Skip to content

Latest commit

 

History

History
419 lines (336 loc) · 18.1 KB

Lexicon.rst

File metadata and controls

419 lines (336 loc) · 18.1 KB
Raw
orphan

Lexicon

This file defines several terms used by the Swift compiler and standard library source code, tests, and commit messages. See also the LLVM lexicon.

Note

This document uses Sphinx-specific features. If you are viewing this on GitHub, you'll have to use raw mode, or download and build the docs yourself.

abstraction pattern

The unsubstituted generic type of a property or function parameter, which sets constraints on its representation in memory. For example, given the following definitions:

struct Foo<T> {
  var value: T
  // Foo.value has abstraction pattern <T> T

struct Bar<T, U> {
  var value: (T) -> U
  // Bar.value has abstraction pattern <T, U> (T) -> U
}
struct Bas {
  var value: (Int) -> String
  // Bas.value has abstraction pattern (Int) -> String
}
let transform: (Int) -> String = { "\($0)" }
let foo = Foo<(Int) -> String>(value: transform)
let bar = Bar<Int, String>(value: transform)
let bas = Bas(value: transform)

although foo.value, bar.value, and bas.value all have the same function type (Int) -> String, they have different abstraction patterns. Because a value of type Foo or Bar may be used in a generic context and invoke value with a parameter or result type of unknown size, the compiler has to pick a more conservative representation for the closure that uses indirect argument passing, whereas Bas.value has a fully concrete closure type so can always use a more specialized direct register-based calling convention. The compiler transparently introduces reabstraction conversions when a value is used with a different abstraction pattern. (This is where the infamous "reabstraction thunk helpers" sometimes seen in Swift backtraces come from.)

archetype

A placeholder for a generic parameter or an associated type within a generic context. Sometimes known as a "rigid type variable" in formal CS literature. Directly stores its conforming protocols and nested archetypes, if any.

AST

"Abstract syntax tree", although in practice it's more of a directed graph. A parsed representation of code used by a compiler.

bitcode

Serialized LLVM IR.

build czar

Apple term for "the person assigned to watch CI this week".

canonical SIL

SIL after the mandatory passes <mandatory passes / mandatory optimizations> have run. This can be used as input to IRGen to generate LLVM IR or object files.

canonical type

A unique representation of a type, with any sugar <sugared type> removed. These can usually be directly compared to test whether two types are the same; the exception is when generics get involved. In this case you'll need a generic environment. Contrast with sugared type.

Clang importer

The part of the compiler that reads C and Objective-C declarations and exposes them as Swift. Essentially contains a small instance of Clang running inside the Swift compiler, which is also used during IRGen.

conformance

A construct detailing how a particular type conforms to a particular protocol. Represented in the compiler by the ProtocolConformance type at the AST level. See also witness table.

contextual type
  1. The expected type for a Swift sub-expression based on the rest of the statement. For example, in the statement print(6 * 9), the contextual type of the expression 6 * 9 is Any.
  2. The type of a value or declaration from inside a potentially generic context. This type may contain archetypes <archetype> and cannot be used directly from outside the context. Compare with interface type.
DI (definite initialization / definitive initialization)

The feature that no uninitialized variables, constants, or properties will be read by a program, or the analysis pass that operates on SIL to guarantee this. This was discussed on Apple's Swift blog__.

__ https://developer.apple.com/swift/blog/?id=28

dup

From "duplicate". As a noun, refers to another filed issue that describes the same bug ("I have a dup of this"); as a verb, the act of marking a bug as a duplicate ("Please dup this to the underlying issue"). Sometimes written "dupe". Pronounced the same way as the first syllable of "duplicate", which for most American English speakers is "doop".

existential

A value whose type is a protocol composition (including a single protocol and zero protocols; the latter is the Any type).

fragile

Describes a type or function where making changes will break binary compatibility. See LibraryEvolution.rst <LibraryEvolution>.

generic environment

Provides context for interpreting a type that may have generic parameters in it. Generic parameter types are normally just represented as "first generic parameter in the outermost context" (or similar), so it's up to the generic environment to note that that type must be a Collection. (Another way of looking at it is that the generic environment connects interface types <interface type> with contextual types <contextual type>).

generic signature

A representation of all generic parameters and their requirements. Like types, generic signatures can be canonicalized <canonical type> to be compared directly.

iff

"if and only if__". This term comes from mathematics.

