[swift-dev] Cross-compiling with Toolchain files

[Jordan Rose]

The main reason we didn't even try to go for the CMake toolchain support is because of the old build/host/target problem. For those who don't know, in old GCC terminology, the build machine is where you build the compiler, the host machine is where you run the compiler, and the target machine is where you run the code that comes out of the compiler.

Right now our CMake project tries to build some binaries for the host machine (the compiler) and some binaries for the target machine (the runtime, standard library, and overlays). On non-Darwin platforms, these are always the same today; for us the host machine is OS X and we build several target standard libraries. CMake really doesn't like this, which is why a CMake-generated Xcode project still can't build for iOS, even though Xcode supports products with different platforms perfectly well.

So from CMake's perspective, we're always building for the host machine, and we pass some funny flags when building the runtime and stdlib. We get away with this because the OS X linker can also link binaries intended for iOS et al.

If you're planning on building a cross-compiler (build = OS X, host = OS X, target = Linux), I think you'd really have the best success by using the same mechanism we use to build multiple stdlibs on Darwin. You'd have to figure out how to trick it into using your linker for just those libraries, but Clang actually does have support for cross-compilation <http://clang.llvm.org/docs/CrossCompilation.html#toolchain-options> (and CMake should be using Clang to link, I think?) so you should have a chance.

Jordan


> On Feb 16, 2016, at 13:48 , Tom Birch via swift-dev <swift-dev at swift.org> wrote:
> 
> I'm working on cross-compiling swift using OS X to build a linux-arm runtime/stdlib, and I've had some success. My change is here:
> 
> https://github.com/froody/swift/commit/bc7ca07c1c7a94a8d07acd635c486388640ee2d2 <https://github.com/froody/swift/commit/bc7ca07c1c7a94a8d07acd635c486388640ee2d2>
> 
> Steps to repro are in cmake/modules/Toolchain-linux-arm.cmake
> 
> It's still a work in progress, but I wanted to get some feedback on whether or not I'm going in the right direction. Toolchain files are the recommended way for cross-compiling with CMake, but it seems like it doesn't mesh well with the current system of building multiple mach-o targets from a single CMake invocation (e.g. OS X, iOS, tvOS and watchOS built on OS X). I've also had to fight the build-system a lot (using many --skip-build-foo flags, and two invocations of ./utils/build-script) in order to avoid building ninja targets that don't exist, and I'm wondering if there's a better way to do this, i.e. to build llvm/clang/swiftc for OS X in llvm-macosx-x86_64/swift-macosx-x86_64 build directories, and use that to build (swiftc?)/stdlib/tests for linux-arm in a swift-linux-armv7 build directory. Maybe this is a good reason to do https://bugs.swift.org/browse/SR-237 <https://bugs.swift.org/browse/SR-237> first/in parallel?
> 
> I also required a change to LLVM:
> 
> https://github.com/froody/swift-llvm/commit/ed54c92943444999f11918f013cca1dba7892da1 <https://github.com/froody/swift-llvm/commit/ed54c92943444999f11918f013cca1dba7892da1>
> 
> to fix this error:
>  
> CMake Error: The inter-target dependency graph contains the following strongly connected component (cycle):
>   "NativeLLVMConfig" of type UTILITY
>     depends on "llvm-config" (strong)
>   "llvm-config" of type EXECUTABLE
>     depends on "NativeLLVMConfig" (strong)
> 
> Maybe Chris Bieneman has an idea about this? I seemed to successfully cross-compile with the "False AND" hack, not sure what's going on.
> 
> Anyway, I'd appreciate any feedback on the direction/next steps.
> 
> cheers,
> Tom
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