Remove usage of external flatc in builds and scripts (#9306)

### Summary

* Remove the dependence of `EXECUTORCH_BUILD_HOST_TARGETS` since we
always build flatc for the host now
* Turn on `EXECUTORCH_BUILD_FLATC` if  `FLATC_EXECUTABLE` is not set
* Build flatc from source for Apple builds

### Test Plan

Now macOS/Linux targeting macOS/Linux/iOS/Android should all build flatc
from source:

```
$ rm -rf pip-out && ./install_executorch.sh
$ rm -rf cmake-out* && ./build/build_apple_frameworks.sh
$ rm -rf cmake-out* && ./build/build_android_library.sh
```

cc @larryliu0820 @lucylq

Author: jathu

.ci/scripts/build_llama_android.sh

@@ -60,6 +60,5 @@ build_llama_runner() {
 
     cmake --build cmake-android-out/examples/models/llama -j4 --config Release
 }
-install_flatc_from_source
 install_executorch_and_backend_lib
 build_llama_runner

.ci/scripts/utils.sh

@@ -80,25 +80,6 @@ install_pytorch_and_domains() {
   sccache --show-stats || true
 }
 
-install_flatc_from_source() {
-  # NB: This function could be used to install flatbuffer from source
-  pushd third-party/flatbuffers || return
-
-  cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release
-  if [ "$(uname)" == "Darwin" ]; then
-    CMAKE_JOBS=$(( $(sysctl -n hw.ncpu) - 1 ))
-  else
-    CMAKE_JOBS=$(( $(nproc) - 1 ))
-  fi
-  cmake --build . -j "${CMAKE_JOBS}"
-
-  # Copy the flatc binary to conda path
-  EXEC_PATH=$(dirname "$(which python)")
-  cp flatc "${EXEC_PATH}"
-
-  popd || return
-}
-
 build_executorch_runner_buck2() {
   # Build executorch runtime with retry as this step is flaky on macos CI
   retry buck2 build //examples/portable/executor_runner:executor_runner

CMakeLists.txt

@@ -460,7 +460,7 @@ endif()
 # tools like `flatc`, along with example executables like `executor_runner` and
 # libraries that it uses, like `gflags`. Disabling this can be helpful when
 # cross-compiling, but some required tools that would have been built need to be
-# provided directly (via, for example, FLATC_EXECUTABLE).
+# provided directly.
 cmake_dependent_option(
   EXECUTORCH_BUILD_HOST_TARGETS "Build host-only targets." ON
   "NOT CMAKE_TOOLCHAIN_IOS" OFF
@@ -471,10 +471,9 @@ cmake_dependent_option(
 #
 cmake_dependent_option(
   EXECUTORCH_BUILD_FLATC "Build the flatc executable." ON
-  "NOT FLATC_EXECUTABLE;EXECUTORCH_BUILD_HOST_TARGETS" OFF
+  "NOT FLATC_EXECUTABLE" OFF
 )
 
-
 set(FLATBUFFERS_BUILD_FLATC OFF CACHE BOOL "")
 set(FLATBUFFERS_BUILD_FLATHASH OFF CACHE BOOL "")
 set(FLATBUFFERS_BUILD_FLATLIB OFF CACHE BOOL "")
@@ -507,6 +506,8 @@ if(EXECUTORCH_BUILD_FLATC)
                -DFLATBUFFERS_BUILD_TESTS=${FLATBUFFERS_BUILD_TESTS}
                -DFLATBUFFERS_INSTALL=${FLATBUFFERS_INSTALL}
                -DCMAKE_CXX_FLAGS="-DFLATBUFFERS_MAX_ALIGNMENT=${FLATBUFFERS_MAX_ALIGNMENT}"
+               # If building for iOS, "unset" these variables to rely on the host (macOS) defaults.
+               $<$<AND:$<BOOL:${CMAKE_TOOLCHAIN_IOS}>,$<BOOL:$<FILTER:${PLATFORM},EXCLUDE,^MAC>>>:-DCMAKE_OSX_SYSROOT=>
     INSTALL_COMMAND ""
     BUILD_BYPRODUCTS <BINARY_DIR>/flatc
   )
@@ -515,6 +516,8 @@ if(EXECUTORCH_BUILD_FLATC)
     # flatbuffers does not use CMAKE_BUILD_TYPE. Internally, the build forces Release
     # config, but from CMake's perspective the build type is always Debug.
     set(FLATC_EXECUTABLE ${BINARY_DIR}/$<CONFIG>/flatc.exe)
+  elseif(CMAKE_GENERATOR STREQUAL "Xcode")
+    set(FLATC_EXECUTABLE ${BINARY_DIR}/$<CONFIG>/flatc)
   else()
     set(FLATC_EXECUTABLE ${BINARY_DIR}/flatc)
   endif()
@@ -528,12 +531,7 @@ if(NOT FLATC_EXECUTABLE)
   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."
-    )
+    message(FATAL_ERROR "FLATC_EXECUTABLE must be set when EXECUTORCH_BUILD_FLATC is disabled.")
   endif()
 endif()
 

