gcc-7: error: unrecognized command line option ‘-m64’

Question

I'm trying to compile C code on a Jetson Nano and I get this error during compiling. I tried removing any occurrence of 'm -64' but it seems like its added automatically. This is the cmd where it fails: /usr/bin/gcc-7 -Wall -Wextra -Wconversion -pedantic -Wshadow -m64 -Wfatal-errors -O0 -g -o CMakeFiles/dir/testCCompiler.c.o -c /home/user/dir/CMakeFiles/CMakeTmp/testCCompiler.c

uname -a: Linux jetson-nano 4.9.140-tegra aarch64 aarch64 aarch64 GNU/Linux
gcc-7 -v: Using built-in specs.
COLLECT_GCC=gcc-7
COLLECT_LTO_WRAPPER=/usr/lib/gcc/aarch64-linux-gnu/7/lto-wrapper
Target: aarch64-linux-gnu
gcc version 7.4.0 (Ubuntu/Linaro 7.4.0-1ubuntu1~18.04.1)

CMAKE_C_COMPILER = gcc-7
CMAKE_CXX_COMPILER = g++-7
CXX_COMPILE_FLAGS =  "-Wall -Werror -Wextra -Wnon-virtual-dtor -Wconversion -Wold-style-cast -pedantic -Wshadow"
C_COMPILE_FLAGS = "-Wall -Wextra -Wconversion -pedantic -Wshadow"

gcc-7: error: unrecognized command line option ‘-m64’

Solution

error: unrecognized command line option ‘-m64’

I believe you are looking for -march=armv8-a (and friends), and not -m64. The GCC arm64 options are available at 3.18.1 AArch64 Options in the manual.

Aarch64 includes ASIMD in the base specification, so there are no extra gyrations needed for it. ASIMD is "Advanced SIMD instructions", and it is what ARM calls NEON on the Aarch32 and Aarch64 architectures.

If you want to enable extensions, like CRC or Crypto, then the option would look like -march=armv8.1-a+crc or -march=armv8.1-a+crypto or -march=armv8.1-a+crc+crypto.

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The equivalent x86 options would be the following. Obviously, the ARM port of GCC does not use the same model as x86. It is confusing for new users (or it was confusing for me).

• -march=armv8-a → -msse2
• -march=armv8.1-a+crc → -msse2 -msse4.1
• -march=armv8.1-a+crypto → -msse2 -mpclmul -maes
• -march=armv8.1-a+crc+crypto → -msse2 -msse4.1 -mpclmul -maes

ARM instruction set includes SHA in crypto, so the x86 options should probably include -msha. The problem is, x86 SHA did not arrive until about 8 years after carryless multiplies and AES.

Also, GCC ARM compilers usually don't understand -march=native. On older GCC compilers, the compiler will just crash. On mid-ranged GCC it is simply ignored. I believe the latest GCC compilers honor it.

Comments

[Jen]

thanks for your explanation! my problem is that i didn't put -m64 anywhere, where would I need to configure -march=armv8-a to make it work?

[jww]

They are compiler options. Compiler options go in CPPFLAGS, CFLAGS and CXXFLAGS. I believe you would use C_COMPILE_FLAGS and CXX_COMPILE_FLAGS for Cmake.

[Jen]

I added it to the flags but no luck. Is there any possibility that it's configured somewhere else or would it have to be in the code somewhere?