[swift-dev] copy-on-write proposal
Erik Eckstein
eeckstein at apple.com
Tue Oct 18 18:38:13 CDT 2016
> On Oct 17, 2016, at 10:21 AM, Dave Abrahams <dabrahams at apple.com> wrote:
>
>
> on Mon Oct 17 2016, Erik Eckstein <eeckstein-AT-apple.com> wrote:
>
>> On Oct 16, 2016, at 2:05 PM, Dave Abrahams via swift-dev <swift-dev at swift.org> wrote:
>>
>>> on Thu Oct 13 2016, Joe Groff <swift-dev-AT-swift.org <http://swift-dev-at-swift.org/>> wrote:
>>>
>>>>> On Oct 11, 2016, at 4:48 PM, Erik Eckstein via swift-dev <swift-dev at swift.org> wrote:
>>>>>
>>>>> This is a proposal for representing copy-on-write buffers in
>>>>> SIL. Actually it’s still a draft for a proposal. It also heavily
>>>>> depends on how we move forward with SIL ownership.
>>>>> <CopyOnWrite.rst>
>>>>> If you have any comments, please let me know.
>>>>
>>>> The SIL-level design seems sensible to me at a glance. At the language
>>>> level, I think it would make more sense to treat this as an attribute
>>>> on class types rather than on properties in structs using the class. I
>>>> don't think many people reuse class definitions as both shared
>>>> reference types and as value type payloads,
>>>
>>> Foundation does, or would if they could.
>>>
>>>> but beyond that, I think that making it an attribute of classes would
>>>> put us into a better position to leverage the borrow model to enforce
>>>> the "mutable-only-when-unique" aspect of COW implementations. John
>>>> alluded to this in the "SIL address types and borrowing" thread:
>>>>
>>>>> I wonder if it would make more sense to make copy-on-write buffer
>>>>> references a move-only type, so that as long as you were just
>>>>> working with the raw reference (as opposed to the CoW aggregate,
>>>>> which would remain copyable) it wouldn't get implicitly copied
>>>>> anymore. You could have mutable and immutable buffer reference
>>>>> types, both move-only, and there could be a consuming checkUnique
>>>>> operation on the immutable one that, I dunno, returned an Either of
>>>>> the mutable and immutable versions.
>>>>>
>>>>> For CoW aggregates, you'd need some @copied attribute on the field
>>>>> to make sure that the CoW attribute was still copyable. Within the
>>>>> implementation of the type, though, you would be projecting out the
>>>>> reference immediately, and thereafter you'd be certain that you were
>>>>> borrowing / moving it around as appropriate.
>>>>
>>>> If 'copy-on-write' were a trait on classes, then we could distinguish
>>>> unique and nonunique references to the class. A unique reference would
>>>> act like a move-only type to prevent accidental loss of uniqueness.
>>>
>>> +1
>>>
>>>> We can also allow a copy-on-write class to have "mutating" methods,
>>>> and only allow mutation on unique references. It seems to me like,
>>>> exploring this direction, we could also come up with a way for the
>>>> high-level value-semantics operations on the struct to statically
>>>> indicate which methods are known to leave the value's buffers in a
>>>> unique state, or which return values that are uniquely owned, which
>>>> would give the optimizer more ability to avoid uniqueness checks
>>>> across calls without relying on inlining and IPO.
>>>
>>> That's pretty cool. However, I think there's nothing to prevent any
>>> mutating method from storing a copy of self in a global, so I think we'd
>>> need some participation from the programmer (either an agreement not to
>>> do that, or an explicit claim of uniqueness on exit) in order to
>>> identify operations that create/preserve uniqueness.
>>
>> If a mutating reference (like self in a mutating method) is move-only
>> then you would not be able to “copy” it to a global.
>
> Yes, a reference to a move-only type would work for this purpose.
>
>
>>> On Oct 16, 2016, at 2:01 PM, Dave Abrahams via swift-dev <swift-dev at swift.org> wrote:
>>>
>>>
>>> on Tue Oct 11 2016, Erik Eckstein <swift-dev-AT-swift.org> wrote:
>>>
>>>> This is a proposal for representing copy-on-write buffers in
>>>> SIL. Actually it’s still a draft for a proposal. It also heavily
>>>> depends on how we move forward with SIL ownership.
