reproduce (this is a W=1 build):
        # https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git/commit/?id=a86e2a52063aaf13992ddd8ba68a42798d6d1faf
        git remote add ebiggers https://git.kernel.org/pub/scm/linux/kernel/git/ebiggers/linux.git
        git fetch --no-tags ebiggers wip-rm-ldmxcsr
        git checkout a86e2a52063aaf13992ddd8ba68a42798d6d1faf
        # apt-get install sparse
        # sparse: v0.6.5-rc1

        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=um olddefconfig
        make W=1 C=1 CF='-fdiagnostic-prefix -D__CHECK_ENDIAN__ -fmax-errors=unlimited -fmax-warnings=unlimited' O=build_dir ARCH=um SHELL=/bin/bash lib/raid/xor/