Attached below is an updated version of the proposal. Again a rendered version is located at:
https://gottesmm.github.io/proposals/high-level-arc-memory-operations.html Michael ---- # Summary This document proposes: 1. adding the following ownership qualifiers to `load`: `[take]`, `[copy]`, `[borrow]`, `[trivial]`. 2. adding the following ownership qualifiers to `store`: `[init]`, `[assign]`, `[trivial]`. 3. requiring all `load` and `store` operations to have ownership qualifiers. 4. banning the use of `load [trivial]`, `store [trivial]` on memory locations of `non-trivial` type. This will allow for: 1. eliminating optimizer miscompiles that occur due to releases being moved into the region in between a `load`/`retain`, `load`/`release`, `store`/`release`. (For a specific example, see the appendix). 2. explicitly modeling `load [trivial]`/`store [trivial]` as having `unsafe unowned` ownership semantics. This will be enforced via the verifier. 3. more aggressive ARC code motion. # Definitions ## ownership qualified load We propose four different ownership qualifiers for load. Define `load [trivial]` as: %x = load [trivial] %x_ptr : $*Int => %x = load %x_ptr : $*Int A `load [trivial]` can not be used to load values of non-trivial type. Define `load [copy]` as: %x = load [copy] %x_ptr : $*C => %x = load %x_ptr : $*C retain_value %x : $C Then define `load [take]` as: %x = load [take] %x_ptr : $*Builtin.NativeObject => %x = load %x_ptr : $*Builtin.NativeObject **NOTE** `load [take]` implies that the loaded from memory location no longer owns the result object (i.e. a take is a move). Loading from the memory location again without reinitialization is illegal. Next we provide `load [borrow]`: %x = load [borrow] %x_ptr : $*Builtin.NativeObject ... endBorrow(%x, %x_ptr) => %x = load %x_ptr : $*Builtin.NativeObject ... endBorrow(%x, %x_ptr) `load [borrow]` implies that in the region between the `load` and the `endBorrow`, the loaded object must semantically remain alive. The `endBorrow` communicates to the optimizer: 1. That the value in `%x_ptr` should not be destroyed before endBorrow. 2. Uses of `%x` should not be sunk past endBorrow since `%x` is only a shallow copy of the value in `%x_ptr` and past that point `%x_ptr` may not remain alive. An example of where this construct is useful is when one has a let binding to a class instance `c` that contains a let field `f`. In that case `c`'s lifetime guarantees `f`'s lifetime meaning that returning `f` over the function call boundary is safe. ## ownership qualified store First define a `store [trivial]` as: store %x to [trivial] %x_ptr : $*Int => store %x to %x_ptr : $*Int The verifier will prevent this instruction from being used on types with non-trivial ownership. Define a `store [assign]` as follows: store %x to [assign] %x_ptr : $*C => %old_x = load %x_ptr : $*C store %new_x to %x_ptr : $*C release_value %old_x : $C *NOTE* `store` is defined as a consuming operation. We also provide `store [init]` in the case where we know statically that there is no previous value in the memory location: store %x to [init] %x_ptr : $*C => store %new_x to %x_ptr : $*C # Implementation ## Goals Our implementation strategy goals are: 1. zero impact on other compiler developers until the feature is fully developed. This implies all work will be done behind a flag. 2. separation of feature implementation from updating passes. Goal 2 will be implemented via a pass that transforms ownership qualified `load`/`store` instructions into unqualified `load`/`store` right after SILGen. ## Plan We begin by adding initial infrastructure for our development. This means: 1. Adding to SILOptions a disabled by default flag called "EnableSILOwnershipModel". This flag will be set by a false by default frontend option called "-enable-sil-ownership-mode". 2. Bots will be brought up to test the compiler with "-enable-sil-ownership-model" set to true. The specific bots are: * RA-OSX+simulators * RA-Device * RA-Linux. The bots will run once a day until the feature is close to completion. Then a polling model will be followed. Now that change isolation is borrow, we develop building blocks for the optimization: 1. Two enums will be defined: `LoadInstOwnershipQualifier`, `StoreInstOwnershipQualifier`. The exact definition of these enums are as follows: enum class LoadOwnershipQualifier { Unqualified, Take, Copy, Borrow, Trivial }; enum class StoreOwnershipQualifier { Unqualified, Init, Assign, Trivial }; *NOTE* `LoadOwnershipQualifier::Unqualified` and `StoreOwnershipQualifier::Unqualified` are only needed for staging purposes. 