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Enforce Exclusive Access to Memory

Introduction

In Swift 3, it is possible to modify a variable while it's being used or modified by another part of the program. This can lead to unexpected and confusing results. It also forces a great deal of conservatism onto the implementation of the compiler and the standard libraries, which must generally ensure the basic soundness of the program (no crashes or undefined behavior) even in unusual circumstances.

We propose that Swift should instead enforce a general rule that potential modifications of variables must be exclusive with any other access to that variable.

This proposal is a core part of the Ownership feature, which was described in the ownership manifesto. That document presents the high-level objectives of Ownership, develops a more rigorous theory of memory access in Swift, and applies it in detail to a variety of different language features. In that document, the rule we're proposing here is called the Law of Exclusivity. We will not be going into that level of detail in this proposal. Instead, we will lay out the basic rule, how it will be enforced, and the implications for programming in Swift. It should be possible to understand this proposal without actually having read the ownership manifesto at all. That said, if you are interested in the technical details, that document is probably the right place to turn.

Motivation

Instantaneous and non-instantaneous accesses

On a basic level, Swift is an imperative language which allows programmers to directly access mutable memory.

Many of the language features that access memory, like simply loading from or assigning to a variable, are "instantaneous". This means that, from the perspective of the current thread, the operation completes without any other code being able to interfere. For example, when you assign to a stored property, the current value is just replaced with the new value. Because arbitrary other code can't run during an instantaneous access, it's never possible for two instantaneous accesses to overlap each other (without introducing concurrency, which we'll talk about later). That makes them very easy to reason about.

However, not all accesses are instantaneous. For example, when you call a mutating method on a stored property, it's really one long access to the property: self just becomes another way of referring to the property's storage. This access isn't instantaneous because all of the code in the method executes during it, so if that code manages to access the same property again, the accesses will overlap. There are several language features like this already in Swift, and Ownership will add a few more.

Examples of problems due to overlap

Here's an example:

// These are simple global variables.
var global: Int = 0
var total: Int = 0

extension Int {
  // Mutating methods access the variable they were called on
  // for the duration of the method.
  mutating func increaseByGlobal() {
    // Any accesses they do will overlap the access to that variable.

    total += self // Might access 'total' through both 'total' and 'self'
    self += global // Might access 'global' through both 'global' and 'self'
  }
}

If self is total or global, the low-level semantics of this method don't change, but the programmer's high-level understanding of it almost certainly does. A line that superficially seems to not change 'global' might suddenly start doubling it! And the data dependencies between the two lines instantly go from simple to very complex. That's very important information for someone maintaining this method, who might be tempted to re-arrange the code in ways that seem equivalent. That kind of maintenance can get very frustrating because of overlap like this.

The same considerations apply to the language implementation. The possibility of overlap means the language has to make pessimistic assumptions about the loads and stores in this method. For example, the following code avoids a seemingly-redundant load, but it's not actually equivalent because of overlap:

    let value = self
    total += value
    self = value + global

Because these variables just have type Int, the cost of this pessimism is only an extra load. If the types were more complex, like String, it might mean doing extra copies of the String value, which would translate to extra retains and releases of the string's buffer; in a more complex example, that could even lead to the underlying data being copied unnecessarily.

In the above examples, we've made the potentially-overlapping accesses obvious, but they don't have to be. For example, here is another method that takes a closure as an argument:

extension Array {
  mutating func modifyElements(_ closure: (inout Element) -> ()) {
    var i = startIndex
    while i != endIndex {
      closure(&self[i])
      i = index(after: i)
    }
  }
}

This method's implementation seems straightforwardly correct, but unfortunately it doesn't account for overlap. Absolutely nothing prevents the closure from modifying self during the iteration, which means that i can suddenly become an invalid index, which could lead to all sorts of unwanted behavior. Even if this never happen in reality, the fact that it's possible means that the implementation is blocked from pursuing all sorts of important optimizations.

