This commit adds initial build system support for macCatalyst,
an Apple technology that enables code targeting iOS
to be recompiled so that it can be executed on macOS while still using
iOS APIs. This is the first in a series of commits building out support for
macCatalyst in the compiler, runtime, standard library, and overlays. Swift
for macCatalyst represents the work of multiple people, including
Devin Coughlin, Ross Bayer, and Brent Royal-Gordon.
Under macCatalyst, compiler-provided shared libraries (including overlays)
are built as one of four kinds (or "flavors") of libraries,
each with different install names and Mach-O load commands. This commit
adds the build system infrastructure to produce these different
library flavors.
**macOS-like Libraries**
A "macOS-like" library (such as the GLKit overlay) is a plain-old macOS library
that can only be loaded into regular macOS processes. It has a macOS slice with
a single load command allowing it to be loaded into normal macOS processes.
**iOS-like Libraries**
An "iOS-like" library, such as the UIKit overlay, is a library with a
macOS slice but with a load command that only allows it be loaded into
macCatalyst processes. iOS-like libraries are produced by passing a new
target tuple to the compiler:
swiftc ... -target x86_64-apple-ios13.0-macabi ...
Here 'ios' (and an iOS version number) is used for OS portion
of the triple, but the 'macabi' environment tells the compiler
that the library is intended for macCatalyst.
**Zippered Libraries**
A "zippered" library can be loaded into either a macCatalyst process or
a standard macOS process. Since macCatalyst does not introduce a new Mach-O
slice, the same code is shared between both processes. Zippered libraries
are usually relatively low level and with an API surface that is similar
between macOS and iOS (for example, both the Foundation overlay and the Swift
Standard Library/Runtime itself are zippered).
Zippered libraries are created by passing both the usual `-target`
flag to the compiler and an additional `-target-variant` flag:
swiftc ... -target x86_64-apple-macos10.15 \
-target-variant x86_64-apple-ios13.0-macabi
Just like the -target flag, -target-variant takes a target tuple.
This tells the compiler to compile the library for the -target tuple but
to add an extra load command, allowing the library to be loaded into processes
of the -target-variant flavor as well.
While a single zippered library and slice is shared between macOS and
macCatalyst, zippered libraries require two separate .swiftinterface/.swiftmodule
files, one for macOS and one for macCatalyst. When a macOS or macCatalyst client
imports the library, it will use module file for its flavor to determine what
symbols are present. This enables a zippered library to expose a subset of its
target APIs to its target-variant.
**Unzippered-Twin Libraries**
"Unzippered Twins" are pairs of libraries with the same name but different
contents and install locations, one for use from macOS processes and one for
use from macCatalyst processes. Unzippered twins are usually libraries that
depend on AppKit on macOS and UIKit on iOS (for example, the MapKit overlay)
and so do not share a common implementation between macOS and macCatalyst.
The macCatalyst version of an unzippered twin is installed in a parallel
directory hierarchy rooted at /System/iOSSupport/. So, for example, while macOS
and zippered Swift overlays are installed in /usr/lib/swift/, iOS-like and
the macCatalyst side of unzippered twins are installed in
/System/iOSSupport/usr/lib/swift. When building for macCatalyst, the build system
passes additional search paths so that the macCatalyst version of libraries is
found before macOS versions.
The add_swift_target_library() funciton now take an
optional MACCATALYST_BUILD_FLAVOR, which enables swift libraries to indicate
which flavor of library they are.
These properties are extraneous. They are never queried, and the
`set(...CACHE)` which precedes them will ensure that the value is
cached. This should be functionally equivalent.
Preconditions have been moved to the top of the file. All the variables set have been grouped by kind and a few converted to cache variables for better control when building. This should cut down on merge-conflicts as well."
CMake defaults to using a special <package>_DIR for finding packages.
Prefer this to import the CMake package for LLVM/Clang/Swift rather than
custom paths.
CMake defaults to using a special <package>_DIR for finding packages.
Prefer this to import the CMake package for LLVM/Clang/Swift rather than
custom paths.
Otherwise, we'll think we don't need to install the API notes.
No test because this configuration isn't (yet) tested publicly
anywhere, but it is being tested within Apple.
...because the Darwin overlay is responsible for installing them.
Context: Within Apple, overlays are sometimes built separately from
the rest of the Swift project. This doesn't currently work with just
the public repo (rdar://problem/47324189), but it could, and I broke
it with my recent API notes cleanup. This should get things working
again.
rdar://problem/47294352