backends/apple/coreml/scripts/build_tests.sh

@@ -32,7 +32,6 @@ cmake "$EXECUTORCH_ROOT_PATH" -B"$CMAKE_EXECUTORCH_BUILD_DIR_PATH" \
 -DCMAKE_TOOLCHAIN_FILE="$IOS_TOOLCHAIN_PATH" \
 -DPLATFORM=MAC_UNIVERSAL \
 -DDEPLOYMENT_TARGET=13.0 \
--DFLATC_EXECUTABLE="$(which flatc)" \
 -DEXECUTORCH_BUILD_EXECUTOR_RUNNER=OFF \
 -DEXECUTORCH_BUILD_XNNPACK=OFF \
 -DEXECUTORCH_BUILD_GFLAGS=OFF

backends/arm/scripts/build_executorch.sh

@@ -84,7 +84,6 @@ if [ "$build_with_etdump" = true ] ; then
         -DEXECUTORCH_ENABLE_EVENT_TRACER=ON               \
         -DEXECUTORCH_SEPARATE_FLATCC_HOST_PROJECT=ON      \
         -DFLATCC_ALLOW_WERROR=OFF                         \
-        -DFLATC_EXECUTABLE="$(which flatc)"               \
         -B"${et_build_host_dir}"                          \
         "${et_root_dir}"
 
@@ -133,7 +132,6 @@ cmake                                                 \
     -DEXECUTORCH_ENABLE_LOGGING=ON                    \
     ${build_devtools_flags}                           \
     ${build_with_etdump_flags}                        \
-    -DFLATC_EXECUTABLE="$(which flatc)"               \
     -B"${et_build_dir}"                               \
     "${et_root_dir}"
 

backends/cadence/build_cadence_fusionG3.sh

@@ -32,7 +32,6 @@ if $STEPWISE_BUILD; then
         -DEXECUTORCH_ENABLE_LOGGING=ON \
         -DEXECUTORCH_USE_DL=OFF \
         -DEXECUTORCH_BUILD_CADENCE=OFF \
-        -DFLATC_EXECUTABLE="$(which flatc)" \
         -DHAVE_FNMATCH_H=OFF \
         -Bcmake-out .
 
@@ -45,7 +44,6 @@ if $STEPWISE_BUILD; then
         -DEXECUTORCH_BUILD_EXECUTOR_RUNNER=ON \
         -DEXECUTORCH_BUILD_PTHREADPOOL=OFF \
         -DEXECUTORCH_BUILD_CADENCE=ON \
-        -DFLATC_EXECUTABLE="$(which flatc)" \
         -DEXECUTORCH_ENABLE_LOGGING=ON \
         -DEXECUTORCH_ENABLE_PROGRAM_VERIFICATION=ON \
         -DEXECUTORCH_USE_DL=OFF \
@@ -71,9 +69,7 @@ else
         -DEXECUTORCH_BUILD_EXECUTOR_RUNNER=ON \
         -DEXECUTORCH_BUILD_PTHREADPOOL=OFF \
         -DEXECUTORCH_BUILD_CPUINFO=OFF \
-        -DEXECUTORCH_BUILD_FLATC=OFF \
         -DEXECUTORCH_BUILD_CADENCE=ON \
-        -DFLATC_EXECUTABLE="$(which flatc)" \
         -DEXECUTORCH_BUILD_EXTENSION_RUNNER_UTIL=ON \
         -DEXECUTORCH_ENABLE_LOGGING=ON \
         -DEXECUTORCH_ENABLE_PROGRAM_VERIFICATION=ON \

backends/cadence/build_cadence_hifi4.sh

@@ -32,7 +32,6 @@ if $STEPWISE_BUILD; then
         -DEXECUTORCH_ENABLE_LOGGING=ON \
         -DEXECUTORCH_USE_DL=OFF \
         -DEXECUTORCH_BUILD_CADENCE=OFF \
-        -DFLATC_EXECUTABLE="$(which flatc)" \
         -Bcmake-out .
 
