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Standard Library Programmers Manual

This is meant to be a guide to people working on the standard library. It covers coding standards, code organization, best practices, internal annotations, and provides a guide to standard library internals. This document is inspired by LLVM's excellent programmer's manual and coding standards.

TODO: Should this subsume or link to StdlibRationales.rst?

TODO: Should this subsume or link to AccessControlInStdlib.rst

In this document, "stdlib" refers to the core standard library (stdlib/public/core), our Swift overlays for system frameworks (stdlib/public/Darwin/*, stdlib/public/Windows/*, etc.), as well as the auxiliary and prototype libraries under stdlib/private.

Coding style

Formatting Conventions

The stdlib currently has a hard line length limit of 80 characters. To break long lines, please closely follow the indentation conventions you see in the existing codebase. (FIXME: Describe.)

We use two spaces as the unit of indentation. We don't use tabs.

Public APIs

Core Standard Library

All new public API additions to the core Standard Library must go through the Swift Evolution Process. The Core Team must have approved the additions by the time we merge them into the stdlib codebase.

All public APIs should come with documentation comments describing their purpose and behavior. It is highly recommended to use big-oh notation to document any guaranteed performance characteristics. (CPU and/or memory use, number of accesses to some underlying collection, etc.)

Note that implementation details are generally outside the scope of the Swift Evolution -- the stdlib is free to change its internal algorithms, internal APIs and data structures etc. from release to release, as long as the documented API (and ABI) remains intact.

For example, since hashValue was always documented to allow changing its return value across different executions of the same program, we were able to switch to randomly seeded hashing in Swift 4.2 without going through the Swift Evolution process. However, the introduction of hash(into:) required a formal proposal. (Note though that highly visible behavioral changes like this are much more difficult to implement now that they were in the early days -- in theory we can still do ABI-preserving changes, but Hyrum's Law makes it increasingly more difficult to change any observable behavior. For example, in some cases we may need to add runtime version checks for the Swift SDK on which the main executable was compiled, to prevent breaking binaries compiled with previous releases.)

We sometimes need to expose some internal APIs as public for technical reasons (such as to interoperate with other system frameworks, and/or to enable testing/debugging certain functionality). We use the Leading Underscore Rule (see below) to differentiate these from the documented stdlib API. Underscored APIs aren't considered part of the public API surface, and as such they don't need to go through the Swift Evolution Process. Regular Swift code isn't supposed to directly call these; when necessary, we may change their behavior in source-incompatible ways or we may even remove them. (However, such changes are technically ABI breaking, so they need to be carefully considered against the risk of breaking existing binaries.)

Overlays and Private Code

The overlays specific to particular platforms generally have their own API review processes. These are outside the scope of Swift Evolution.

Anything under stdlib/private can be added/removed/changed with the simple approval of a stdlib code owner.

The Leading Underscore Rule

All APIs that aren't part of the stdlib's official public API must include at least one underscored component in their fully qualified names. This includes symbols that are technically declared public but that aren't considered part of the public stdlib API, as well as @usableFromInline internal, plain internal and [file]private types and members.

The underscored component need not be the last. For example, Swift.Dictionary._worble() is a good name for an internal helper method, but so is Swift._NativeDictionary.worble() -- the _NativeDictionary type already has the underscore.

Initializers don't have a handy base name on which we can put the underscore; instead, we put the underscore on the first argument label, adding one if necessary: e.g., init(_ value: Int) may become init(_value: Int). If the initializer doesn't have any parameters, then we add a dummy parameter of type Void with an underscored label: for example, UnsafeBufferPointer.init(_empty: ()).

This rule ensures we don't accidentally clutter the public namespace with @usableFromInline things (which could prevent us from choosing the best names for newly public API later), and it also makes it easy to see at a glance if a piece of stdlib code uses any non-public entities.

Explicit Access Modifiers

We prefer to explicitly spell out all access modifiers in the stdlib codebase. (I.e., we type internal even if it's the implicit default.) Additionally, we put the access level on each individual entry point rather than inheriting them from the extension they are in:

public extension String {
  // 😢👎
  func blanch() { ... }
  func roast() { ... }
}

extension String {
  // 😊👍
  public func blanch() { ... }
  public func roast() { ... }
}

This makes it trivial to identify the access level of every definition without having to scan the context it appears in.

