Introduce return_borrow and unchecked_ownership instructions for use with borrow accessors #85040
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This instruction can be used to disable ownership verification on it's result and will be allowed only in raw SIL. Sometimes SILGen can produce invalid ownership SSA, that cannot be resolved until mandatory passes run. We have a few ways to piecewise disable verification. With unchecked_ownership instruction we can provide a uniform way to disable ownership verification for a value.
SILGen may produce a borrow accessor result from within a local borrow scope. Such as: ``` %ld = load_borrow %self %fwd = unchecked_ownership %ld %ex = struct_extract %fwd, #Struct.storedProperty end_borrow %ld return %ex ``` This is illegal OSSA, since the return uses a value outside it's borrow scope. Add a new SILGenCleanup transform, to turn this into valid OSSA: ``` %ld = load_borrow %self %ex = struct_extract %ld, #Struct.storedProperty return_borrow %ex from_scopes %ld ```
When return_borrow has both lifetime ending and non-lifetime ending use, consider it lifetime ending.
SILGen inserts mark_unresolved_noncopyable_value at the introducer in some cases and at uses in other cases. This inconsistency can causes insertion of a redundant mark* instruction which crashes the move-only checker. This change avoids inserting a redundant mark* instruction.
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Currently borrow accessor SILGen uses
unchecked_ownership_conversioninstructions to produce a@guaranteedreturn value from an address in some cases:unchecked_ownership_conversionsilences the ownership verifier, but is an unsafe SIL operation which will get treated conservatively in the optimizer.Introduce a new
return_borrowinstruction and add a new SILGenCleanup transform, to turn this into valid OSSA:return_borrow generation becomes straightforward after the SILGen phase, introduce a raw SIL only -
unchecked_ownershipinstruction for SILGen which will get eliminated during SILGenCleanup.There are many different markers used today when we cannot resolve ownership right after SILGen. These markers are present until a mandatory pass resolves them.
unchecked_ownershipcan be used as a uniform marker for this purpose in the future.