MAX_ALIGNMENT=${FLATBUFFERS_MAX_ALIGNMENT}"
+    INSTALL_COMMAND ""
+  )
+  ExternalProject_Get_Property(flatbuffers BINARY_DIR)
+  set(FLATC_EXECUTABLE ${BINARY_DIR}/flatc)
+  set(FLATC_EXECUTABLE_BUILT_FROM_SOURCE YES)
 endif()
+
 if(NOT FLATC_EXECUTABLE)
   message(
-    FATAL_ERROR
-      "FLATC_EXECUTABLE must be set when EXECUTORCH_BUILD_FLATC is disabled. "
-      "Note that EXECUTORCH_BUILD_FLATC may be disabled implicitly when "
-      "cross-compiling or when EXECUTORCH_BUILD_HOST_TARGETS is disabled."
+    WARNING "FLATC_EXECUTABLE not specified, looking for flatc"
   )
+  find_program(FLATC_EXECUTABLE flatc)
+
+  if(NOT FLATC_EXECUTABLE)
+    message(
+      FATAL_ERROR
+        "FLATC_EXECUTABLE must be set when EXECUTORCH_BUILD_FLATC is disabled. "
+        "Note that EXECUTORCH_BUILD_FLATC may be disabled implicitly when "
+        "cross-compiling or when EXECUTORCH_BUILD_HOST_TARGETS is disabled."
+    )
+  endif()
+endif()
+
+add_executable(flatc IMPORTED GLOBAL)
+set_target_properties(flatc PROPERTIES IMPORTED_LOCATION ${FLATC_EXECUTABLE})
+
+if(FLATC_EXECUTABLE_BUILT_FROM_SOURCE)
+  add_dependencies(flatc flatbuffers)
 endif()
 
 #

backends/apple/mps/CMakeLists.txt

@@ -22,10 +22,6 @@ if(NOT PYTHON_EXECUTABLE)
   resolve_python_executable()
 endif()
 
-if(NOT FLATC_EXECUTABLE)
-  set(FLATC_EXECUTABLE flatc)
-endif()
-
 set(_common_compile_options -Wno-deprecated-declarations)
 set(_common_include_directories ${EXECUTORCH_ROOT}/..)
 
@@ -50,6 +46,7 @@ add_custom_command(
     "${_mps_schema__include_dir}/executorch/backends/apple/mps"
     ${_mps_schema__srcs}
   WORKING_DIRECTORY ${EXECUTORCH_ROOT}
+  DEPENDS flatc
   COMMENT "Generating mps_schema headers"
   VERBATIM
 )

backends/qualcomm/CMakeLists.txt

@@ -39,16 +39,13 @@ if(${ANDROID})
   find_library(android_log log)
 endif()
 
-if(NOT FLATC_EXECUTABLE)
-  set(FLATC_EXECUTABLE flatc)
-endif()
-
 set(qcir_schema_include_dir ${CMAKE_CURRENT_LIST_DIR}/aot/ir)
 set(qcir_schema_output ${qcir_schema_include_dir}/qcir_generated.h)
 add_custom_command(
   OUTPUT qcir_schema_output
   COMMAND ${FLATC_EXECUTABLE} --cpp --cpp-std c++11 --scoped-enums -o
           ${qcir_schema_include_dir} ${qcir_schema_include_dir}/qcir.fbs
+  DEPENDS flatc
   COMMENT "Generating qualcomm ir schema headers"
   VERBATIM
 )
@@ -100,6 +97,7 @@ add_custom_command(
     "${_qnn_schema__include_dir}/executorch/backends/qualcomm"
     ${_qnn_schema__srcs}
   WORKING_DIRECTORY ${EXECUTORCH_SOURCE_DIR}
+  DEPENDS flatc
   COMMENT "Generating qnn_schema headers"
   VERBATIM
 )

backends/vulkan/CMakeLists.txt

@@ -28,10 +28,6 @@ if(NOT PYTHON_EXECUTABLE)
   set(PYTHON_EXECUTABLE python3)
 endif()
 
-if(NOT FLATC_EXECUTABLE)
-  set(FLATC_EXECUTABLE flatc)
-endif()
-
 # Include this file to access target_link_options_shared_lib This is required to
 # provide access to target_link_options_shared_lib which allows libraries to be
 # linked with the --whole-archive flag. This is required for libraries that
@@ -92,6 +88,7 @@ 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 flatc
   COMMENT "Generating vulkan_schema headers"
   VERBATIM
 )

backends/xnnpack/CMakeLists.txt

@@ -18,10 +18,6 @@ if(NOT CMAKE_CXX_STANDARD)
   set(CMAKE_CXX_STANDARD 17)
 endif()
 