For example, the compiler has an optimization that "hoists" the uniqueness check on a copy-on-write collection from the inside of a loop (where it's run on each iteration) to the outside (so that it's only checked once, before the loop begins). But that optimization can't be applied in this example because the closure might change or copy self. The only realistic way to tell the compiler that that can't happen is to enforce exclusivity on self.

The same considerations that apply to self in a mutating method also apply to inout parameters. For example:

open class Person {
  open var title: String
}

func collectTitles(people: [Person], into set: inout Set<String>) {
  for person in people {
    set.insert(person.title)
  }
}

This function mutates a set of strings, but it also repeatedly calls a class method. The compiler cannot know how this method is implemented, because it is open and therefore overridable from an arbitrary module. Therefore, because of overlap, the compiler must pessimistically assume that each of these method calls might somehow find a way to modify the original variable that set was bound to. (And if the method did manage to do so, the resulting strange behavior would probably be seen as a bug by the caller of collectTitles.)

Eliminating non-instantaneous accesses?

If non-instantaneous accesses create all of these problems with overlapping accesses, should we just eliminate non-instantaneous accesses completely? Well, no, and there's two big reasons why not. In order to make something like a mutating method not access the original storage of self for the duration of the method, we would need to make it access a temporary copy instead, which we would assign back to the storage after the method is complete. That is, suppose we had the following Swift code:

var numbers = [Int]()
numbers.appendABunchOfStuff()

Currently, behind the scenes, this is implemented somewhat like the following C code:

struct Array numbers = _Array_init();
_Array_appendABunchOfStuff(&numbers);

You can see clearly how _Array_appendABunchOfStuff will be working directly with the storage of numbers, creating the abstract possibility of overlapping accesses to that variable. To prevent this in general, we would need to pass a temporary copy instead:

struct Array numbers = _Array_init();
struct Array temp = _Array_copy(numbers);
_Array_appendABunchOfStuff(&temp);
_Array_assign(&numbers, temp);

Like we said, there's two big problems with this.

The first problem is that it's awful for performance. Even for a normal type, doing extra copies is wasteful, but doing it with Array is even worse because it's a copy-on-write type. The extra copy here means that there will be multiple references to the buffer, which means that _Array_appendABunchOfStuff will be forced to copy the buffer instead of modifying it in place. Removing these kinds of copies, and making it easier to reason about when they happen, is a large part of the goal of the Ownership feature.

The second problem is that it doesn't even eliminate the potential confusion. Suppose that _Array_appendABunchOfStuff somehow reads or writes to numbers (perhaps because numbers is captured in a closure, or it's actually a global variable or a class property or something else that can be potentially accessed from anywhere). Because the method is now modifying the copy in temp, any reads it makes from numbers won't see any of the changes it's made to temp, and any changes it makes to numbers will be silently lost when it returns and the caller unconditionally overwrites numbers with temp.

Consequences of non-instantaneous accesses

So we have to accept that accesses can be non-instantaneous. That means programmers can write code that would naturally cause overlapping accesses to the same variable. We currently allow this to happen and make a best effort to live with the consequences. The costs, in general, are a lot of complexity and lost performance.

For example, the Array type has an optimization in its subscript operator which allows callers to directly access the storage of array elements. This is a very important optimization which, among other things, allows arrays to efficiently hold values of copy-on-write types. However, because the caller can execute arbitrary code while they're working with the array element storage, and that code might do something like assign a new value to the original array variable and therefore drop the last reference to the array buffer, this optimization has to create a new strong reference to the buffer until the caller is done with the element, which itself causes a whole raft of complexity.

Similarly, when the compiler is optimizing a mutating method, it has to assume that an arbitrary call might completely rewrite self. This makes it very difficult to perform any meaningful optimization at all, especially in generic code. It also means that the compiler must generally emit a large number of conservative copies just in case things are modified in unexpected ways.

Furthermore, the possibility of overlapping accesses has a continued impact on language evolution. Many of the features laid out in the Ownership manifesto rely on static guarantees that Swift simply cannot make without stronger rules about when a variable can be modified.

Therefore we think it best to simply disallow overlapping accesses as best as we can.