2. Creating a `LoadInst`, `StoreInst` will be changed to require an ownership qualifier. At this stage, this argument will default to `Unqualified`. "Bare" `load`, `store` when parsed via textual SIL will be considered to be unqualified. This implies that the rest of the compiler will not have to be changed as a result of this step. 3. Support will be added to SIL, IRGen, Serialization, SILPrinting, and SIL Parsing for the rest of the qualifiers. SILGen will not be modified at this stage. 4. A pass called the "OwnershipModelEliminator" will be implemented. It will blow up all `load`, `store` instructions with non `*::Unqualified` ownership into their constituant ARC operations and `*::Unqualified` `load`, `store` insts. 3. An option called "EnforceSILOwnershipMode" will be added to the verifier. If the option is set, the verifier will assert if: a. `load`, `store` operations with trivial ownership are applied to memory locations with non-trivial type. b. `load`, `store` operation with unqualified ownership type are present in the IR. Finally, we wire up the building blocks: 1. If SILOption.EnableSILOwnershipModel is true, then the after SILGen SIL verification will be performed with EnforceSILOwnershipModel set to true. 2. If SILOption.EnableSILOwnershipModel is true, then the pass manager will run the OwnershipModelEliminator pass right after SILGen before the normal pass pipeline starts. 3. SILGen will be changed to emit non-unqualified ownership qualifiers on load, store instructions when the EnableSILOwnershipModel flag is set. We will use the verifier throwing to guarantee that we are not missing any specific cases. Then once all of the bots are green, we change SILOption.EnableSILOwnershipModel to be true by default. After a cooling off period, we move all of the code behind the SILOwnershipModel flag in front of the flag. We do this so we can reuse that flag for further SILOwnershipModel changes. ## Optimizer Changes Since the SILOwnershipModel eliminator will eliminate the ownership qualifiers on load, store instructions right after ownership verification, there will be no immediate affects on the optimizer and thus the optimizer changes can be done in parallel with the rest of the ARC optimization work. But, in the long run, we want to enforce these ownership invariants all throughout the SIL pipeline implying these ownership qualified `load`, `store` instructions must be processed by IRGen, not eliminated by the SILOwnershipModel eliminator. Thus we will need to update passes to handle these new instructions. The main optimizer changes can be separated into the following areas: memory forwarding, dead stores, ARC optimization. In all of these cases, the necessary changes are relatively trivial to respond to. We give a quick taste of two of them: store->load forwarding and ARC Code Motion. ### store->load forwarding Currently we perform store->load forwarding as follows: store %x to %x_ptr : $C ... NO SIDE EFFECTS THAT TOUCH X_PTR ... %y = load %x_ptr : $C use(%y) => store %x to %x_ptr : $C ... NO SIDE EFFECTS THAT TOUCH X_PTR ... use(%x) In a world, where we are using ownership qualified load, store, we have to also consider the ownership implications. *NOTE* Since we are not modifying the store, `store [assign]` and `store [init]` are treated the same. Thus without any loss of generality, lets consider solely `store`. store %x to [assign] %x_ptr : $C ... NO SIDE EFFECTS THAT TOUCH X_PTR ... %y = load [copy] %x_ptr : $C use(%y) => store %x to [assign] %x_ptr : $C ... NO SIDE EFFECTS THAT TOUCH X_PTR ... strong_retain %x use(%x) ### ARC Code Motion If ARC Code Motion wishes to move the ARC semantics of ownership qualified `load`, `store` instructions, it must now consider read/write effects. On the other hand, it will be able to now not consider the side-effects of destructors when moving retain/release operations. ### Normal Code Motion Normal code motion will lose some effectiveness since many of the load/store operations that it used to be able to move now must consider ARC information. We may need to consider running ARC code motion earlier in the pipeline where we normally run Normal Code Motion to ensure that we are able to handle these cases. ### ARC Optimization The main implication for ARC optimization is that instead of eliminating just retains, releases, it must be able to recognize ownership qualified `load`, `store` and set their flags as appropriate. ### Function Signature Optimization Semantic ARC affects function signature optimization in the context of the owned to borrow optimization. Specifically: 1. A `store [assign]` must be recognized as a release of the old value that is being overridden. In such a case, we can move the `release` of the old value into the caller and change the `store [assign]` into a `store [init]`. 