choosenim breaks user code on M1 Mac

[mode80]

choosenim seems to break packages and user code when installing Nim on an M1 mac (Apple silicone, ARM64) .

The issue seems to be that choosenim compiles the nim compiler as an x86 executable. Checking this, I get:

$ file `which nim`
/Users/mode80/.nimble/bin/nim: Mach-O 64-bit executable x86_64

When packages and user code make use of code like when defined(arm64),
it incorrectly reports false, and instead reports true for defined(amd64). The program usually crashes thereafter because whatever code path was needed for M1 doesn't run. (For example, the threading module using processor-specific assembly instructions based on this kind of check. )

When I install nim with homebrew, and test, I get

$ file `which nim`
/opt/homebrew/bin/nim: Mach-O 64-bit executable arm64

Then things work as expected.

[quantimnot]

I just wasted a lot of time on this myself.

Here is what I learned:

• The default arch on macOS is that of the calling process.
• Choosenim doesn't have an arm64 build.
• The choosenim proxy (~/.nimble/bin/nim) calling an arm64 nim compiler, will target x86_64.
• hostCPU is not determined at runtime, but is inherited from the bootstrapping compiler.
• Passing --cpu:arm64 doesn't help.
• x86_64 emulation can be detected at runtime
• Adding this configuration to the global nim.cfg will make this work for clang and ld:

  @if macosx:
    clang.options.always %= "${clang.options.always} -arch arm64"
    clang.options.linker %= "${clang.options.linker} -arch arm64"
  @end

See Also

https://developer.apple.com/documentation/apple-silicon/addressing-architectural-differences-in-your-macos-code