>>>>
>>>> :orphan:
>>>>
>>>> .. highlight:: sil
>>>>
>>>> ===================================
>>>> Copy-On-Write Representation in SIL
>>>> ===================================
>>>>
>>>> .. contents::
>>>>
>>>> Overview
>>>> ========
>>>>
>>>> This document proposes:
>>>>
>>>> - An ownership attribute to define copy-on-write (COW) buffers in Swift data
>>>> types.
>>>>
>>>> - A representation of COW buffers in SIL so that optimizations can take benefit
>>>> of it.
>>>>
>>>> The basic idea is to enable the SIL optimizer to reason about COW data types
>>>> in the same way as a programmer can do.
>>>> This means: a COW buffer can only be modified by its owning SIL value, because
>>>> either it's uniquely referenced or the buffer is copied before modified.
>>>>
>>>> .. note::
>>>> In the following the term "buffer" refers to a Swift heap object.
>>>> It can be any heap object, not necessarily a “buffer” with e.g. tail-allocated elements.
>>>>
>>>> COW Types
>>>> =========
>>>>
>>>> The basic structure of COW data types can be simplified as follows::
>>>>
>>>> class COWBuffer {
>>>> var someData: Int
>>>> ...
>>>> }
>>>>
>>>> struct COWType {
>>>> var b : COWBuffer
>>>>
>>>> mutating func change_it() {
>>>> if (!isUniquelyReferenced(b)) {
>>>> b = copy_buffer(b)
>>>> }
>>>> b.someData = ...
>>>> }
>>>> }
>>>>
>>>> Currently the COW behavior of such types is just defined by their implementation.
>>>> But there is no representation of this special behavior in the SIL.
>>>> So the SIL optimizer has no clue about it and cannot take advantage of it.
>>>>
>>>> For example::
>>>>
>>>> func foo(arr : [Int]) {
>>>> x = arr[0]
>>>> opaque_function()
>>>> y = arr[0] // can RLE replace this with y = x?
>>>> }
>>>>
>>>> If opaque_function() wants to change the contents of the array buffer it first
>>>> has to copy it.
>>>
>>> ...or determine that it's uniquely-referenced.
>>
>> In this specific example, if opqaue_function holds a reference to arr’s buffer, the buffer is not
>> uniquely-referenced.
>
> Right.
>
>>>
>>>> But the optimizer does not know it so it has to conservatively assume
>>>> that opaque_function() will write to the location of arr[0].
>>>>
>>>> Copy-on-write Ownership Attribute
>>>> =================================
>>>>
>>>> This section proposes an ownership attribute to define a copy-on-write buffer.
>>>>
>>>> Swift Syntax
>>>> ------------
>>>>
>>>> A COW buffer reference can be defined with a new ownership attribute for the
>>>> buffer variable declaration (similar to “weak” and “unowned”)::
>>>>
>>>> struct COWType {
>>>> copy_on_write var b : COWBuffer
>>>>
>>>> // ...
>>>> }
>>>>
>>>> The ``copy_on_write`` attribute is purely used for optimization purposes.
>>>> It does not change the semantics of the program.
>>>
>>> Presumably, it changes what code you can execute on `b` without invoking
>>> traps or undefined behavior. Otherwise, the optimizer wouldn't be able
>>> to do anything differently to take advantage of the annotation.
>>
>> That’s true.
>>
>>> What are the rules for writing code that uses `copy_on_write`?
>>
>> See below ("The rules for using ``copy_on_write`` and the built-ins are:”)
>
> Yeah, I got there, eventually. But just saying “doesn't change
> semantics” at this point in the proposal leaves a gap, because it does
> change semantic *requirements*. You should mention that.
>
>>>> .. note::
>>>>
>>>> “copy_on_write” is a working title. TODO: decide on the name.
>>>> Maybe it should be a @-attribute, like @copy_on_write?
>>>> Another question is if we should open this attribute for the public or just
>>>> use it internally in the library, because violating the implied rules
>>>> (see below) could break memory safety.
>>>>
>>>> Implementation
>>>> --------------
>>>>
>>>> The ``copy_on_write`` references can be represented in the AST as a special
>>>> ``StorageType``, just like how ``unowned`` and ``weak`` is represented.
>>>> The canonical type of a ``CopyOnWriteStorageType`` would be the referenced
>>>> buffer class type.
>>>>
>>>> In SIL the buffer reference will have type::
>>>>
>>>> $@sil_cow COWBuffer
>>>>
>>>> where ``COWBuffer`` is the type of the referenced heap object.