2. A `load [copy]` must be recognized as a retain in the callee. Then function signature optimization will transform the `load [copy]` into a `load [borrow]`. This would require the addition of a new `@borrow` return value convention. # Appendix ## Partial Initialization of Loadable References in SIL In SIL, a value of non-trivial loadable type is loaded from a memory location as follows: %x = load %x_ptr : $*S ... retain_value %x_ptr : $S At first glance, this looks reasonable, but in truth there is a hidden drawback: the partially initialized zone in between the load and the retain operation. This zone creates a period of time when an "evil optimizer" could insert an instruction that causes x to be deallocated before the copy is finished being initialized. Similar issues come up when trying to perform a store of a non-trival value into a memory location. Since this sort of partial initialization is allowed in SIL, the optimizer is forced to be overly conservative when attempting to move releases passed retains lest the release triggers a deinit that destroys a value like `%x`. Lets look at two concrete examples that show how semantically providing ownership qualified load, store instructions eliminate this problem. **NOTE** Without any loss of generality, we will speak of values with reference semantics instead of non-trivial values. ## Case Study: Partial Initialization and load [copy] ### The Problem Consider the following swift program: func opaque_call() final class C { var int: Int = 0 deinit { opaque_call() } } final class D { var int: Int = 0 } var GLOBAL_C : C? = nil var GLOBAL_D : D? = nil func useC(_ c: C) func useD(_ d: D) func run() { let c = C() GLOBAL_C = c let d = D() GLOBAL_D = d useC(c) useD(d) } Notice that both `C` and `D` have fixed layouts and separate class hierarchies, but `C`'s deinit has a call to the function `opaque_call` which may write to `GLOBAL_D` or `GLOBAL_C`. Additionally assume that both `useC` and `useD` are known to the compiler to not have any affects on instances of type `D`, `C` respectively and useC assigns `nil` to `GLOBAL_C`. Now consider the following valid SIL lowering for `run`: sil_global GLOBAL_D : $D sil_global GLOBAL_C : $C final class C { var x: Int deinit } final class D { var x: Int } sil @useC : $@convention(thin) () -> () sil @useD : $@convention(thin) () -> () sil @run : $@convention(thin) () -> () { bb0: %c = alloc_ref $C %global_c = global_addr @GLOBAL_C : $*C strong_retain %c : $C store %c to %global_c : $*C (1) %d = alloc_ref $D %global_d = global_addr @GLOBAL_D : $*D strong_retain %d : $D store %d to %global_d : $*D (2) %c2 = load %global_c : $*C (3) strong_retain %c2 : $C (4) %d2 = load %global_d : $*D (5) strong_retain %d2 : $D (6) %useC_func = function_ref @useC : $@convention(thin) (@owned C) -> () apply %useC_func(%c2) : $@convention(thin) (@owned C) -> () (7) %useD_func = function_ref @useD : $@convention(thin) (@owned D) -> () apply %useD_func(%d2) : $@convention(thin) (@owned D) -> () (8) strong_release %d : $D (9) strong_release %c : $C (10) } Lets optimize this function! First we perform the following operations: 1. Since `(2)` is storing to an identified object that can not be `GLOBAL_C`, we can store to load forward `(1)` to `(3)`. 2. Since a retain does not block store to load forwarding, we can forward `(2)` to `(5)`. But lets for the sake of argument, assume that the optimizer keeps such information as an analysis and does not perform the actual load->store forwarding. 