Flatc should be built for the host rather than the target when cross-compiling

[fiberflow]

🐛 Describe the bug

executorch/CMakeLists.txt

Line 478 in a6841d5

add_subdirectory(third-party/flatbuffers)

It could be fixed with something like this.

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 46fec4cf0..62dfa6e62 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -475,12 +475,24 @@ if(EXECUTORCH_BUILD_FLATC)
       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)
+  if(CMAKE_CROSSCOMPILING)
+    set(FLATC_EXECUTABLE  ${CMAKE_BINARY_DIR}/_host_build_flatbuffers/flatc)
+    execute_process(
+      COMMAND
+        ${CMAKE_COMMAND} ${CMAKE_CURRENT_SOURCE_DIR}/third-party/flatbuffers -DCMAKE_C_FLAGS=-DFLATBUFFERS_MAX_ALIGNMENT=1024 -B${CMAKE_BINARY_DIR}/_host_build_flatbuffers
+    )
+    execute_process(
+      COMMAND ${CMAKE_COMMAND} --build ${CMAKE_BINARY_DIR}/_host_build_flatbuffers -j
+    )
+  else()
+    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)
+  endif()
 endif()
 if(NOT FLATC_EXECUTABLE)
   message(

Versions

Collecting environment information...
PyTorch version: 2.6.0.dev20241112+cpu
Is debug build: False
CUDA used to build PyTorch: Could not collect
ROCM used to build PyTorch: N/A

OS: Ubuntu 24.04.1 LTS (x86_64)
GCC version: (Ubuntu 13.2.0-23ubuntu4) 13.2.0
Clang version: Could not collect
CMake version: version 3.31.1
Libc version: glibc-2.39

Python version: 3.12.3 (main, Nov  6 2024, 18:32:19) [GCC 13.2.0] (64-bit runtime)
Python platform: Linux-6.8.0-48-generic-x86_64-with-glibc2.39
Is CUDA available: False
CUDA runtime version: 11.5.119
CUDA_MODULE_LOADING set to: N/A
GPU models and configuration: Could not collect
Nvidia driver version: Could not collect
cuDNN version: /usr/lib/x86_64-linux-gnu/libcudnn.so.7.5.0
HIP runtime version: N/A
MIOpen runtime version: N/A
Is XNNPACK available: True

CPU:
Architecture:                         x86_64
CPU op-mode(s):                       32-bit, 64-bit
Address sizes:                        43 bits physical, 48 bits virtual
Byte Order:                           Little Endian
CPU(s):                               12
On-line CPU(s) list:                  0-11
Vendor ID:                            AuthenticAMD
Model name:                           AMD Ryzen 5 1600 Six-Core Processor
CPU family:                           23
Model:                                1
Thread(s) per core:                   2
Core(s) per socket:                   6
Socket(s):                            1
Stepping:                             1
Frequency boost:                      enabled
CPU(s) scaling MHz:                   51%
CPU max MHz:                          3200.0000
CPU min MHz:                          1550.0000
BogoMIPS:                             6388.26
Flags:                                fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm constant_tsc rep_good nopl nonstop_tsc cpuid extd_apicid aperfmperf rapl pni pclmulqdq monitor ssse3 fma cx16 sse4_1 sse4_2 movbe popcnt aes xsave avx f16c rdrand lahf_lm cmp_legacy extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw skinit wdt tce topoext perfctr_core perfctr_nb bpext perfctr_llc mwaitx cpb hw_pstate ssbd vmmcall fsgsbase bmi1 avx2 smep bmi2 rdseed adx smap clflushopt sha_ni xsaveopt xsavec xgetbv1 clzero irperf xsaveerptr arat npt lbrv svm_lock nrip_save tsc_scale vmcb_clean flushbyasid decodeassists pausefilter pfthreshold avic v_vmsave_vmload vgif overflow_recov succor smca sev
L1d cache:                            192 KiB (6 instances)
L1i cache:                            384 KiB (6 instances)
L2 cache:                             3 MiB (6 instances)
L3 cache:                             16 MiB (2 instances)
NUMA node(s):                         1
NUMA node0 CPU(s):                    0-11
Vulnerability Gather data sampling:   Not affected
Vulnerability Itlb multihit:          Not affected
Vulnerability L1tf:                   Not affected
Vulnerability Mds:                    Not affected
Vulnerability Meltdown:               Not affected
Vulnerability Mmio stale data:        Not affected
Vulnerability Reg file data sampling: Not affected
Vulnerability Retbleed:               Mitigation; untrained return thunk; SMT vulnerable
Vulnerability Spec rstack overflow:   Vulnerable: Safe RET, no microcode
Vulnerability Spec store bypass:      Mitigation; Speculative Store Bypass disabled via prctl
Vulnerability Spectre v1:             Mitigation; usercopy/swapgs barriers and __user pointer sanitization
Vulnerability Spectre v2:             Mitigation; Retpolines; STIBP disabled; RSB filling; PBRSB-eIBRS Not affected; BHI Not affected
Vulnerability Srbds:                  Not affected
Vulnerability Tsx async abort:        Not affected

Versions of relevant libraries:
[pip3] numpy==2.2.0
[pip3] torch==2.6.0.dev20241112+cpu
[pip3] torchaudio==2.5.0.dev20241112+cpu
[pip3] torchsr==1.0.4
[pip3] torchvision==0.20.0.dev20241112+cpu
[conda] numpy                     1.21.6                   pypi_0    pypi
[conda] torch                     1.12.1                   pypi_0    pypi
[conda] torchvision               0.9.0                    pypi_0    pypi