>>>>
>>>> Two conversion instructions are needed to convert from a ``@sil_cow`` reference
>>>> type to a regular reference type::
>>>>
>>>> cow_to_ref
>>>> ref_to_cow
>>>>
>>>> Again, this is similar to ``ref_to_unowned`` and ``unowned_to_ref``.
>>>>
>>>> For example the SIL code for::
>>>>
>>>> var c: COWType
>>>> let x = c.b.someData
>>>>
>>>> would be::
>>>>
>>>> %1 = struct_extract %1 : COWType, #COWType.b
>>>> %2 = cow_to_ref %1 : $@sil_cow COWBuffer
>>>> %3 = ref_element_addr %2 : $COWBuffer, #COWBuffer.someData
>>>> %4 = load %3 : $*Int
>>>>
>>>> The ``ref_to_cow`` instruction is needed to store a new buffer reference into a
>>>> COW type.
>>>>
>>>> COW Buffers and the Optimizer
>>>> =============================
>>>>
>>>> A reference to a COW buffer gives the optimizer additional information:
>>>>
>>>> *A buffer, referenced by a @sil_cow reference is considered to be immutable
>>>> during the lifetime of the reference.*
>>>
>>> This seems like much too broad a rule to allow inplace mutations of
>>> uniquely referenced buffers.
>>
>> The point is that all mutations must be guarded by an is_unique, which
>> takes the _address_ of the buffer reference as argument.
>> And the optimizer considers this instruction as a potential write to the buffer reference.
>> The effect is that the lifetime of a buffer reference (as a SIL value)
>> will not outlive a is_unique - regardless if this is inside a called
>> function or inlined.
>
> I don't see how that allows me to mutate a uniquely referenced buffer held
> in a @sil_cow reference, given what you wrote above.
You would not be able to get a reference to a mutable buffer by reading the COW type’s @sil_cow field.
Instead you would only get such a reference as a result of the is_unique instruction/builtin. Or, of course, by creating a new buffer.
I’m not sure if this was the question, though.
Plus: we will have an explicit conversion instruction (start_unique) to convert an immutable reference to a mutable referece.
A SIL optimization can replace an is_unique with this instruction if it can prove that the reference is already unique at that point.
>
>
>>> Unless you mean the reference is
>>> immutable, rather than the storage being referred to by it.
>>>
>>>> This means any address derived from a ``cow_to_ref`` instruction can be
>>>> considered to point to immutable memory.
>>>>
>>>> Some examples of optimizations which will benefit from copy-on-write
>>>> representation in SIL:
>>>>
>>>> - Redundant load elimination
>>>>
>>>> RLE can assume that opaque code does not modify a COW buffer.
>>>
>>> How do you distinguish “opaque code” from “code that is meant to
>>> modify the buffer and might do so in place if it's uniquely-referenced?”
>>
>> Again, the is_unique which takes the address of the reference, will
>> guarantee that during the lifetime of a buffer there are no
>> modifications of the buffer.
>
> Again, that sounds like it rules out inplace modification of uniquely
> referenced buffers.
>
>>
>>
>>>
>>>> Example::
>>>>
>>>> %2 = cow_to_ref %1 : $@sil_cow COWBuffer
>>>> %3 = ref_element_addr %2 : $COWBuffer, #someData
>>>> %4 = load %3 : $*Int
>>>> %5 = apply %foo() // Cannot overwrite memory location %3
>>>> %6 = load %3 : $*Int // Can be replaced by %4
>>>>
>>>> Currently we do some ad-hoc optimizations for array, based on semantics,
>>>> like array count propagation. These hacks would not be needed
>>>> anymore.
>>>
>>> W0000000000000000000000t.
>>>
>>>> Note that it’s not required to check if a ``cow_to_ref`` reference (or a
>>>> projected address) escapes. Even if it escapes, it will reference immutable
>>>> memory.
>>>>
>>>> - CSE, loop hoisting
>>>>
>>>> Similar to RLE: the optimizer can assume that opaque code cannot modify a
>>>> COW buffer
>>>
>>> Same question here as above, then.
