GCC 8 Cross Compiler outputs ARMv7 executable instead of ARMv6

Question

I'm trying to compile a C++ application for a Raspberry Pi Zero using GCC 8.2.1 .
I'm using this for a relatively large C++17 project that is being built using CMake, and I'm trying to cross-compile it on my x86-64 laptop.

Even with the simplest code possible, I'm not able to compile it for ARMv6:

int main() {}

$ arm-linux-gnueabihf-g++ test.cpp -static -march=armv6 -mfpu=vfp -mfloat-abi=hard

When running the file on the Pi, I get an Illegal instruction error, and readelf returns the following:

$ arm-linux-gnueabihf-readelf -A a.out
Attribute Section: aeabi
File Attributes
  Tag_CPU_name: "7-A"
  Tag_CPU_arch: v7
  Tag_CPU_arch_profile: Application
  Tag_ARM_ISA_use: Yes
  Tag_THUMB_ISA_use: Thumb-2
  Tag_FP_arch: VFPv3
  Tag_Advanced_SIMD_arch: NEONv1
  Tag_ABI_PCS_wchar_t: 4
  Tag_ABI_FP_rounding: Needed
  Tag_ABI_FP_denormal: Needed
  Tag_ABI_FP_exceptions: Needed
  Tag_ABI_FP_number_model: IEEE 754
  Tag_ABI_align_needed: 8-byte
  Tag_ABI_align_preserved: 8-byte, except leaf SP
  Tag_ABI_enum_size: int
  Tag_ABI_VFP_args: VFP registers
  Tag_CPU_unaligned_access: v6

GCC seems to ignore my architecture flags.

When simply compiling it into an object file, it seems to work just fine, but the linking stage always uses ARMv7:

$ arm-linux-gnueabihf-g++ test.cpp -static -march=armv6 -mfpu=vfp -mfloat-abi=hard -c
$ arm-linux-gnueabihf-readelf -A test.o
Attribute Section: aeabi
File Attributes
  Tag_CPU_name: "6"
  Tag_CPU_arch: v6
  Tag_ARM_ISA_use: Yes
  Tag_THUMB_ISA_use: Thumb-1
  Tag_FP_arch: VFPv2
  Tag_ABI_PCS_wchar_t: 4
  Tag_ABI_FP_denormal: Needed
  Tag_ABI_FP_exceptions: Needed
  Tag_ABI_FP_number_model: IEEE 754
  Tag_ABI_align_needed: 8-byte
  Tag_ABI_align_preserved: 8-byte, except leaf SP
  Tag_ABI_enum_size: int
  Tag_ABI_VFP_args: VFP registers
  Tag_ABI_optimization_goals: Aggressive Debug
  Tag_CPU_unaligned_access: v6

What am I doing wrong?

Answer 1

I ended up compiling GCC from source, following this post . I didn't need all of the steps (I compiled everything using GCC 8 instead of compiling GCC 6.3 first, and I didn't edit any source files.)

I posted a Dockerfile with all build steps onGitHub .

The architecture of the executables produced is correct now, but I can't test it on target yet to check if it actually runs.

Answer 2

By default, newer GCC versions do not create correct binaries for ARMv6. Even though you pass the correct -mcpu= flag to gcc, it will create startup code for the newer ARMv7 architecture. Running them on your RasPI Zero will cause an "Illegal Instruction" exception.

this info is mentioned here https://github.com/Pro/raspi-toolchain