__ https://en.wikipedia.org/wiki/If_and_only_if

interface type

The type of a value or declaration outside its generic context. These types are written using "formal" generic types, which only have meaning when combined with a particular generic declaration's "generic signature". Unlike contextual types <contextual type>, interface types store conformances and requirements in the generic signature and not in the types themselves. They can be compared across declarations but cannot be used directly from within the context.

irrefutable pattern

A pattern that always matches. These patterns either bind to a variable or perform structural modification, e.x.:

  1. case _:.
  2. case let x:.
  3. case (_, _):.
IR
  1. "intermediate representation": a generic term for a format representing code in a way that is easy for a compiler or tool to manipulate.
  2. "LLVM IR": a particular IR used by the LLVM libraries for optimization and generation of machine code.
IUO (implicitly unwrapped optional)

A type like Optional, but it implicitly converts to its wrapped type. If the value is nil during such a conversion, the program traps just as it would when a normal Optional is force-unwrapped. IUOs implicitly convert to and from normal Optionals with the same wrapped type.

IWYU (include what you use)

The accepted wisdom that implementation files (.cpp, .c, .m, .mm) should explicitly #include or #import the headers they use. Doing so prevents compilation errors when header files are included in a different order, or when header files are modified to use forward declarations instead of direct includes.

LGTM

"Looks good to me." Used in code review to indicate approval with no further comments.

LLVM IR

See IR.

lvalue

Pronounced "L-value". Refers to an expression that can be assigned to or passed inout. The term originally comes from C; the "L" refers to the "l"eft side of an assignment operator. See also rvalue.

main module

The module for the file or files currently being compiled.

mandatory passes / mandatory optimizations

Transformations over SIL that run immediately after SIL generation. Once all mandatory passes have run (and if no errors are found), the SIL is considered canonical <canonical SIL>.

metatype

The type of a value representing a type. Greg Parker has a good explanation of Objective-C's "metaclasses"__; because Swift has types that are not classes, a more general term is used.

We also sometimes refer to a value representing a type as a "metatype object" or just "metatype", usually within low-level contexts like IRGen and LLDB. This is technically incorrect (it's just a "type object"), but the malapropism happened early in the project and has stuck around.

__ http://sealiesoftware.com/blog/archive/2009/04/14/objc_explain_Classes_and_metaclasses.html

model

A type that conforms to a particular protocol. Sometimes "concrete model". Example: "Array and Set are both models of CollectionType".

module

Has many uses in the Swift world. We may want to rename some of them. #1 and #2 are the most common.

  1. A unit of API distribution and grouping. The import declaration brings modules into scope. Represented as ModuleDecl in the compiler.
  2. A compilation unit; that is, source files that are compiled together. These files may contain cross-references. Represented as "the main module" (a specific ModuleDecl).
  3. (as "SIL module") A container for SIL to be compiled together, along with various context for the compilation.
  4. (as "LLVM module") A collection of LLVM IR to be compiled together. Always created in an LLVMContext.
  5. A file containing serialized AST and SIL information for a source file or entire compilation unit. Often "swiftmodule file", with "swiftmodule" pronounced as a single word.
  6. (as "Clang module") A set of self-contained C-family header files. Represented by a ClangModuleUnit in the Swift compiler, each of which is contained in its own ModuleDecl. For more information, see Clang's documentation for Modules__.
  7. Shorthand for a "precompiled module file"; effectively "precompiled headers" for an entire Clang module. Never used directly by Swift. See also module cache.

__ http://clang.llvm.org/docs/Modules.html

module cache

Clang's cache directory for precompiled module files. As cache files, these are not forward-compatible, and so cannot be loaded by different versions of Clang (or programs using Clang, like the Swift compiler). Normally this is fine, but occasionally a development compiler will not have proper version information and may try to load older module files, resulting in crashes in clang::ASTReader.

NFC

"No functionality change." Written in commit messages that are intended to have no change on the compiler or library's behavior, though for some this refers to having the same implementation and for others merely an equivalent one. "NFC" is typically used to explain why a patch has no included testcase, since the Swift project requires testcases for all patches that change functionality.

open existential

An existential value with its dynamic type pulled out, so that the compiler can do something with it.

overlay

A library that is imported whenever a C library or framework by the same name is imported. The purpose of an overlay is to augment and extend a library on the system when the library on the system cannot be modified. Apple has a number of overlays for its own SDKs in stdlib/public/SDK/.

PCH

Precompiled header, a type of file ending in .pch. A precompiled header is like a precompiled module, in the sense that it's the same file format and is just a cache file produced by clang and read by clang::ASTReader. The difference is that PCH files are not "modular": they do not correspond to a named module, and cannot be read in any order or imported by module-name; rather they must be the first file parsed by the compiler. PCHs are used only to accelerate the process of reading C/C++/Objective-C headers, such as the bridging headers read in by the -import-objc-header command-line flag to swiftc.