>>>>
>>>> - ARC optimization
>>>>
>>>> Knowing that some opaque code cannot overwrite a reference in the COW buffer
>>>> can remove retain/release pairs across such code::
>>>>
>>>> %2 = cow_to_ref %1 : $@sil_cow COWBuffer
>>>> %3 = ref_element_addr %2 : $COWBuffer, #someRef
>>>> %4 = load_strong %3 : $*MyClass // Can do a load_strong [guarantee]
>>>> %5 = apply %foo() // Cannot overwrite someRef and dealloc the object
>>>> // Use %4
>>>> destroy_value %4 : $MyClass
>>>>
>>>> Scoping instructions
>>>> --------------------
>>>>
>>>> To let the optimizer reason about the immutability of the COW buffer, it is
>>>> important to *bind* the lifetime of the buffer content to the lifetime of the
>>>> buffer reference. For example::
>>>>
>>>> %b1 = load %baddr : $@sil_cow COWBuffer // load the buffer reference
>>>> // load something from %b1
>>>> %a = apply %foo(%baddr : $@sil_cow COWBuffer)
>>>> %b2 = load %baddr : $@sil_cow COWBuffer // load the buffer reference again
>>>> // load something from %b2
>>>>
>>>> The question is: can RLE forward the load of the buffer reference and replace
>>>> ``%b2`` with ``%b1``? It must not be able to do so if ``foo()`` modifies the
>>>> buffer.
>>>>
>>>> To enforce this restriction, the scope of any buffer modification must be
>>>> enclosed in a pair of SIL instructions. Those instructions define the scope
>>>> of the mutation. Both instructions take the *address* of the buffer
>>>> reference as operand and act as a potential write to the buffer reference.
>>>>
>>>> The purpose of the scoping instructions is to strictly separate the liferanges
>>>> of references to an immutable buffer and references to the mutable buffer.
>>>
>>> Looks reasonable.
>>>
>>>> The following example shows why the scoping instructions (specifically the
>>>> end-of-scope instruction) are required to prevent loop-hoisting from
>>>> interleaving mutable and immutable liferanges::
>>>>
>>>> // there should be a begin-of-scope %baddr
>>>> %mut_b = load %baddr
>>>> store %x to %mut_b // modification of the buffer
>>>> // there should be a end-of-scope %baddr
>>>>
>>>> loop {
>>>> %b = load %baddr
>>>> %y = load %b // load from the buffer
>>>> ...
>>>> }
>>>>
>>>> If there is no end-of-scope instruction, loop hoisting could do::
>>>>
>>>> %mut_b = load %baddr
>>>> %b = load %baddr // moved out of the loop
>>>> store %x to %mut_b
>>>>
>>>> loop {
>>>> %y = load %b
>>>> ...
>>>> }
>>>>
>>>> Now the optimizer assumes that ``%b`` references an immutable buffer, so it could
>>>> also hoist the load::
>>>>
>>>> %mut_b = load %baddr
>>>> %b = load %baddr
>>>> %y = load %b // Wrong! Will be overwritten by the following store
>>>> store %x to %mut_b
>>>>
>>>> loop {
>>>> ...
>>>> }
>>>>
>>>>
>>>> The following sections describe two alternatives to implement the scoping.
>>>>
>>>> Scoping Alternative 1: Explicit Built-ins
>>>> -----------------------------------------
>>>>
>>>> SIL instructions
>>>> ^^^^^^^^^^^^^^^^
>>>>
>>>> The existing ``is_unique`` instruction is changed to a terminator instruction::
>>>>
>>>> bb0:
>>>> is_unique_addr_br %0 : $*@sil_cow COWBuffer, bb1, bb2 // %0 is the address of the COWBuffer reference
>>>> bb1(%1 : $COWBuffer): // the true-block. The payload %1 is the unique reference. Physically identical to "load %0”
>>>> // usually empty
>>>> br bb3(%1 : $COWBuffer)
>>>> bb2: // the false-block
>>>> // usually contains:
>>>> %2 = apply %copy_buffer
>>>> %3 = cow_to_ref %2
>>>> store_strong %3 to %0 : $*@sil_cow COWBuffer
>>>> br bb3(%2 : $COWBuffer)
>>>> bb3(%4 : $COWBuffer):
>>>> // Modify the buffer referenced by %4
>>>> // ...
>>>>
>>>> The end-of-scope instruction is::
>>>>
>>>> end_unique_addr %0 : $*COWBuffer
>>>>
>>>> It is important that the references to the unique buffers (``%1``, ``%2``) must
>>>> not outlive ``end_unique_addr``. In most cases this can be check by the SIL
>>>> verifier.
>>>>
>>>> The two instructions must be paired properly but not necessarily in the
>>>> same function.
>>>>
>>>> The purpose of an ``is_unique_addr_br`` - ``end_unique_addr`` pair is to
>>>> separate the lifetimes of mutable and immutable accesses to the COW buffer.