3. Even though we do not foward `(2)` to `(5)`, we can still move `(4)` over `(6)` so that `(4)` is right before `(7)`. This yields (using the ' marker to designate that a register has had load-store forwarding applied to it): sil @run : $@convention(thin) () -> () { bb0: %c = alloc_ref $C %global_c = global_addr @GLOBAL_C : $*C strong_retain %c : $C store %c to %global_c : $*C (1) %d = alloc_ref $D %global_d = global_addr @GLOBAL_D : $*D strong_retain %d : $D store %d to %global_d : $*D (2) strong_retain %c : $C (4') %d2 = load %global_d : $*D (5) strong_retain %d2 : $D (6) %useC_func = function_ref @useC : $@convention(thin) (@owned C) -> () apply %useC_func(%c) : $@convention(thin) (@owned C) -> () (7') %useD_func = function_ref @useD : $@convention(thin) (@owned D) -> () apply %useD_func(%d2) : $@convention(thin) (@owned D) -> () (8) strong_release %d : $D (9) strong_release %c : $C (10) } Then by assumption, we know that `%useC` does not perform any releases of any instances of class `D`. Thus `(6)` can be moved past `(7')` and we can then pair and eliminate `(6)` and `(9)` via the rules of ARC optimization using the analysis information that `%d2` and `%d` are th same due to the possibility of performing store->load forwarding. After performing such transformations, `run` looks as follows: sil @run : $@convention(thin) () -> () { bb0: %c = alloc_ref $C %global_c = global_addr @GLOBAL_C : $*C strong_retain %c : $C store %c to %global_c : $*C (1) %d = alloc_ref $D %global_d = global_addr @GLOBAL_D : $*D strong_retain %d : $D store %d to %global_d : $*D %d2 = load %global_d : $*D (5) strong_retain %c : $C (4') %useC_func = function_ref @useC : $@convention(thin) (@owned C) -> () apply %useC_func(%c) : $@convention(thin) (@owned C) -> () (7') %useD_func = function_ref @useD : $@convention(thin) (@owned D) -> () apply %useD_func(%d2) : $@convention(thin) (@owned D) -> () (8) strong_release %c : $C (10) } Now by assumption, we know that `%useD_func` does not touch any instances of class `C` and `%c` does not contain any ivars of type `D` and is final so none can be added. At first glance, this seems to suggest that we can move `(10)` before `(8')` and then pair/eliminate `(4')` and `(10)`. But is this a safe optimization perform? Absolutely Not! Why? Remember that since `useC_func` assigns `nil` to `GLOBAL_C`, after `(7')`, `%c` could have a reference count of 1. Thus `(10)` _may_ invoke the destructor of `C`. Since this destructor calls an opaque function that _could_ potentially write to `GLOBAL_D`, we may be be passing `%d2`, an already deallocated object to `%useD_func`, an illegal optimization! Lets think a bit more about this example and consider this example at the language level. Remember that while Swift's deinit are not asychronous, we do not allow for user level code to create dependencies from the body of the destructor into the normal control flow that has called it. This means that there are two valid results of this code: - Operation Sequence 1: No optimization is performed and `%d2` is passed to `%useD_func`. - Operation Sequence 2: We shorten the lifetime of `%c` before `%useD_func` and a different instance of `$D` is passed into `%useD_func`. The fact that 1 occurs without optimization is just as a result of an implementation detail of SILGen. 2 is also a valid sequence of operations. Given that: 1. As a principle, the optimizer does not consider such dependencies to avoid being overly conservative. 2. We provide constructs to ensure appropriate lifetimes via the usage of constructs such as fix_lifetime. We need to figure out how to express our optimization such that 2 happens. Remember that one of the optimizations that we performed at the beginning was to move `(6)` over `(7')`, i.e., transform this: %d = alloc_ref $D %global_d_addr = global_addr GLOBAL_D : $D %d = load %global_d_addr : $*D (5) strong_retain %d : $D (6) // Call the user functions passing in the instances that we loaded from the globals. %useC_func = function_ref @useC : $@convention(thin) (@owned C) -> () apply %useC_func(%c) : $@convention(thin) (@owned C) -> () (7') into: %global_d_addr = global_addr GLOBAL_D : $D %d2 = load %global_d_addr : $*D (5) // Call the user functions passing in the instances that we loaded from the globals. %useC_func = function_ref @useC : $@convention(thin) (@owned C) -> () apply %useC_func(%c) : $@convention(thin) (@owned C) -> () (7') strong_retain %d2 : $D (6) This transformation in Swift corresponds to transforming: let d = GLOBAL_D useC(c) to: let d_raw = load_d_value(GLOBAL_D) useC(c) let d = take_ownership_of_d(d_raw) This is clearly an instance where we have moved a side-effect in between the loading of the data and the taking ownership of such data, that is before the `let` is fully initialized. What if instead of just moving the retain, we moved the entire let statement? This would then result in the following swift code: useC(c) let d = GLOBAL_D and would correspond to the following SIL snippet: %global_d_addr = global_addr GLOBAL_D : $D // Call the user functions passing in the instances that we loaded from the globals. %useC_func = function_ref @useC : $@convention(thin) (@owned C) -> () apply %useC_func(%c) : $@convention(thin) (@owned C) -> () (7') %d2 = load %global_d_addr : $*D (5) strong_retain %d2 : $D (6) Moving the load with the strong_retain to ensure that the full initialization is performed even after code motion causes our SIL to look as follows: sil @run : $@convention(thin) () -> () { bb0: %c = alloc_ref $C %global_c = global_addr @GLOBAL_C : $*C strong_retain %c : $C store %c to %global_c : $*C (1) %d = alloc_ref $D %global_d = global_addr @GLOBAL_D : $*D strong_retain %d : $D store %d to %global_d : $*D strong_retain %c : $C (4') %useC_func = function_ref @useC : $@convention(thin) (@owned C) -> () apply %useC_func(%c) : $@convention(thin) (@owned C) -> () (7') %d2 = load %global_d : $*D (5) %useD_func = function_ref @useD : $@convention(thin) (@owned D) -> () apply %useD_func(%d2) : $@convention(thin) (@owned D) -> () (8) strong_release %c : $C (10) } Giving us the exact result that we want: Operation Sequence 2! ### Defining load [copy] Given that we wish the load, store to be tightly coupled together, it is natural to express this operation as a `load [copy]` instruction. Lets define the `load [copy]` instruction as follows: %1 = load [copy] %0 : $*C => %1 = load %0 : $*C retain_value %1 : $C Now lets transform our initial example to use this instruction: Notice how now if we move `(7)` over `(3)` and `(6)` now, we get the following SIL: sil @run : $@convention(thin) () -> () { bb0: %c = alloc_ref $C %global_c = global_addr @GLOBAL_C : $*C strong_retain %c : $C store %c to %global_c : $*C (1) %d = alloc_ref $D %global_d = global_addr @GLOBAL_D : $*D strong_retain %d : $D store %d to %global_d : $*D (2) %c2 = load [copy] %global_c : $*C (3) %d2 = load [copy] %global_d : $*D (5) %useC_func = function_ref @useC : $@convention(thin) (@owned C) -> () apply %useC_func(%c2) : $@convention(thin) (@owned C) -> () (7) %useD_func = function_ref @useD : $@convention(thin) (@owned D) -> () apply %useD_func(%d2) : $@convention(thin) (@owned D) -> () (8) strong_release %d : $D (9) strong_release %c : $C (10) } We then perform the previous code motion: sil @run : $@convention(thin) () -> () { bb0: %c = alloc_ref $C %global_c = global_addr @GLOBAL_C : $*C strong_retain %c : $C store %c to %global_c : $*C (1) %d = alloc_ref $D %global_d = global_addr @GLOBAL_D : $*D strong_retain %d : $D store %d to %global_d : $*D (2) %c2 = load [copy] %global_c : $*C (3) %useC_func = function_ref @useC : $@convention(thin) (@owned C) -> () apply %useC_func(%c2) : $@convention(thin) (@owned C) -> () (7) strong_release %d : $D (9) %d2 = load [copy] %global_d : $*D (5) %useD_func = function_ref @useD : $@convention(thin) (@owned D) -> () apply %useD_func(%d2) : $@convention(thin) (@owned D) -> () (8) strong_release %c : $C (10) } We then would like to eliminate `(9)` and `(10)` by pairing them with `(3)` and `(8)`. Can we still do so? One way we could do this is by introducing the `[take]` flag. The `[take]` flag on a `load [take]` says that one is semantically loading a value from a memory location and are taking ownership of the value thus eliding the retain. In terms of SIL this flag is defined as: %x = load [take] %x_ptr : $*C => %x = load %x_ptr : $*C Why do we care about having such a `load [take]` instruction when we could just use a `load`? The reason why is that a normal `load` has unsafe unowned ownership (i.e. it has no implications on ownership). We would like for memory that has non-trivial type to only be able to be loaded via instructions that maintain said ownership. We will allow for casting to trivial types as usual to provide such access if it is required. Thus we have achieved the desired result: sil @run : $@convention(thin) () -> () { bb0: %c = alloc_ref $C %global_c = global_addr @GLOBAL_C : $*C strong_retain %c : $C store %c to %global_c : $*C (1) %d = alloc_ref $D %global_d = global_addr @GLOBAL_D : $*D strong_retain %d : $D store %d to %global_d : $*D (2) %c2 = load [take] %global_c : $*C (3) %useC_func = function_ref @useC : $@convention(thin) (@owned C) -> () apply %useC_func(%c2) : $@convention(thin) (@owned C) -> () (7) %d2 = load [take] %global_d : $*D (5) %useD_func = function_ref @useD : $@convention(thin) (@owned D) -> () apply %useD_func(%d2) : $@convention(thin) (@owned D) -> () (8) } > On Oct 6, 2016, at 3:03 PM, John McCall <rjmcc...@apple.com> wrote: > >> On Oct 5, 2016, at 4:48 PM, Michael Gottesman <mgottes...@apple.com >> <mailto:mgottes...