Build flatc for the host instead of the target platform (#9077)

### Summary

* Fixes #7260
* We use flatc as part of the build step to generate some files from
FlatBuffer definitions. This implies the tool runs on the host. However,
we currently build flatc as part of the main build — which propagates
the target CXX flags to flatc. Thus, flatc gets built targeting the
target platform. To ensure flatc is built for the host, we can use
include flatc as an
[ExternalProject](https://cmake.org/cmake/help/latest/module/ExternalProject.html).
This does not propagate the CXX flags
* Some targets implicitly depended on flatc, we now make that
requirement explicit
* We currently spread the `FLATC_EXECUTABLE` defaulting across the
project. Let's just centralize this at the root

### Test plan

```bash
$ ./install_executorch.sh

# Previously flatc was built against the Android target, now they target the host
$ ./build/build_android_library.sh
$ file /Users/jathu/executorch/cmake-out-android-arm64-v8a/third-party/flatbuffers/flatc
/Users/jathu/executorch/cmake-out-android-arm64-v8a/third-party/flatbuffers/flatc: Mach-O 64-bit executable arm64

# Apple builds work as usual, but they use flatc from pip
$ ./build/build_apple_frameworks.sh
```

cc @larryliu0820 @lucylq @swolchok @dbort

Author: jathu

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",