>>>> Both instructions take an address to the COW buffer reference and are
>>>> considered as potential stores to the reference.
>>>> This makes sure that the SIL optimizer cannot mix-up buffer reference lifetimes
>>>> across these instructions.
>>>> For example, RLE cannot combine two buffer loads which are interleaved with
>>>> a ``is_unique_addr_br``::
>>>>
>>>> %1 = load_strong %0 : $*@sil_cow COWBuffer
>>>> // do something with %1
>>>> …
>>>> is_unique_addr_br %0 : $*@sil_cow COWBuffer
>>>> …
>>>> %2 = load_strong %0 : $*@sil_cow COWBuffer // RLE cannot replace this with %1
>>>>
>>>> Another important thing is that the COW buffer can only be mutated by using the
>>>> reference of the ``is_unique_addr_br`` true-block argument.
>>>> The COW buffer cannot be modified by simply loading/extracting the reference
>>>> from the COWType.
>>>> Example::
>>>>
>>>> %1 = load_strong %0 : $*COWBuffer
>>>> %2 = cow_to_ref %1 : $@sil_cow COWBuffer
>>>> %3 = ref_element_addr %2 : $COWBuffer, #someData
>>>> store %7 : $Int to %3 : $*Int // Violation!
>>>>
>>>> Most obvious violations to this constraint can be catched by the SILVerifier.
>>>>
>>>> The ``_addr`` variants of the instructions also have a non-addr counterpart::
>>>>
>>>> is_unique_br %0 : $COWBuffer, bb1, bb2. // consumes %0 and produces the true-block arg as owned
>>>>
>>>> %1 = end_unique %0 : $COWBuffer // consumes %0 and produces %1 as owned
>>>>
>>>> These instructions are generated by Mem2reg (or a similar optimization)
>>>> in case the COW value is stored (in a temporary alloc_stack location)
>>>> just for the sake of passing an address to ``is_unique_addr_br`` and
>>>> ``end_unique_addr``.
>>>> For example in the following code, where the COW data is passed as-value and
>>>> all the mutating functions are inlined::
>>>>
>>>> func foo(arr : [Int], x: Int) {
>>>> arr[0] = 27
>>>> …
>>>> y = arr[x]
>>>> …
>>>> }
>>>>
>>>> Finally it’s probably a good idea to add an instruction for converting an
>>>> immutable reference to a mutable reference::
>>>>
>>>> %1 = start_unique %0 : $COWBuffer // consumes %0 and produces %1 : $COWBuffer as owned
>>>>
>>>> which is basically just a simpler representation of the following pattern::
>>>>
>>>> bb0:
>>>> is_unique_br %0 : $@sil_cow COWBuffer, bb1, bb2
>>>> bb1(%1 : $COWBuffer):
>>>> … // main control flow continues here
>>>> bb2:
>>>> unreachable
>>>>
>>>> An optimizations, which eliminate uniqueness checks, would replace a
>>>> ``is_unique_br`` by a ``start_unique``.
>>>>
>>>> Built-ins
>>>> ^^^^^^^^^
>>>>
>>>> A COW type implementor can generate the new instructions by using a set of built-ins::
>>>>
>>>> func isUnique<BufferType>(_ buffer: inout BufferType) -> BufferType?
>>>> func endUnique<BufferType>(_ buffer: inout BufferType)
>>>>
>>>> For example::
>>>>
>>>> struct COWType {
>>>> copy_on_write var b : COWBuffer
>>>>
>>>> mutating func makeMutable() -> COWBuffer {
>>>> if let uniqueBuffer = isUnique(&self.b) {
>>>> return uniqueBuffer
>>>> }
>>>> let copiedBuffer = copyBuffer(self.b)
>>>> self.b = copiedBuffer
>>>> return copiedBuffer
>>>> }
>>>>
>>>> mutating func setSomeData(x: Int) {
>>>> let uniqueBuffer = makeMutable()
>>>> uniqueBuffer.someData = x
>>>> endUnique(&self.b)
>>>> }
>>>> }
>>>
>>> This seems reasonable, but it also looks like the compiler could do the
>>> `endUnique` dance for us based, e.g., on the mutability of methods.
>>
>> I agree, that would be ideal, e.g. the compiler could insert the endUnique at the end of an inout
>> scope.
>>
>>>
>>>> The ``isUnique`` built-in returns an optional unique buffer reference.