Proposed solution

We should add a rule to Swift that two accesses to the same variable are not allowed to overlap unless both accesses are reads. By "variable", we mean any kind of mutable memory: global variables, local variables, class and struct properties, and so on.

This rule should be enforced as strongly as possible, depending on what sort of variable it is:

  • Local variables, inout parameters, and struct properties can generally enforce the rule statically. The compiler can analyze all the accesses to the variable and emit an error if it sees any conflicts.

  • Class properties and global variables will have to enforce the rule dynamically. The runtime can keep track of what accesses are underway and report any conflicts. Local variables will sometimes have to use dynamic enforcement when they are captured in closures.

  • Unsafe pointers will not use any active enforcement; it is the programmer's responsibility to follow the rule.

  • No enforcement is required for immutable memory, like a let binding or property, because all accesses must be reads.

Examples:

var x = 0, y = 0

// NOT A CONFLICT.  These two accesses to 'x' are both reads.
// Each completes instantaneously, so the accesses do not overlap and
// therefore do not conflict.  Even if they were not instantaneous, they
// are both reads and therefore do no conflict.
let z = x + x

// NOT A CONFLICT.  The right-hand side of the assignment is a read of
// 'x' which completes instantaneously.  The assignment is a write to 'x'
// which completes instantaneously.  The accesses do not overlap and
// therefore do not conflict.
x = x

// NOT A CONFLICT.  The right-hand side is a read of 'x' which completes
// instantaneously.  Calling the operator involves passing 'x' as an inout
// argument; this is a write access for the duration of the call, but it does
// not begin until immediately before the call, after the right-hand side is
// fully evaluated.  Therefore the accesses do not overlap and do not conflict.
x += x

// CONFLICT.  Passing 'x' as an inout argument is a write access for the
// duration of the call.  Passing the same variable twice means performing
// two overlapping write accesses to that variable, which therefore conflict.
swap(&x, &x)

extension Int {
  mutating func assignResultOf(_ function: () -> Int) {
    self = function()  
  }
}

// CONFLICT.  Calling a mutating method on a value type is a write access
// that lasts for the duration of the method.  The read of 'x' in the closure
// is evaluated while the method is executing, which means it overlaps
// the method's formal access to 'x'.  Therefore these accesses conflict.
x.assignResultOf { x + 1 }

Detailed design

Concurrency

Swift has always considered read/write and write/write races on the same variable to be undefined behavior. It is the programmer's responsibility to avoid such races in their code by using appropriate thread-safe programming techniques.

We do not propose changing that. Dynamic enforcement is not required to detect concurrent conflicting accesses, and we propose that by default it should not make any effort to do so. This should allow the dynamic bookkeeping to avoid synchronizing between threads; for example, it can track accesses in a thread-local data structure instead of a global one protected by locks. Our hope is that this will make dynamic access-tracking cheap enough to enable by default in all programs.

The implementation should still be permitted to detect concurrent conflicting accesses, of course. Some programmers may wish to use an opt-in thread-safe enforcement mechanism instead, at least in some build configurations.

Any future concurrency design in Swift will have the elimination of such races as a primary goal. To the extent that it succeeds, it will also define away any specific problems for exclusivity.

Value types

Calling a method on a value type is an access to the entire value: a write if it's a mutating method, a read otherwise. This is because we have to assume that a method might read or write an arbitrary part of the value. Trying to formalize rules like "this method only uses these properties" would massively complicate the language.

For similar reasons, using a computed property or subscript on a value type generally has to be treated as an access to the entire value. Whether the access is a read or write depends on how the property/subscript is used and whether either the getter or the setter is mutating.

Accesses to different stored properties of a struct or different elements of a tuple are allowed to overlap. However, note that modifying part of a value type still requires exclusive access to the entire value, and that acquiring that access might itself prevent overlapping accesses. For example:

struct Pair {
  var x: Int
  var y: Int
}

class Paired {
  var pair = Pair(x: 0, y: 0)
}

let object = Paired()
swap(&object.pair.x, &object.pair.y)

Here, initiating the write-access to object.pair for the first argument will prevent the write-access to object.pair for the second argument from succeeding because of the dynamic enforcement used for the property. Attempting to make dynamic enforcement aware of the fact that these accesses are modifying different sub-components of the property would be prohibitive, both in terms of the additional performance cost and in terms of the complexity of the implementation.

