Cross-compiling from aarch64 to x86_64 fails (unrecognized command-line option '-m64)

[erikschul]

Setup code
The runner should ideally setup correctly on either platform, and compile to either target.
Using debian12, running in docker.
Using cross is not an option (docker-in-docker etc.).

apt-get install -y gcc make \
    gcc-aarch64-linux-gnu \
    gcc-i686-linux-gnu \
    gcc-arm-linux-gnueabi \
    binutils \
    binutils-aarch64-linux-gnu \
    g++-aarch64-linux-gnu \
    g++-x86-64-linux-gnu \
    libc6-dev-arm64-cross

rustup update
rustup target add x86_64-unknown-linux-gnu
rustup target add aarch64-unknown-linux-gnu

Error

cargo build --target=x86_64-unknown-linux-gnu

fails with

cc: error: unrecognized command-line option '-m64'

which makes sense if it's using cc, which only supports these targets:
cc -E -march=help -xc /dev/null

# 0 "/dev/null"
cc1: error: unknown value ‘help’ for ‘-march’
cc1: note: valid arguments are: armv8-a armv8.1-a armv8.2-a armv8.3-a armv8.4-a armv8.5-a armv8.6-a armv8.7-a armv8.8-a armv8-r armv9-a native

So I tried installing x86-64 gcc and setting it:

export CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_LINKER=/usr/bin/aarch64-linux-gnu-gcc

but its valid targets are:

nocona core2 nehalem corei7 westmere sandybridge corei7-avx ivybridge core-avx-i haswell core-avx2 broadwell skylake skylake-avx512 cannonlake icelake-client rocketlake icelake-server cascadelake tigerlake cooperlake sapphirerapids alderlake bonnell atom silvermont slm goldmont goldmont-plus tremont knl knm x86-64 x86-64-v2 x86-64-v3 x86-64-v4 eden-x2 nano nano-1000 nano-2000 nano-3000 nano-x2 eden-x4 nano-x4 k8 k8-sse3 opteron opteron-sse3 athlon64 athlon64-sse3 athlon-fx amdfam10 barcelona bdver1 bdver2 bdver3 bdver4 znver1 znver2 znver3 btver1 btver2

so I need to change the target to -mx86-64.

I tried overriding with RUSTFLAGS but that didn't work.

Problem

• how can I override the -m target?
• is there a better way?

Alternatives
It works with zigbuild: (I haven't tried to run the x86_64 binary)

cargo install cargo-zigbuild
cargo zigbuild --target aarch64-unknown-linux-gnu.2.17
cargo zigbuild --target x86_64-unknown-linux-gnu.2.17

[adamgemmell]

"g++-x86-64-linux-gnu" is the cross-compiler to x86_64 - so you want export CARGO_TARGET_X86_64_UNKNOWN_LINUX_GNU_LINKER=/usr/bin/x86_64-linux-gnu-gcc (rather than aarch64-linux-gnu-gcc)

This is enough to get cargo build --target=x86_64-unknown-linux-gnu working.

However something's not quite adding up since https://packages.debian.org/stable/devel/g++-12-aarch64-linux-gnu isn't even available for aarch64 since it's a cross compiler TO aarch64. Are you definitely running docker on an aarch64 host?

[erikschul]

@adamgemmell Thanks Adam, that seems to work!
I'm surprised that 64 is not in the targets list (/usr/bin/x86_64-linux-gnu-gcc -E -march=help -xc /dev/null), so I assumed it was an invalid argument.

It seems you're right about the package; apt performs a substitution:

apt-get install -y g++-aarch64-linux-gnu
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Note, selecting 'g++' instead of 'g++-aarch64-linux-gnu'
The following additional packages will be installed:
  binutils binutils-aarch64-linux-gnu binutils-common cpp cpp-12 fontconfig-config fonts-dejavu-core g++-12 gcc gcc-12 ...

I feel it would make sense to make cross-compilation easier, by auto-detecting cross-platform, using relevant search paths, and auto-detecting missing dependencies? But that's a separate issue for the cargo project.

I will close this issue.

[adamgemmell]

@erikschul -m64 is a separate command line option from -march.

The former enables code generation for x86-64, while the latter chooses a version of x86-64 to target. -mcpu is even more specific, letting you choose a certain CPU to optimise for.

I think you're also mixing up the flags - -m isn't a flag, the m is part of the name of the flag. Treat it as a prefix to signify something about the flag (maybe it stands for "machine"?)