>>>> Physically this is the COW buffer which is passed as the inout argument.
>>>> The result is nil if the buffer is not uniquely referenced.
>>>> In this case usually the original buffer is copied and the reference to the
>>>> copy is written back to the original buffer reference location
>>>> (``self.b = copiedBuffer``).
>>>> Starting at the point of the write-back, the reference to the copy also becomes
>>>> a unique buffer reference.
>>>>
>>>> The ``isUnique`` built-in is lowered to the ``is_unique_addr_br`` pattern which
>>>> constructs the Optional in the successor blocks. Using ``isUnique`` in an
>>>> if-let (as shown above) will end up in two consecutive CFG "diamonds".
>>>> Simplify-CFG can combine those into a single ``is_unique_addr_br`` diamond.
>>>>
>>>> .. note::
>>>> This makes the definition of the unique buffer location lifetime a little bit
>>>> problematic, because the false-branch of ``isUnique`` is not equivalent to
>>>> the false-branch of the ``is_unique_addr_br`` instruction (before SimplifyCFG
>>>> can do its job).
>>>
>>> I don't know what the implications of these diamonds and the problem
>>> described above might be, FWIW.
>>>
>>>> The rules for using ``copy_on_write`` and the built-ins are:
>>>>
>>>> 1. ``isUnique`` must be paired with ``endUnique``, but not necessarily in the
>>>> same function.
>>>>
>>>> 2. The COW buffer may only be mutated by using the unique buffer reference.
>>>>
>>>> 3. The COW buffer must not be mutated outside the ``isUnique`` - ``endUnique``
>>>> pair.
>>>>
>>>> 4. During the lifetime of the unique buffer reference, the original COW buffer
>>>> reference must not be used in any way, e.g. for reading from the buffer.
>>>>
>>>> Note that the lifetime of the unique buffer reference does not include the
>>>> part between the begin of the ``isUnique`` false-branch and the write-back
>>>> of the copy. This means is okay to read from the buffer (using ``self.b``)
>>>> for the purpose of copying.
>>>>
>>>> Examples::
>>>>
>>>> mutating func setSomeData(x: Int) {
>>>> let uniqueBuffer = makeMutable()
>>>> uniqueBuffer.someData = x
>>>> // violates rule 1
>>>> }
>>>>
>>>> mutating func setSomeData(x: Int) {
>>>> makeMutable()
>>>> self.b.someData = x // violates rule 2
>>>> endUnique(&self.b)
>>>> }
>>>>
>>>> mutating func setSomeData(x: Int) {
>>>> let uniqueBuffer = makeMutable()
>>>> uniqueBuffer.someData = x
>>>> endUnique(&self.b)
>>>> uniqueBuffer.someData = 27 // violates rule 3
>>>> }
>>>>
>>>> mutating func incrementSomeData() {
>>>> let uniqueBuffer = makeMutable()
>>>> uniqueBuffer.someData = self.b.someData + 1 // violates rule 4
>>>> endUnique(&self.b)
>>>> }
>>>
>>> It would be instructive to write down the *correct* code for these
>>> operations.
>>
>> added to my todo list.
>>
>>>
>>>> The intention of the rules is to ensure that there is no overlap of a
>>>> "read-only" life-range with a "mutable" life-range of the buffer reference.
>>>> It’s the responsibility of the implementor to follow the rules.
>>>> But the compiler (a mandatory diagnostics pass and the SIL verifier) can
>>>> statically detect rule violations in obvious cases (with inter-procedural
>>>> analysis maybe even in most cases).
>>>>
>>>> This approach would require to change some of the internals of our
>>>> current COW data structures in the stdlib (Array, Dictionary, etc.).
>>>> For example, the Array make_mutable semantic functions currently do not return
>>>> the unique buffer.
>>>
>>> No big deal.
>>>
>>>> Scoping Alternative 2: Implicit Inout Scopes
>>>> --------------------------------------------
>>>>
>>>> There is an idea (proposal?) to change the representation of inout variables
>>>> in SIL. This is independent of this proposal, but can be helpful for the
>>>> purpose of defining the scope of a COW mutation.
>>>>
>>>> The basic idea is that SILGen inserts scoping instructions for *all* inout
>>>> variables. And those scoping instructions can be used to define the mutating
>>>> scope of a COW buffer.