However, this limitation can be worked around by binding object.pair to an inout parameter:

func modifying<T>(_ value: inout T, _ function: (inout T) -> ()) {
  function(&value)
}

modifying(&object.pair) { pair in swap(&pair.x, &pair.y) }

This works because now there is only a single access to object.pair and because, once the the inout parameter is bound to that storage, accesses to the parameter within the function can use purely static enforcement.

We expect that workarounds like this will only rarely be required.

Note that two different properties can only be assumed to not conflict when they are both known to be stored. This means that, for example, it will not be allowed to have overlapping accesses to different properties of a resilient value type. This is not expected to be a significant problem for programmers.

Arrays

Collections do not receive any special treatment in this proposal. For example, Array's indexed subscript is an ordinary computed subscript on a value type. Accordingly, mutating an element of an array will require exclusive access to the entire array, and therefore will disallow any other simultaneous accesses to the array, even to different elements. For example:

var array = [[1,2], [3,4,5]]

// NOT A CONFLICT.  These accesses to the elements of 'array' each
// complete instantaneously and do not overlap each other.  Even if they
// did overlap for some reason, they are both reads and therefore
// do not conflict.
print(array[0] + array[1])

// NOT A CONFLICT.  The access done to read 'array[1]' completes
// before the modifying access to 'array[0]' begins.  Therefore, these
// accesses do not conflict.
array[0] += array[1]

// CONFLICT.  Passing 'array[i]' as an inout argument performs a
// write access to it, and therefore to 'array', for the duration of
// the call.  This call makes two such accesses to the same array variable,
// which therefore conflict.
swap(&array[0], &array[1])

// CONFLICT.  Calling a non-mutating method on 'array[0]' performs a
// read access to it, and thus to 'array', for the duration of the method.
// Calling a mutating method on 'array[1]' performs a write access to it,
// and thus to 'array', for the duration of the method.  These accesses
// therefore conflict.
array[0].forEach { array[1].append($0) }

It's always been somewhat fraught to do simultaneous accesses to an array because of copy-on-write. The fact that you should not create an array and then fork off a bunch of threads that assign into different elements concurrently has been independently rediscovered by a number of different programmers. (Under this proposal, we will still not be reliably detecting this problem by default, because it is a race condition; see the section on concurrency.) The main new limitation here is that some idioms which did work on a single thread are going to be forbidden. This may just be a cost of progress, but there are things we can do to mitigate the problem.

In the long term, the API of Array and other collections should be extended to ensure that there are good ways of achieving the tasks that exclusivity enforcement has made difficult. It will take experience living with exclusivity in order to understand the problems and propose the right API additions. In the short term, these problems can be worked around with withUnsafeMutableBufferPointer.

We do know that swapping two array elements will be problematic, and accordingly we are (separately proposing)[https://github.com/apple/swift-evolution/blob/master/proposals/0173-swap-indices.md] to add a swapAt method to MutableCollection that takes two indices rather than two inout arguments. The Swift 3 compatibility mode should recognize the swap-of-elements pattern and automatically translate it to use swapAt, and the 3-to-4 migrator should perform this rewrite automatically.

Class properties

Unlike value types, calling a method on a class doesn't formally access the entire class instance. In fact, we never try to enforce exclusivity of access on the whole object at all; we only enforce it for individual stored properties. Among other things, this means that an access to a class property never conflicts with an access to a different property.

There are two major reasons for this difference between value and reference types.

The first reason is that it's important to allow overlapping method calls to a single class instance. It's quite common for an object to have methods called on it concurrently from different threads. These methods may access different properties, or they may synchronize their accesses to the same properties using locks, dispatch queues, or some other thread-safe technique. Regardless, it's a widespread pattern.

