[feature request] Cross compiling support

[beakthoven]

It would be great to be able to cross-compile the toolchain for a specific platform, rather than having to run the script directly on that platform.

[beakthoven]

I guess things are simple when building llvm with clang and lld since we could add a check to see if the given compiler supports the cross compile target and we need to enable the cross compiling flag in llvm configuration.

For other combinations and compilation of binutils which both use gcc or bfd/gold linkers we need to check for the availability of the prefixed gcc and binutils. (An extension can be added to readme's dependency section for the same)

[beakthoven]

I guess things are simple when building llvm with clang and lld since we could add a check to see if the given compiler supports the cross compile target and we need to enable the cross compiling flag in llvm configuration.

For other combinations and compilation of binutils which both use gcc or bfd/gold linkers we need to check for the availability of the prefixed gcc and binutils. (An extension can be added to readme's dependency section for the same)

Actually... We could instead repurpose the stage1 build and enable the required cross compile target and build it then we can use stage1 binaries to compile the final cross compiled stage.
This way we can use both gcc and clang for cross compilation of llvm and we don't need any extra dependency.

I'm still unsure about cross compilation of binutils.

[nathanchance]

Right, I would definitely suggest only implementing cross compiling via a two stage build so that clang is being used for cross compiling. I think trying to make the logic work otherwise would be way too complicated. PGO would probably not work as well in that scenario (if at all) either.

LLVM has some official documentation on this: https://llvm.org/docs/HowToCrossCompileLLVM.html

I probably will not be able to work on this for some time but I would be happy to review and test patches. This could likely be done purely within build-llvm.py if it is just cmake switches.

[beakthoven]

Right, I would definitely suggest only implementing cross compiling via a two stage build so that clang is being used for cross compiling. I think trying to make the logic work otherwise would be way too complicated. PGO would probably not work as well in that scenario (if at all) either.

LLVM has some official documentation on this: https://llvm.org/docs/HowToCrossCompileLLVM.html

I probably will not be able to work on this for some time but I would be happy to review and test patches. This could likely be done purely within build-llvm.py if it is just cmake switches.

Sounds good to me! I might try working on it in my spare time. I'd appreciate your help with testing and reviewing when I have something ready.

[nathanchance]

This documentation got rewritten recently to be more clear, which may help with wiring this up.

llvm/llvm-project@ba11e1e