>>>>
>>>> The scoping instructions which are inserted by SILGen for an inout scope are::
>>>>
>>>> begin_exclusive
>>>> end_exclusive
>>>>
>>>> Simliar to ``is_unique_addr_br`` and ``end_unique_addr``, those instructions take the
>>>> address of the inout variable as argument. For the optimizer those instructions
>>>> look like potential writes to the inout variable.
>>>>
>>>> The implementor of a COW type has to follow the rule that the COW buffer may
>>>> only be modified in mutating functions of the COW type. But this is the case
>>>> anyway because any modification needs a uniqueness check and this can only be
>>>> done in mutating functions.
>>>>
>>>> Example::
>>>>
>>>> // > mutating func setSomeData(x: Int) {
>>>> // Accepts a unique reference to the array value (avoiding refcount operations)
>>>> sil @setSomeData : $(Int, @inout Array) -> () {
>>>> bb_entry(%x : Int, %arrayref : $*Array<T>) // Begin scope #0
>>>>
>>>> // > makeMutable() (inlined)
>>>> // Forward the unique reference to the `self` array value, still avoiding refcount operations.
>>>> // Begin the inlined exclusive scope (could be trivially removed).
>>>> begin_exclusive %arrayref : $*Array<T> // Begin scope #1
>>>>
>>>> // > if !isUnique(&self._storage) {
>>>> // Extract a unique inout reference to the class reference to the array storage.
>>>> // This begins the isUnique() argument's exclusive scope. The memory is already exclusive
>>>> // but the scope helps ensure this is the only alias to _storage.
>>>> %arrayref._storageref = struct_element_addr [exclusive] %arrayref, #Array._storage
>>>>
>>>> // Uniqueness checking requires an inout reference to the class reference.
>>>> // The is_unique instruction does not need to create a new storage reference.
>>>> // It's only purpose is to check the RC count, ensure that the checked reference
>>>> // is inout, and prevent the inout scope from being optimized away.
>>>> %isuniq = is_unique %arrayref._storageref : $*@sil_cow ArrayStorage<T>
>>>>
>>>> // End the isUnique argument's exclusive scope (can also be trivially removed).
>>>> end_exclusive %arrayref._storageref : $*@sil_cow ArrayStorage<T>
>>>>
>>>> br %isuniq, bb_continue, bb_slow
>>>>
>>>> bb_slow:
>>>> // > self._storage = copyBuffer(self._storage)
>>>> // Produce a new class reference to storage with verifiably unique RC semantics.
>>>> %copied_storage_class = alloc_ref ...
>>>> // A begin/end exclusive scope is implicit in store [assign].
>>>> store [assign] %copied_storage_class to %arrayref._storageref
>>>> br bb_continue
>>>>
>>>> bb_continue:
>>>>
>>>> // This marks the end of makeMutable's inout `self` scope. Because Array
>>>> // contains a "copy_on_write" property, the SIL verifier needs to
>>>> // prove that %arrayref.#_storage has not escaped at this point. This
>>>> // is equivalent to checking that %arrayref itself is not copied, and
>>>> // checking each projection of the "copy_on_write" storage property
>>>> // (%arrayref._storageref) is not copied. Or, if any copies are present,
>>>> // they must be consumed within this scope.
>>>> end_exclusive %arrayref : $*Array<T> // End scope #1
>>>>
>>>> // > self._storage.someData = x
>>>> // An _addr instruction with one load/store use doesn't really need its own scope.
>>>> %arrayref._storageref = struct_element_addr %arrayref, #Array._storage
>>>>
>>>> // ARC optimization can promote this to a borrow, replacing strong_release with end_borrow.
>>>> %arrayref.cow_storage = load [copy] %arrayref._storageref : $*@sil_cow ArrayStorage
>>>> %arrayref._storage = cow_to_ref %arrayref.cow_storage : $@sil_cow ArrayStorage
>>>>
>>>> // Write some data into the CoW buffer.
>>>> // (For simplicity, pretend ArrayStorage has a "someData" field).
>>>> // A single-use _addr instruction, so no scope.
>>>> %somedata_addr = ref_element_addr %arrayref._storage, #someData
>>>> // A store with an implicit [exclusive] scope.
>>>> store [assign] %x to %somedata_addr
>>>>
>>>> strong_release %arrayref._storage : $*ArrayStorage<T>
>>>>
>>>> // End the isUnique argument's exclusive scope.
>>>> // The same verification is needed here, but the inner scope would be eliminated.
>>>> end_exclusive %arrayref : $*Array<T> // End scope #0
>>>>
>>>> In general this approach looks more "user-friendly" than the first
>>>> alternative.