The second reason is that there's no benefit to trying to enforce exclusivity of access to the entire class instance. For a value type to be mutated, it has to be held in a variable, and it's often possible to reason quite strongly about how that variable is used. That means that the exclusivity rule that we're proposing here allows us to make some very strong guarantees for value types, generally making them an even tighter, lower-cost abstraction. In contrast, it's inherent to the nature of reference types that references can be copied pretty arbitrarily throughout a program. The assumptions we want to make about value types depend on having unique access to the variable holding the value; there's no way to make a similar assumption about reference types without knowing that we have a unique reference to the object, which would radically change the programming model of classes and make them unacceptable for the concurrent patterns described above.

Disabling dynamic enforcement.

We could add an attribute which allows dynamic enforcement to be downgraded to an unsafe-pointer-style undefined-behavior rule on a variable-by-variable basis. This would allow programmers to opt out of the expense of dynamic enforcement when it is known to be unnecessary (e.g. because exclusivity is checked at some higher level) or when the performance burden is simply too great.

There is some concern that adding this attribute might lead to over-use and that we should only support it if we are certain that the overheads cannot be reduced in some better way.

Since the rule still applies, and it's merely no longer being checked, it makes sense to borrow the "checked" and "unchecked" terminology from the optimizer settings.

class TreeNode {
  @exclusivity(unchecked) var left: TreeNode?
  @exclusivity(unchecked) var right: TreeNode?
}

Closures

A closure (including both local function declarations and closure expressions, whether explicit or autoclosure) is either "escaping" or "non-escaping". Currently, a closure is considered non-escaping only if it is:

  • a closure expression which is immediately called,

  • a closure expression which is passed as a non-escaping function argument, or

  • a local function which captures something that is not allowed to escape, like an inout parameter.

It is likely that this definition will be broadened over time.

A variable is said to be escaping if it is captured in an escaping closure; otherwise, it is non-escaping.

Escaping variables generally require dynamic enforcement instead of static enforcement. This is because Swift cannot reason about when an escaping closure will be called and thus when the variable will be accessed. There are some circumstances where static enforcement may still be allowed, for example when Swift can reason about how the variable will be used after it is escaped, but this is only possible as a best-effort improvement for special cases, not as a general rule.

In contrast, non-escaping variables can always use static enforcement. (In order to achieve this, we must impose a new restriction on recursive uses of non-escaping closures; see below.) This guarantee aligns a number of related semantic and performance goals. For example, a non-escaping variable does not need to be allocated on the heap; by also promising to only use static enforcement for the variable, we are essentially able to guarantee that the variable will have C-like performance, which can be important for some kinds of program. This guarantee also ensures that only static enforcement is needed for inout parameters, which cannot be captured in escaping closures; this substantially simplifies the implementation model for capturing inout parameters.

Diagnosing dynamic enforcement violations statically

In general, Swift is permitted to upgrade dynamic enforcement to static enforcement when it can prove that two accesses either always or never conflict. This is analogous to Swift's rules about integer overflow.

For example, if Swift can prove that two accesses to a global variable will always conflict, then it can report that error statically, even though global variables use dynamic enforcement:

var global: Int
swap(&global, &global) // Two overlapping modifications to 'global'

Swift is not required to prove that both accesses will actually be executed dynamically in order to report a violation statically. It is sufficient to prove that one of the accesses cannot ever be executed without causing a conflict. For example, in the following example, Swift does not need to prove that mutate actually calls its argument function:

// The inout access lasts for the duration of the call.
global.mutate { return global + 1 }

When a closure is passed as a non-escaping function argument or captured in a closure that is passed as a non-escaping function argument, Swift may assume that any accesses made by the closure will be executed during the call, potentially conflicting with accesses that overlap the call.

Restrictions on recursive uses of non-escaping closures

In order to achieve the goal of guaranteeing the use of static enforcement for variables that are captured only by non-escaping closures, we do need to impose an additional restriction on the use of such closures. This rule is as follows:

A non-escaping closure A may not be recursively invoked during the execution of a non-escaping closure B which captures the same local variable or inout parameter unless:

  • A is defined within B or

  • A is a local function declaration which is referenced directly by B.