@apple.com>> wrote: >>> On Oct 5, 2016, at 4:40 PM, Michael Gottesman via swift-dev >>> <swift-dev@swift.org <mailto:swift-dev@swift.org>> wrote: >>> >>>> >>>> On Oct 4, 2016, at 1:04 PM, John McCall <rjmcc...@apple.com >>>> <mailto:rjmcc...@apple.com>> wrote: >>>> >>>>> >>>>> On Sep 30, 2016, at 11:54 PM, Michael Gottesman via swift-dev >>>>> <swift-dev@swift.org <mailto:swift-dev@swift.org>> wrote: >>>>> >>>>> The document attached below contains the first "Semantic ARC" mini >>>>> proposal: the High Level ARC Memory Operations Proposal. >>>>> >>>>> An html rendered version of this markdown document is available at the >>>>> following URL: >>>>> >>>>> https://gottesmm.github.io/proposals/high-level-arc-memory-operations.html >>>>> >>>>> <https://gottesmm.github.io/proposals/high-level-arc-memory-operations.html> >>>>> >>>>> ---- >>>>> >>>>> # Summary >>>>> >>>>> This document proposes: >>>>> >>>>> 1. adding the `load_strong`, `store_strong` instructions to SIL. These >>>>> can only >>>>> be used with memory locations of `non-trivial` type. >>>> >>>> I would really like to avoid using the word "strong" here. Under the >>>> current proposal, these instructions will be usable with arbitrary >>>> non-trivial types, not just primitive class references. Even if you think >>>> of an aggregate that happens to contain one or more strong references as >>>> some sort of aggregate strong reference (which is questionable but not >>>> completely absurd), we already have loadable non-strong class references >>>> that this operation would be usable with, like native unowned references. >>>> "load_strong %0 : $*@sil_unowned T" as an operation yielding a scalar >>>> "@sil_unowned T" is ridiculous, and it will only get more ridiculous when >>>> we eventually allow this operation to work with types that are currently >>>> address-only, like weak references. >>>> >>>> Brainstorming: >>>> >>>> Something like load_copy and store_copy would be a bit unfortunate, since >>>> store_copy doesn't actually copy the source operand and we want to have a >>>> load_copy [take]. >>>> >>>> load_value and store_value seem excessively generic. It's not like >>>> non-trivial types aren't values. >>>> >>>> One question that comes to mind: do we actually need new instructions here >>>> other than for staging purposes? We don't actually need new instructions >>>> for pseudo-linear SIL to work; we just need to say that we only enforce >>>> pseudo-linearity for non-trivial types. >>>> >>>> If we just want the instruction to be explicit about ownership so that we >>>> can easily distinguish these cases, we can make the rule always explicit, >>>> e.g.: >>>> load [take] %0 : $*MyClass >>>> load [copy] %0 : $*MyClass >>>> load [trivial] %0 : $*Int >>>> >>>> store %0 to [initialization] %1 : $*MyClass >>>> store %0 to [assignment] %1 : $*MyClass >>>> store %0 to [trivial] %1 : $*Int >>>> >>>> John. >>> >>> The reason why I originally suggested to go the load_strong route is that >>> we already have load_weak, load_unowned instructions. If I could add a >>> load_strong instruction, then it would make sense to assign an engineer to >>> do a pass over all 3 of these instructions and combine them into 1 load >>> instruction. That is, first transform into a form amenable for >>> canonicalization and then canonicalize all at once. >>> >>> As you pointed out, both load_unowned and load_weak involve representation >>> changes in type (for instance the change of weak pointers to Optional<T>). >>> Such a change would be against the "spirit" of a load instruction to >>> perform such representation changes versus ownership changes. >>> >>> In terms of the properties that we actually want here, what is important is >>> that we can verify that no non-trivially typed values are loaded in an >>> unsafe unowned manner. That can be done also with ownership flags on >>> load/store. >>> >>> Does this sound reasonable: >>> >>> 1. We introduce two enums that define memory ownership changes, one for >>> load and one for store. Both of these enums will contain a [trivial] >>> ownership. >>> 2. We enforce in the verifier that non-trivial types must have a >>> non-trivial ownership modifier on any memory operations that they are >>> involved in. >> >> Sorry for not being explicit. I will not add new instructions, just >> modifiers. Assuming that this is agreeable to you, I am going to prepare a >> quick additional version of the proposal document. > > That sounds great, thanks. > > John.
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