Cross-compilation issue with projects that use RocksDB built using Conan or Cmake on macOS with x86_64 CPU and targeted for arm64 CPUs about rocksdb

[adamretter]

We currently support cross-compilation on macOS using Make. See the target rocksdbjavastaticosx_ub in Makefile.

We might consider adding support in CMake. We don't use Conan. Can you provide the exact CMake command that you ran that will allow us to reproduce your problem please?

[shpala]

@adamretter The code seems to successfully cross-compile when using Make, but fails when using Cmake.
The command I'm using is:
cmake -G "Unix Makefiles" -DCMAKE_SYSTEM_PROCESSOR=arm64 ..
The main CMakeLists.txt checks this variable in order to check for whether the compiler supports the "-march=armv8-a+crc+crypto" flag and consequently set the HAS_ARMV8_CRC variable.
Without setting HAS_ARMV8_CRC, the generated makefile won't compile the required crc32c_arm64.cc .

The reason this fails is that on macOS Cmake, calls to project() in CMakeLists.txt resets CMAKE_SYSTEM_PROCESSOR to the host CPU architecture.
For this reason it's more common to use CMAKE_OSX_ARCHITECTURES on macOS systems.
The main drawback of this, is that CMAKE_OSX_ARCHITECTURES might be a list of architectures, which we really don't want to support.

In the PR I provided, I modified CMakeLists.txt to check:
Only when running on Apple systems, if CMAKE_SYSTEM_PROCESSOR is different from CMAKE_OSX_ARCHITECTURES,
then set CMAKE_SYSTEM_PROCESSOR to be the same as CMAKE_OSX_ARCHITECTURES.
The build will return an error in cases when CMAKE_OSX_ARCHITECTURES will contain more than one value