For clarity, we will call this rule the Non-Escaping Recursion Restriction, or NRR. The NRR is sufficient to prove that non-escaping variables captured by B will not be interfered with unless B delegates to something which is locally known by B to have access to those variables. This, together with the fact that the uses of B itself can be statically analyzed by its defining function, is sufficient to allow static enforcement for the non-escaping variables it captures. (It also enables some powerful analyses of captured variables within non-escaping closures; we do not need to get into that here.)

Because of the tight restrictions on how non-escaping closures can be used in Swift today, it's already quite difficult to violate the NRR. The following user-level restrictions are sufficient to ensure that the NRR is obeyed:

  • A function may not call a non-escaping function parameter passing a non-escaping function parameter as an argument.

    For the purposes of this rule, a closure which captures a non-escaping function parameter is treated the same as the parameter.

    We will call this rule the Non-Escaping Parameter Call Restriction, or NPCR.

  • Programmers using withoutActuallyEscaping should take care not to allow the result to be recursively invoked.

The NPCR is a conservative over-approximation: that is, there is code which does not violate the NRR which will be considered ill-formed under the NPCR. This is unfortunate but inevitable.

Here is an example of the sort of code that will be disallowed under the NPCR:

func recurse(fn: (() -> ()) -> ()) {
  // Invoke the closure, passing a closure which, if invoked,
  // will invoke the closure again.
  fn { fn { } }
}

func apply<T>(argProvider: () -> T, fn: (() -> T) -> T) {
  // Pass the first argument function to the second.
  fn(argProvider)
}

Note that it's quite easy to come up with ways to use these functions that wouldn't violate the NRR. For example, if either argument to apply is not a closure, the call cannot possibly violate the NRR. Nonetheless, we feel that the NPCR is a reasonable restriction:

  • Functions like recurse that apply a function to itself are pretty much just of theoretical interest. Recursion is an important programming tool, but nobody writes it like this because it's just unnecessarily more difficult to reason about.

  • Functions like apply that take two closures are not uncommon, but they're likely to either invoke the closures sequentially, which would not violate the NPCR, or else be some sort of higher-order combinator, which would require the closures to be @escaping and thus also not violate the NPCR.

Note that passing two non-escaping functions as arguments to the same call does not violate the NPCR. This is because the NPCR will be enforced, recursively, in the callee. (Imported C functions which take non-escaping block parameters can, of course, easily violate the NPCR. They can also easily allow the block to escape. We do not believe there are any existing functions or methods on our target platforms that directly violate the NPCR.)

In general, programmers who find the NPCR an unnecessarily overbearing restriction can simply declare their function parameter to be @escaping or, if they are certain that their code will not violate the NRR, use withoutActuallyEscaping to disable the NPCR check.

Source compatibility

In order to gain the performance and language-design benefits of exclusivity, we will have to enforce it in all language modes. Therefore, exclusivity will eventually demand a source break.

We can mitigate some of the impact of this break by implicitly migrating code matching certain patterns to use different patterns that are known to satisfy the exclusivity rule. For example, it would be straightforward to automatically translate calls like swap(&array[i], &array[j]) to array.swapAt(i, j). Whether this makes sense for any particular migration remains to be seen; for example, swap does not appear to be used very often in practice outside of specific collection algorithms.

Overall, we do not expect that a significant amount of code will violate exclusivity. This has been borne out so far by our testing. Often the examples that do violate exclusivity can easily be rewritten to avoid conflicts. In some of these cases, it may make sense to do the rewrite automatically to avoid source-compatibility problems.

Effect on ABI stability and resilience

In order to gain the performance and language-desing benefits of exclusivity, we must be able to assume that it is followed faithfully in various places throughout the ABI. Therefore, exclusivity must be enforced before we commit to a stable ABI, or else we'll be stuck with the current conservatism around inout and mutating methods forever.