For historical reasons, the existing codebase generally uses internal as the catch-all non-public access level. However, it is okay to use private/fileprivate when appropriate.

Availability

Every entry point in the standard library that has an ABI impact must be applied an @available attribute that describes the earliest ABI-stable OS releases that it can be deployed on. (Currently this only applies to macOS, iOS, watchOS and tvOS, since Swift isn't ABI stable on other platforms yet.)

Unlike access control modifiers, we prefer to put @available attributes on the extension context rather than duplicating them on every individual entry point. This is an effort to fight against information overload: the @available attribute is information dense and it's generally easier to review/maintain if applied to a group of entry points all at once.

// 👍
@available(macOS 10.6, iOS 10, watchOS 3, tvOS 12, *)
extension String {
  public func blanch() { ... }
  public func roast() { ... }
}

Features under development that haven't been released yet must be marked with the placeholder version number 9999. This special version is always considered available in custom builds of the Swift toolchain (including development snapshots), but not in any ABI-stable production release.

@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
public struct FutureFeature {
  ...
}

On these platforms, the Swift Standard Library ships as an integrated part of the operating system; as such, it is the platform owners' responsibility to update these placeholder version numbers to actual versions as part of their release process.

Internals

Unwrapping Optionals

Optionals can be unwrapped with !, which triggers a trap on nil. Alternatively, they can be .unsafelyUnwrapped(), which will check and trap in debug builds of user code. Internal to the standard library is ._unsafelyUnwrappedUnchecked() which will only check and trap in debug builds of the standard library itself. These correspond directly with _precondition, _debugPrecondition, and _internalInvariant. See that section for details.

UnsafeBitCast and Casting References

In general unsafeBitCast should be avoided because its correctness relies on subtle assumptions that will never be enforced, and it indicates a bug in Swift's type system that should be fixed. It's less bad for non-pointer trivial types. Pointer casting should go through one of the memory binding API instead as a last resort.

Reference casting is more interesting. References casting can include converting to an Optional reference and converting from a class constrained existential.

The regular as operator should be able to convert between reference types with full dynamic checking.

unsafeDownCast is just as capable, but is only dynamically checked in debug mode or if the cast requires runtime support.

_unsafeUncheckedDowncast is the same but is only dynamically checked in the stdlib asserts build, or if the cast requires runtime support.

_unsafeReferenceCast is only dynamically checked if the cast requires runtime support. Additionally, it does not impose any static AnyObject constraint on the incoming reference. This is useful in a generic context where the object-ness can be determined dynamically, as done in some bridged containers.

Builtins

_fastPath and _slowPath

_fastPath returns its argument, wrapped in a Builtin.expect. This informs the optimizer that the vast majority of the time, the branch will be taken (i.e. the then branch is “hot”). The SIL optimizer may alter heuristics for anything dominated by the then branch. But the real performance impact comes from the fact that the SIL optimizer will consider anything dominated by the else branch to be infrequently executed (i.e. “cold”). This means that transformations that may increase code size have very conservative heuristics to keep the rarely executed code small.

The probabilities are passed through to LLVM as branch weight metadata, to leverage LLVM’s use of GCC style builtin_expect knowledge (e.g. for code layout and low-level inlining).

_fastPath probabilities are compounding, see the example below. For this reason, it can actually degrade performance in non-intuitive ways as it marks all other code (including subsequent _fastPaths) as being cold. Consider _fastPath as basically spraying the rest of the code with a Mr. Freeze-style ice gun.

_slowPath is the same as _fastPath, just with the branches swapped.

Example:

if _fastPath(...) {
  // 90% of the time we execute this: aggressive inlining
  ...
  return
}
// 10% of the time we execute this: very conservative inlining
...
if _fastPath(...) {
  // 9% of the time we execute this: very conservative inlining
    ...
    return
}

// 1% of the time we execute this: very conservative inlining
...
return

_onFastPath

This should be rarely used. It informs the SIL optimizer that any code dominated by it should be treated as the innermost loop of a performance critical section of code. It cranks optimizer heuristics to 11. Injudicious use of this will degrade performance and bloat binary size.