>>>
>>> Well, I can't really tell, because you haven't shown the Swift code that
>>> generates this SIL.
>>>
>>>> But it depends on implementing the general feature to insert the inout
>>>> scoping instructions. Also, we still have to think through all the
>>>> details of this approach.
>>>
>>> FWIW, I am convinced we will need (and get) a stricter inout model that
>>> would be conducive to inserting the scoping instructions.
>>>
>>>
>>>> Dependency between a buffer reference to the scope-begin
>>>> --------------------------------------------------------
>>>
>>> You can only have a dependency between two things, but as phrased “a
>>> buffer reference to the scope-begin” sounds like one thing. s/to/and/
>>> would fix it.
>>>
>>>> With both alternatives there is no explicit dependency from a buffer reference
>>>> to a scope-begin instruction::
>>>>
>>>> %b_cow = load %baddr
>>>> %b = cow_to_ref %b_cow
>>>> %x = load %b // No dependency between this...
>>>> ...
>>>> begin_exclusive %baddr // ... and this instruction.
>>>> ...
>>>>
>>>> So in theory the optimizer is free to reschedule the instructions::
>>>>
>>>> %b_cow = load %baddr
>>>> %b = cow_to_ref %b_cow
>>>> ...
>>>> begin_exclusive %baddr
>>>> %x = load %b // Wrong! Buffer could be modified here
>>>> ...
>>>>
>>>> We still have to figure out how to cope with this.
>>>>
>>>> - We could add an end-of-lifetime instruction for a COW buffer reference, which
>>>> the optimizer may not move over a begin-of-scope instruction.
>>>>
>>>> - Or we just define the implicit rule for the optimizer that any use of a COW
>>>> reference may not be moved over a begin-of-scope instruction.
>>>>
>>>> Preconditions
>>>> =============
>>>>
>>>> To benefit from COW optimizations in the stdlib Array, Set and Dictionary data
>>>> structures we first need eager bridging, meaning getting rid of the bridged
>>>> buffer.
>>>
>>> As you know I'm very much in favor of eager bridging, but I don't see
>>> why this would be dependent on it.
>>
>> We could use copy_on_write with eager bridging, but I don’t think it will give any benefits to the
>> optimizer.
>> For example, the SIL code to get from an Array to a native
>> ContiguousArrayStorage reference is pretty hard to understand for the
>> optimizer (involves low level bit operations, etc.).
>
> It wouldn't need to do low-level bit operations if our enums were
> capable/controllable enough. I'm just saying, there's no reason we
> couldn't give the optimizer something to work with that has higher level
> semantics than what we currently do.
>
>>>> At least for Array this is implemented as low-level bit operations and
>>>> optimizations cannot reason about it (e.g. finding a reasonable
>>>> RC-root for the buffer reference).
>>>>
>>>> Another thing is that we currently cannot use ``copy_on_write`` for Array
>>>> because of pinning. Array pins it’s buffer when passing an element address to
>>>> an inout parameter. This allows the array buffer to be modified even if its
>>>> reference count is > 1. With ``copy_on_write``, a programmer could break memory
>>>> safety when violating the inout rule. Example::
>>>>
>>>> var arr = [MyClass()] // a global array
>>>>
>>>> foo(&arr[0]) // Pins the buffer of arr during the call
>>>>
>>>> func foo(_ x: inout MyClass) -> Int {
>>>> let b = arr // The ref-count of the buffer is not incremented, because it is pinned!
>>>> let r = b[0] // optimizer removes the retain of r because it thinks the following code cannot modify b
>>>> arr.removeAll() // does not copy the array buffer and thus de-allocates r
>>>> return r.i // use-after-free!
>>>> }
>>>
>>> I only know of one way to resolve inout and pinning:
>>>
>>> * Semantically, references are replaced with a trap value when entering
>>> an inout context so that all inout values are provably unique
>>> references in the absence of unsafe code. We drop pinning and provide
>>> explicit operations that provide simultaneous lvalue accesses to
>>> distinct regions, e.g. c.swap(i1, i2) where i1 and i2 are indices.
>>>
>>> If there are other ideas out there, I'd like to hear them. If not, we
>>> should probably decide that this is what we're doing so that we can move
>>> forward without this looming uncertainty.
>>>
>>> --
>>> -Dave
>>>
>>> _______________________________________________
>>> swift-dev mailing list
>>> swift-dev at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-dev
>>
>
> --
> -Dave
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