     echo "Building any Cadence-specific binaries on top"
@@ -44,7 +43,6 @@ if $STEPWISE_BUILD; then
         -DEXECUTORCH_BUILD_EXECUTOR_RUNNER=ON \
         -DEXECUTORCH_BUILD_PTHREADPOOL=OFF \
         -DEXECUTORCH_BUILD_CADENCE=ON \
-        -DFLATC_EXECUTABLE="$(which flatc)" \
         -DEXECUTORCH_ENABLE_LOGGING=ON \
         -DEXECUTORCH_ENABLE_PROGRAM_VERIFICATION=ON \
         -DEXECUTORCH_USE_DL=OFF \
@@ -69,9 +67,7 @@ else
         -DEXECUTORCH_BUILD_EXECUTOR_RUNNER=ON \
         -DEXECUTORCH_BUILD_PTHREADPOOL=OFF \
         -DEXECUTORCH_BUILD_CPUINFO=OFF \
-        -DEXECUTORCH_BUILD_FLATC=OFF \
         -DEXECUTORCH_BUILD_CADENCE=ON \
-        -DFLATC_EXECUTABLE="$(which flatc)" \
         -DEXECUTORCH_BUILD_EXTENSION_RUNNER_UTIL=ON \
         -DEXECUTORCH_ENABLE_LOGGING=ON \
         -DEXECUTORCH_ENABLE_PROGRAM_VERIFICATION=ON \

build/build_android_library.sh

@@ -12,7 +12,6 @@ OUTPUT="cmake-out"
 MODES=()
 TOOLCHAIN=""
 PYTHON=$(which python3)
-FLATC=$(which flatc)
 COREML=OFF
 CUSTOM=OFF
 MPS=OFF
@@ -83,7 +82,6 @@ usage() {
   echo "  --Release            Build Release version."
   echo "  --toolchain=FILE     CMake toolchain file. Default: '\$SOURCE_ROOT_DIR/third-party/ios-cmake/ios.toolchain.cmake'"
   echo "  --python=FILE        Python executable path. Default: Path of python3 in \$PATH"
-  echo "  --flatc=FILE         FlatBuffers Compiler executable path. Default: Path of flatc in \$PATH"
   echo "  --coreml             Build the Core ML backend."
   echo "  --custom             Build the Custom kernels."
   echo "  --mps                Build the Metal Performance Shaders backend."
@@ -113,7 +111,6 @@ for arg in "$@"; do
         ;;
       --toolchain=*) TOOLCHAIN="${arg#*=}" ;;
       --python=*) PYTHON="${arg#*=}" ;;
-      --flatc=*) FLATC="${arg#*=}" ;;
       --coreml) COREML=ON ;;
       --custom) CUSTOM=ON ;;
       --mps) MPS=ON ;;
@@ -168,7 +165,6 @@ check_command() {
 check_command cmake
 check_command rsync
 check_command "$PYTHON"
-check_command "$FLATC"
 check_command "$BUCK2"
 
 echo "Building libraries"
@@ -190,7 +186,6 @@ cmake_build() {
         -DCMAKE_C_FLAGS="-ffile-prefix-map=$SOURCE_ROOT_DIR=/executorch -fdebug-prefix-map=$SOURCE_ROOT_DIR=/executorch" \
         -DCMAKE_CXX_FLAGS="-ffile-prefix-map=$SOURCE_ROOT_DIR=/executorch -fdebug-prefix-map=$SOURCE_ROOT_DIR=/executorch" \
         -DPYTHON_EXECUTABLE="$PYTHON" \
-        -DFLATC_EXECUTABLE="$FLATC" \
         -DEXECUTORCH_BUILD_COREML=$COREML \
         -DEXECUTORCH_BUILD_MPS=$MPS \
         -DEXECUTORCH_BUILD_XNNPACK=$XNNPACK \