-if(NOT FLATC_EXECUTABLE)
-  set(FLATC_EXECUTABLE flatc)
-endif()
-
 # Source root directory for executorch.
 if(NOT EXECUTORCH_ROOT)
   set(EXECUTORCH_ROOT ${CMAKE_CURRENT_SOURCE_DIR}/../..)
@@ -82,6 +78,7 @@ add_custom_command(
     ${_xnnpack_schema__srcs}
   COMMAND mv ${_xnnpack_flatbuffer__outputs} ${_xnnpack_schema__outputs}
   WORKING_DIRECTORY ${EXECUTORCH_ROOT}
+  DEPENDS flatc
   COMMENT "Generating xnnpack_schema headers"
   VERBATIM
 )

devtools/CMakeLists.txt

@@ -38,10 +38,6 @@ if(NOT PYTHON_EXECUTABLE)
   resolve_python_executable()
 endif()
 
-if(NOT FLATC_EXECUTABLE)
-  set(FLATC_EXECUTABLE flatc)
-endif()
-
 # Paths to headers generated from the .fbs files. set(_etdump_schemas
 # etdump_schema_flatcc.fbs scalar_type.fbs)
 
@@ -205,7 +201,7 @@ 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 flatc ${_bundled_program_schema__srcs}
   COMMENT "Generating bundled_program headers"
   VERBATIM
 )

examples/apple/mps/CMakeLists.txt

@@ -18,10 +18,6 @@ if(NOT CMAKE_CXX_STANDARD)
   set(CMAKE_CXX_STANDARD 17)
 endif()
 
-if(NOT FLATC_EXECUTABLE)
-  set(FLATC_EXECUTABLE flatc)
-endif()
-
 # Source root directory for executorch.
 if(NOT EXECUTORCH_ROOT)
   set(EXECUTORCH_ROOT ${CMAKE_CURRENT_SOURCE_DIR}/../../..)

extension/flat_tensor/serialize/CMakeLists.txt

@@ -9,10 +9,6 @@
 # cmake-format -i CMakeLists.txt
 # ~~~
 
-if(NOT FLATC_EXECUTABLE)
-  set(FLATC_EXECUTABLE flatc)
-endif()
-
 # The include directory that will contain the generated schema headers.
 set(_flat_tensor_schema__include_dir "${CMAKE_BINARY_DIR}/extension/flat_tensor/include")
 set(_flat_tensor_schema__output_dir "${_flat_tensor_schema__include_dir}/executorch/extension/flat_tensor/serialize")
@@ -37,7 +33,7 @@ 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 flatc ${_schema_srcs}
     COMMENT "Generating ${_schema_name} headers"
     VERBATIM
   )
@@ -49,7 +45,7 @@ function(generate_flat_tensor_schema _schema_srcs _schema_name)
   # and some users need an alignment larger than the default, which is typically
   # 32.
   target_compile_definitions(
-    ${_schema_name} INTERFACE FLATBUFFERS_MAX_ALIGNMENT=1024
+    ${_schema_name} INTERFACE FLATBUFFERS_MAX_ALIGNMENT=${FLATBUFFERS_MAX_ALIGNMENT}
   )
 
   target_include_directories(

schema/CMakeLists.txt

@@ -9,10 +9,6 @@
 # cmake-format -i CMakeLists.txt
 # ~~~
 
-if(NOT FLATC_EXECUTABLE)
-  set(FLATC_EXECUTABLE flatc)
-endif()
-
 # The include directory that will contain the generated schema headers.
 set(_program_schema__include_dir "${CMAKE_BINARY_DIR}/schema/include")
 set(_program_schema__output_dir "${_program_schema__include_dir}/executorch/schema")
@@ -37,7 +33,7 @@ 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 flatc ${_schema_srcs}
     COMMENT "Generating ${_schema_name} headers"
     VERBATIM
   )
@@ -49,7 +45,7 @@ function(generate_program_schema _schema_srcs _schema_name)
   # and some users need an alignment larger than the default, which is typically
   # 32.
   target_compile_definitions(
-    ${_schema_name} INTERFACE FLATBUFFERS_MAX_ALIGNMENT=1024
+    ${_schema_name} INTERFACE FLATBUFFERS_MAX_ALIGNMENT=${FLATBUFFERS_MAX_ALIGNMENT}
   )
 
   target_include_directories(

setup.py

@@ -652,10 +652,6 @@ def run(self):
 
         build_args = [f"-j{self.parallel}"]
 
-        # TODO(dbort): Try to manage these targets and the cmake args from the
-        # extension entries themselves instead of hard-coding them here.
-        build_args += ["--target", "flatc"]
-
         if ShouldBuild.pybindings():
             cmake_args += [
                 "-DEXECUTORCH_BUILD_PYBIND=ON",