The key thing is that the move checker will not consider the explicit copy value
to be a copy_value that can be rewritten, ensuring that any uses of the result
of the explicit copy_value (consuming or other wise) are not checked.
Similar to the _move operator I recently introduced, this is a transparent
function so we can perform one level of specialization and thus at least be
generic over all concrete types.
This abbreviation for "if and only if" is confusing to those not coming
from a background in formal mathematics, and is frequently reported as a
type by developers reading the documentation.
This commit also changes doc comments in internal and private members,
which don't become part of the public documentation, because omitting
"iff" everywhere makes it much easier to check for any later changes
that reintroduce it.
This patch introduces a new stdlib function called _move:
```Swift
@_alwaysEmitIntoClient
@_transparent
@_semantics("lifetimemanagement.move")
public func _move<T>(_ value: __owned T) -> T {
#if $ExperimentalMoveOnly
Builtin.move(value)
#else
value
#endif
}
```
It is a first attempt at creating a "move" function for Swift, albeit a skleton
one since we do not yet perform the "no use after move" analysis. But this at
leasts gets the skeleton into place so we can built the analysis on top of it
and churn tree in a manageable way. Thus in its current incarnation, all it does
is take in an __owned +1 parameter and returns it after moving it through
Builtin.move.
Given that we want to use an OSSA based analysis for our "no use after move"
analysis and we do not have opaque values yet, we can not supporting moving
generic values since they are address only. This has stymied us in the past from
creating this function. With the implementation in this PR via a bit of
cleverness, we are now able to support this as a generic function over all
concrete types by being a little clever.
The trick is that when we transparent inline _move (to get the builtin), we
perform one level of specialization causing the inlined Builtin.move to be of a
loadable type. If after transparent inlining, we inline builtin "move" into a
context where it is still address only, we emit a diagnostic telling the user
that they applied move to a generic or existential and that this is not yet
supported.
The reason why we are taking this approach is that we wish to use this to
implement a new (as yet unwritten) diagnostic pass that verifies that _move
(even for non-trivial copyable values) ends the lifetime of the value. This will
ensure that one can write the following code to reliably end the lifetime of a
let binding in Swift:
```Swift
let x = Klass()
let _ = _move(x)
// hypotheticalUse(x)
```
Without the diagnostic pass, if one were to write another hypothetical use of x
after the _move, the compiler would copy x to at least hypotheticalUse(x)
meaning the lifetime of x would not end at the _move, =><=.
So to implement this diagnostic pass, we want to use the OSSA infrastructure and
that only works on objects! So how do we square this circle: by taking advantage
of the mandatory SIL optimzier pipeline! Specifically we take advantage of the
following:
1. Mandatory Inlining and Predictable Dead Allocation Elimination run before any
of the move only diagnostic passes that we run.
2. Mandatory Inlining is able to specialize a callee a single level when it
inlines code. One can take advantage of this to even at -Onone to
monomorphosize code.
and then note that _move is such a simple function that predictable dead
allocation elimination is able to without issue eliminate the extra alloc_stack
that appear in the caller after inlining without issue. So we (as the tests
show) get SIL that for concrete types looks exactly like we just had run a
move_value for that specific type as an object since we promote away the
stores/loads in favor of object operations when we eliminate the allocation.
In order to prevent any issue with this being used in a context where multiple
specializations may occur, I made the inliner emit a diagnostic if it inlines
_move into a function that applies it to an address only value. The diagnostic
is emitted at the source location where the function call occurs so it is easy
to find, e.x.:
```
func addressOnlyMove<T>(t: T) -> T {
_move(t) // expected-error {{move() used on a generic or existential value}}
}
moveonly_builtin_generic_failure.swift:12:5: error: move() used on a generic or existential value
_move(t)
^
```
To eliminate any potential ABI impact, if someone calls _move in a way that
causes it to be used in a context where the transparent inliner will not inline
it, I taught IRGen that Builtin.move is equivalent to a take from src -> dst and
marked _move as always emit into client (AEIC). I also took advantage of the
feature flag I added in the previous commit in order to prevent any cond_fails
from exposing Builtin.move in the stdlib. If one does not pass in the flag
-enable-experimental-move-only then the function just returns the value without
calling Builtin.move, so we are safe.
rdar://83957028
Adds two new IRGen-level builtins (one for allocating, the other for deallocating), a stdlib shim function for enhanced stack-promotion heuristics, and the proposed public stdlib functions.
We have already done the hard work of unwrapping the optionals
that represent the first and last pointer, and calling `self.count` does it all over again. Use the distance between the unwrapped pointers instead.
This is for the 'freestanding' build to stop assuming the platform has argc/argv.
- Introduce a new sub-library, libswiftCommandLineSupport.a
- Move stubs/CommandLine.cpp into this library
- Conditionally embed it into libswiftCore
- Conditionally embed it into libswiftPrivateLibcExtras if not in libswiftCore to support testing
- Add SWIFT_STDLIB_HAS_COMMANDLINE CMake (and build-script) flag
And set this option in various presets for buildbots.
Don't enable the checks by default because when linking against the OS library (which does not support COW checking) it will result in unresolved symbol errors.
So far it was handled by an availability checks against 9999 (which was a hack), but this does not work anymore.
Note, all this is only relevant for assert builds of the stdlib.
rdar://83673798
Adding build modes for libswift: off, hosttools, bootstrapping, bootstrapping-with-hostlibs
The two bootstrapping modes are new. For details see libswift/README.md
Before this change, `RawRepresentable`'s custom `hashValue` implementation used to forward to `rawValue.hashValue`. (See https://github.com/apple/swift/pull/20705.) This sort of made sense at the time (`hash(into:)` was very new and `hashValue` was still in heavy use): it allowed raw representable values to return the exact same hash value as their `rawValue`, which some code used to (mistakenly) rely on. The big drawback of this is that to customize the Hashable implementation of a RawRepresentable type, people need to implement both `hashValue` and `hash(into:)` -- the former does not otherwise pick up customizations to the latter.
This change makes the default `RawRepresentable.hashValue` implementation call `self.hash(into:)`, just like it would on non-RawRepresentable types.
rdar://82651116