_precondition, _debugPrecondition, and _internalInvariant

These three functions are assertions that will trigger a run time trap if violated.

  • _precondition executes in all build configurations. Use this for invariant enforcement in all user code build configurations
  • _debugPrecondition will execute when user code is built with assertions enabled. Use this for invariant enforcement that's useful while debugging, but might be prohibitively expensive when user code is configured without assertions.
  • _internalInvariant will execute when standard library code is built with assertions enabled. Use this for internal only invariant checks that useful for debugging the standard library itself.

_fixLifetime

A call to _fixLifetime is considered a use of its argument, meaning that the argument is guaranteed to live at least up until the call. It is otherwise a nop. This is useful for guaranteeing the lifetime of a value while inspecting its physical layout. Without a call to _fixLifetime, the last formal use may occur while the value's bits are still being munged.

Example:

var x = ...
defer { _fixLifetime(x) } // Guarantee at least lexical lifetime for x
let theBits = unsafeBitCast(&x, ...)
... // use of theBits in ways that may outlive x if it weren't for the _fixLifetime call

Annotations

@_transparent

Should only be used if necessary. This has the effect of forcing inlining to occur before any dataflow analyses take place. Unless you specifically need this behavior, use @_inline(__always) or some other mechanism. Its primary purpose is to force the compiler's static checks to peer into the body for diagnostic purposes.

Use of this attribute imposes limitations on what can be in the body. For more details, refer to the documentation.

@unsafe_no_objc_tagged_pointer

This is currently used in the standard library as an additional annotation applied to @objc protocols signifying that any objects which conform to this protocol are not tagged. This means that (on Darwin platforms) such references, unlike AnyObject, have spare bits available from things like restricted memory spaces or alignment.

@_silgen_name

This attribute specifies the name that a declaration will have at link time. It is used for two purposes, the second of which is currently considered bad practice and should be replaced with shims:

  1. To specify the symbol name of a Swift function so that it can be called from Swift-aware C. Such functions have bodies.
  2. To provide a Swift declaration which really represents a C declaration. Such functions do not have bodies.
Using @_silgen_name to call Swift from Swift-aware C

Rather than hard-code Swift mangling into C code, @_silgen_name is used to provide a stable and known symbol name for linking. Note that C code still must understand and use the Swift calling convention (available in swift-clang) for such Swift functions (if they use Swift's CC). Example:

@_silgen_name("_destroyTLS")
internal func _destroyTLS(_ ptr: UnsafeMutableRawPointer?) {
  // ... implementation ...
}
SWIFT_CC(swift) SWIFT_RUNTIME_STDLIB_INTERNAL
void _destroyTLS(void *);

// ... C code can now call _destroyTLS on a void * ...
Using @_silgen_name to call C from Swift

The standard library cannot import the Darwin module (much less an ICU module), yet it needs access to these C functions that it otherwise wouldn't have a decl for. For that, we use shims. But, @_silgen_name can also be used on a body-less Swift function declaration to denote that it's an external C function whose symbol will be available at link time, even if not available at compile time. This usage is discouraged.

Internal structures

_FixedArray16

The standard library has an internal array type of fixed size 16. This provides fast random access into contiguous (usually stack-allocated) memory. See FixedArray.swift for implementation.

Thread Local Storage

The standard library utilizes thread local storage (TLS) to cache expensive computations or operations in a thread-safe fashion. This is currently used for tracking some ICU state for Strings. Adding new things to this struct is a little more involved, as Swift lacks some of the features required for it to be expressed elegantly (e.g. move-only structs):

  1. Add the new member to _ThreadLocalStorage and a static getMyNewMember method to access it. getMyNewMember should be implemented using getPointer.
  2. If the member is not trivially initializable, update _initializeThreadLocalStorage and _ThreadLocalStorage.init.
  3. If the field is not trivially destructable, update _destroyTLS to properly destroy the value.

See ThreadLocalStorage.swift for more details.

