LLVM build options for ARM / RaspberryPi

Question

I'm seeking clarification of what the best build options are for LLVM 3.2 on Raspbian

I've fetched the LLVM source of 3.2, and my config is:

cd llvm-3.2.src
./configure --build=armv6-unknown-linux-gnueabi \
--host=armv6-unknown-linux-gnueabi \
--target=armv6-unknown-linux-gnueabi --with-cpu=arm1176jzf-s \
--with-float=hard --with-abi=aapcs-vfp --with-fpu=neon \
--enable-targets=arm --enable-optimized --enable-assertions
make REQUIRES_RTTI=1
sudo make install

I've heard it takes quite a number of hours to build. It's been running a few hours now, happy to just let it run though.

It's a long term build project so I have no qualms about redoing the build if needed :)

Answer 1

I'd recommend cross-compiling LLVM for Raspbian. Building it on Pi itself takes ages.

First, get a working root fs using debootstrap (alternatively, just clone your existing Raspbian installation or unpack a downloaded image). You may need to replace some absolute symlinks with relative, watch out for build errors.

Make sure you have a working and recent enough installation of clang (don't forget to build it with ARM target support).

Then, cross-compile LLVM with the following CMake toolchain definition (you should not chroot into your root fs at this stage!):

set(toolchain_dir /path/to/your/chroot-raspbian-armhf/ )
set(toolchain_bin_dir ${toolchain_dir}/usr/bin)
set(toolchain_inc_dir ${toolchain_dir}/usr/include) # was /include
set(toolchain_lib_dir ${toolchain_dir}/usr/lib)

set(CMAKE_SYSTEM_NAME Linux CACHE INTERNAL "system name")
set(CMAKE_SYSTEM_PROCESSOR arm CACHE INTERNAL "processor")
set(CMAKE_C_COMPILER clang)
set(CMAKE_CXX_COMPILER clang++)
set(CMAKE_C_FLAGS "-O2 -integrated-as -target armv6-linux-gnueabihf -mfloat-abi=hard --sysroot=${toolchain_dir}" CACHE INTERNAL "c compiler flags")
set(CMAKE_CXX_FLAGS "-O2 -integrated-as -target armv6-linux-gnueabihf -mfloat-abi=hard --sysroot=${toolchain_dir}" CACHE INTERNAL "cxx compiler flags")

set(link_flags "-L${toolchain_lib_dir} -ldl")

set(CMAKE_EXE_LINKER_FLAGS ${link_flags} CACHE INTERNAL "exe link flags")
set(CMAKE_MODULE_LINKER_FLAGS ${link_flags} CACHE INTERNAL "module link flags")
set(CMAKE_SHARED_LINKER_FLAGS ${link_flags} CACHE INTERNAL "shared link flags")
set(CMAKE_FIND_ROOT_PATH ${toolchain_lib_dir} CACHE INTERNAL "cross root directory")
set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM BOTH CACHE INTERNAL "")
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY CACHE INTERNAL "")
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY CACHE INTERNAL "")

(use cmake -DCMAKE_TOOLCHAIN_FILE=/path/to/the/file/above.txt)

Please note that target should be armv6-linux-gnueabihf, not armv6-linux-gnueabi.