Cross-architecture compilation on Linux

[Finagolfin]

@taylorswift

i did not know what to supply for the -tools-directory

That flag is only required for Android, a way to use their patched clang when linking, so it shouldn't be needed on linux.

@taylorswift

also did not know where to obtain the glibc sysroot you mentioned. so i tried installing gcc-aarch64-linux-gnu and using the following JSON

The glibc sysroot for AArch64 should be automatically installed with that gcc-aarch64 package, else you would not be able to use it, so it's probably one of its dependencies.

Your JSON config file looks fine, with the exception of the sdk. The Swift compiler expects that path to have two directories, <sdk>/usr/include/, containing the glibc headers, and <sdk>/usr/lib/, containing the glibc library and object files.

Ubuntu probably uses the same headers in /usr/include/ across all arches it supports, so you want the sdk to be /, then it depends where your gcc-aarch toolchain places the AArch64 libc.so.

Try cross-compiling hello world in C using that gcc-aarch64 toolchain with the -v verbose flag, then look for which libc.so it links against. If it's linking against one in /usr/lib64/aarch64-linux-gnu/ instead, add "-L", "/usr/lib64/aarch64-linux-gnu/" to your extra-swiftc-flags, just as I do for Android with a similar non-standard cross-compilation layout.

@taylorswift

i tried to compile swift-nio-ssl, which did not work

That package is a bit more complicated, as it also links against libstdc++, which most Swift packages don't. I suggest you try a package like swift-argument-parser instead, as shown in my Android doc, and only try a more complicated package like NIO SSL once that is working.

Don't forget to add the right symlink in the Swift resource directory, as mentioned in my linked post. The four -Xlinker -rpath flags should not be necessary if you are then running the resulting binary in a linux AArch64 environment.

I suggest you add the --build-tests flag to build the Swift package's test runner, then copy it over to linux AArch and run it, exactly as I document doing so for Android AArch64 in my doc.

@taylorswift

i came across Cross-compiling packages which contain macros doesn't work · Issue #6950 · apple/swift-package-manager · GitHub which suggests projects that use macros do not support cross-compilation on 5.10. while i didn’t get far enough to try on my own, that’s obviously a showstopper because the application i am trying to build uses macros

That issue was closed this week, so the next trunk snapshot toolchain may work for this.

@taylorswift

as for compiling in the cloud, i soon found that this is only really viable for debug builds. compiling an application of sufficient complexity in release mode uses too much memory and i was not able to get anywhere close to a successful release build, at least on a t4g.small instance.

T4g.large instances appear to be available for $20/month: that may be your only option to build Swift for linux AArch64 now, if you don't have your own local AArch64 hardware.