Working with Resilience

Maintaining ABI compatibility with previously released versions of the standard library makes things more complicated. This section details some of the extra rules to remember and patterns to use.

The Curiously Recursive Inlinable Switch Pattern (CRISP)

When inlinable code switches over a non-frozen enum, it has to handle possible future cases (since it will be inlined into a module outside the standard library). You can see this in action with the implementation of round(_:) in FloatingPointTypes.swift.gyb, which takes a FloatingPointRoundingRule. It looks something like this:

@_transparent
public mutating func round(_ rule: FloatingPointRoundingRule) {
  switch rule {
  case .toNearestOrAwayFromZero:
    _value = Builtin.int_round_FPIEEE${bits}(_value)
  case .toNearestOrEven:
    _value = Builtin.int_rint_FPIEEE${bits}(_value)
  // ...
  @unknown default:
    self._roundSlowPath(rule)
  }
}

Making round(_:) inlinable but still have a default case is an attempt to get the best of both worlds: if the rounding rule is known at compile time, the call will compile down to a single instruction in optimized builds; and if it dynamically turns out to be a new kind of rounding rule added in Swift 25 (e.g. .towardFortyTwo), there's a fallback function, _roundSlowPath(_:), that can handle it.

So what does _roundSlowPath(_:) look like? Well, it can't be inlinable, because that would defeat the purpose. It could just look like this:

@usableFromInline
internal mutating func _roundSlowPath(_ rule: FloatingPointRoundingRule) {
  switch rule {
  case .toNearestOrAwayFromZero:
    _value = Builtin.int_round_FPIEEE${bits}(_value)
  case .toNearestOrEven:
    _value = Builtin.int_rint_FPIEEE${bits}(_value)
  // ...
  }
}

...i.e. exactly the same as round(_:) but with no default case. That's guaranteed to be up to date if any new cases are added in the future. But it seems a little silly, since it's duplicating code that's in round(_:). We could omit cases that have always existed, but there's a better answer:

// Slow path for new cases that might have been inlined into an old
// ABI-stable version of round(_:) called from a newer version. If this is
// the case, this non-inlinable function will call into the _newer_ version
// which _will_ support this rounding rule.
@usableFromInline
internal mutating func _roundSlowPath(_ rule: FloatingPointRoundingRule) {
  self.round(rule)
}

Because _roundSlowPath(_:) isn't inlinable, the version of round(_:) that gets called at run time will always be the version implemented in the standard library dylib. And since FloatingPointRoundingRule is also defined in the standard library, we know it'll never be out of sync with this version of round(_:).

Maybe some day we'll have special syntax in the language to say "call this method without allowing inlining" to get the same effect, but for now, this Curiously Recursive Inlinable Switch Pattern allows for safe inlining of switches over non-frozen enums with less boilerplate than you might otherwise have. Not none, but less.

(Meta): List of wants and TODOs for this guide

  1. Library Organization
    1. What files are where
      1. Brief about CMakeLists
      2. Brief about GroupInfo.json
    2. What tests are where
      1. Furthermore, should there be a split between whitebox tests and blackbox tests?
    3. What benchmarks are where
      1. Furthermore, should there be benchmarks, microbenchmarks, and nanobenchmarks (aka whitebox microbenchmarks)?
    4. What SPIs exist, where, and who uses them
    5. Explain test/Prototypes, and how to use that for rapid (relatively speaking) prototyping
  2. Library Concepts
    1. Protocol hierarchy
      1. Customization hooks
    2. Use of classes, COW implementation, buffers, etc
    3. Compatibility, @available, etc.
    4. Resilience, ABI stability, @inlinable, @usableFromInline, etc
    5. Strings and ICU
    6. Lifetimes
      1. withExtendedLifetime, withUnsafe...,
    7. Shims and stubs
  3. Coding Standards
    1. High level concerns
    2. Best practices
    3. Formatting
  4. Internals
    1. @inline(__always) and @inline(never)
    2. @semantics(...)
    3. Builtins
      1. Builtin.addressof, _isUnique, etc
  5. Dirty hacks
    1. Why all the underscores and extra protocols?
    2. How does the ... ranges work?
  6. Frequently Encountered Issues