A ‘forwarding module’ is a YAML file that’s meant to stand in for a .swiftmodule file and provide an up-to-date description of its dependencies, always using modification times.
When a ‘prebuilt module’ is first loaded, we verify that it’s up-to-date by hashing all of its dependencies. Since this is orders of magnitude slower than reading mtimes, we’ll install a `forwarding module` containing the mtimes of the now-validated dependencies.
If the frontend is invoked with
-build-module-from-parseable-interface, we might be trying to persist
and distribute the swiftmodule that gets built. In that case, any
dependencies we list might not be relevant.
This probably isn't really the final answer here; what we want is some
way to say /which/ dependencies are relevant, and how they're related
to how the swiftmodule that gets used. Most likely the right answer
here is to limit this to dependencies within the SDK or something.
Otherwise, the top-level compilation gets the benefit of the prebuilt
cache path, but the sub-invocations for swiftinterfaces that /do/
need to be compiled do not.
This is a little trickier than it sounds because we have 'friend'
access into the FrontendInputsAndOutputs structure, which means all
the helpers need to be declared in the header file. But it makes the
two use sites simpler, and does slightly less work in the cache hit
path.
The previous 'openModuleFiles' interface in SerializedModuleLoaderBase
still assumed that swiftmodule files and swiftdoc files would be found
next to each other, but that's not true anymore with
swiftinterfaces-built-to-modules. Give up on this assumption (and on
the minor optimization of passing down a scratch buffer) and split out
the interface into the customization point
'findModuleFilesInDirectory' and the implementation 'openModuleFiles'.
The latter now takes two full paths: one for the swiftmodule, one for
the swiftdoc.
Makes it easier to test the caching behavior, and may also be useful
for "prebuilding" swiftinterfaces in the future, or having the Driver
kick off a bunch of separate builds as proper tasks.
The goal here is to separate the parts that compute an output file
name from the parts that do the actual compilation, so that we can
test the swiftinterface -> swiftmodule behavior more directly. No
functionality change in this commit; the next will take advantage
of the refactoring.
- Use the name for the cached module, so that we don't end up with a
zillion "x86_64-XXXXXXXX.swiftmodule" files in the cache when we're
working with architecture-specific swiftmodules.
- Diagnose if the expected name is different from the name specified
in the swiftinterface.
- Emit all diagnostics at the location of the import, instead of
without any location at all.
