[swift-dev] Cross-compile for Android armv7 from OSX

[Dmitri Gribenko]

On Tue, Jan 19, 2016 at 2:50 AM, Geordie Jay <geojay at gmail.com> wrote:
> Ok I'm confused as to what you mean now. I can't imagine how it'd make sense
> to run CMake multiple times to build from one host for its own platform

To build a Swift distribution, we need to build two pieces: the Swift
compiler and the standard library.

When cross-compiling, you can build two different kinds of compilers:
a compiler that runs on your host machine, or the compiler that runs
on the target.

To build the standard library, you need to use the Swift compiler that
runs on the host and a Clang that runs on the host, but both
configured to target your target platform.

Thus, if you want to cross-compile to Linux arm from OS X, you would
need to use two or three CMake invocations:

(1) to build a Swift compiler that runs on OS X,

(2) use (1) to build a standard library that runs on Linux arm,

(3) build a Swift compiler that runs on Linux arm.

There is a dependency edge between (1) and (2), but they are in
different CMake invocations.

When cross-compiling for iOS, tvOS and watchOS, there are many CMake
invocations of the second kind -- in fact, multiple for every platform
because of many slices.  And, even more, since those platforms use fat
libraries, there needs to be a step that creates them from separate
slices.

That's why I'm comparing this to "recursive make": if we use the CMake
approach to cross-compiling with multiple CMake invocations, since we
are building the compiler itself, we would be crating dependencies in
the graph that CMake does not know about, and we would require some
code to orchestrate the build.  Just like recursive make does in its
Makefiles.

Using build-script to orchestrate this build would be a natural
choice, but it would break many of the current workflows, where
developers rely on quickly iterating in one build directory, invoking
'ninja check-swift' directly to rebulid the compiler, all the
downstream dependencies, and run tests.

Dmitri

-- 
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(j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/