Fixed layout classes should still have resilient vtables [5.0; ABI] #20888
Conversation
We want to be able to define classes with a fixed storage layout, but a resilient (opaque) vtable. If the class is also generic, we still have to load field offsets from the metadata if they are dependent. So put the field offsets after the generic arguments and before the vtable. This is an ABI break for @_fixed_layout classes, which are defined by the stdlib.
…t-metadata from resilient-storage We want @_fixed_layout classes to have non-resilient storage, but still have resilient metadata.
slavapestov
changed the title
|
@swift-ci Please test |
|
Build failed |
|
Build failed |
|
The test failure was a bad cherry-pick. Let's try again |
|
Build failed |
|
@swift-ci Please test |
|
Build failed |
|
Build failed |
slavapestov
force-pushed
the
fixed-layout-classes-5.0
branch
from
d879b2a to
34e998b
Compare
|
Another test typo... should've tested locally |
|
@swift-ci Please test |
|
Build failed |
|
Build failed |
@_fixed_layout classes have resilient vtables now, so we can add and re-order methods. Removing overrides is not legal though because they are subject to devirtualization.
The result type mismatch comes up when we're calling a covariant override of a class method resiliently as well.
slavapestov
force-pushed
the
fixed-layout-classes-5.0
branch
from
34e998b to
9b3758e
Compare
|
@swift-ci Please test |
1 similar comment
|
@swift-ci Please test |
|
Build failed |
|
Build failed |
|
Build failed |
To achieve this, re-organize the metadata so that the field offset vector comes immediately after the generic parameter area, instead of after the vtable. This means the vtable can change resiliently.
Note that for non-generic
@_fixed_layoutclasses without resiliently-sized fields, field offsets are constant and will be inlined directly, but we need the more general access pattern to work if the@_fixed_layoutclass has generic layout.Also, use method dispatch thunks to call vtable methods instead of accessing the vtable directly, as with resilient classes.
Since the metadata size is not known at compile time, we use the most general metadata instantiation strategy now.