PR
  1. "Problem Report": An issue reported in LLVM's bug tracker__. See also SR.
  2. "pull request"

__ https://llvm.org/bugs/

primary file

The file currently being compiled, as opposed to the other files that are only needed for context. See also Whole-Module Optimization <WMO (whole-module optimization)>.

QoI

"Quality of implementation." The term is meant to describe not how well-engineered a particular implementation is, but how much value it provides to users beyond a sort of minimum expectation. Good diagnostics are a matter of QoI, as is good unoptimized performance. For example, a comment like "FIXME: QoI could be improved here" is suggesting that there's some sort of non-mandatory work that could be done that would improve the behavior of the compiler--it is not just a general statement that the code needs to be improved.

It's possible that this term was originally "quality of life", written as "Qol", referring to the experience of end users. At some point along its history, the lowercase "L" was misinterpreted as an uppercase "i", and a new meaning derived. Swift inherited this term from LLVM, which got it from GCC.

Radar

Apple's bug-tracking system__, or an issue reported on that system.

__ https://bugreport.apple.com

raw SIL

SIL just after being generated, not yet in a form that can be used for IR generation. See mandatory passes <mandatory passes / mandatory optimizations>.

reabstraction

An implicit representation change that occurs when a value is used with a different abstraction pattern from its current representation.

refutable pattern

A pattern that may not always match. These include patterns such as:

  1. Isa check, e.g. case let x as String:.
  2. Enum case check: e.g. case .none:.
  3. Expr pattern: e.g. case foo():.
resilient

Describes a type or function where making certain changes will not break binary compatibility. See LibraryEvolution.rst <LibraryEvolution>.

runtime

Code that implements a language's dynamic features that aren't just compiled down to plain instructions. For example, Swift's runtime library includes support for dynamic casting and for the Mirror-based reflection.

rvalue

Pronounced "R-value". Represents an expression that can be used as a value; in Swift this is nearly every expression, so we don't use the term very often. The term originally comes from C; the "R" refers to the "r"ight side of an assignment operator. Contrast with lvalue.

script mode

The parsing mode that allows top-level imperative code in a source file.

Sema

Short for 'Semantic Analysis', the compiler pass that performs type checking, validation, and expression rewriting before SILGen.

SIL

"Swift Intermediate Language". A high-level IR used by the Swift compiler for flow-sensitive diagnostics, optimization, and LLVM IR generation.

SR

An issue reported on bugs.swift.org. A backronym for "Swift Report"; really the name is derived from LLVM's idiomatic use of "PR" ("Problem Report") for its bugs. We didn't go with "PR" for Swift because we wanted to be able to unambiguously reference LLVM bugs.

stdlib

"Standard library". Sometimes this just means the "Swift" module (also known as "swiftCore"); sometimes it means everything in the stdlib/ directory. Pronounced "stid-lib" or "ess-tee-dee-lib".

sugared type

A type that may have been written in a more convenient way, using special language syntax or a typealias. (For example, Int? is the sugared form of Optional<Int>.) Sugared types preserve information about the form and use of the type even though the behavior usually does not change (except for things like access control). Contrast with canonical type.

thunk

In the Swift compiler, a synthesized function whose only purpose is to perform some kind of adjustment in order to call another function. For example, Objective-C and Swift have different calling conventions, so the Swift compiler generates a thunk for use in Objective-C that calls through to the real Swift implementation.

trap

A deterministic runtime failure. Can be used as both as a noun ("Using an out-of-bounds index on an Array results in a trap") and a verb ("Force-unwrapping a nil Optional will trap").

type metadata

The runtime representation of a type, and everything you can do with it. Like a Class in Objective-C, but for any type.

USR

A Unified Symbol Resolution (USR) is a string that identifies a particular entity (function, class, variable, etc.) within a program. USRs can be compared across translation units to determine, e.g., when references in one translation refer to an entity defined in another translation unit.

value witness table

A runtime structure that describes how to do basic operations on an unknown value, like "assign", "copy", and "destroy". (For example, does copying this value require any retains?)

Only conceptually related to a witness table.

vtable (virtual dispatch table)

A map attached to a class of which implementation to use for each overridable method in the class. Unlike an Objective-C method table, vtable keys are just offsets, making lookup much simpler at the cost of dynamism and duplicated information about non-overridden methods.

witness

The value or type that satisfies a protocol requirement.

witness table

The SIL (and runtime) representation of a conformance; essentially a vtable <vtable (virtual dispatch table)> but for a protocol instead of a class.

Only conceptually related to a value witness table.

WMO (whole-module optimization)

A compilation mode where all files in a module are compiled in a single process. In this mode there is no primary file; all files are parsed, type-checked, and optimized together at the SIL level. LLVM optimization and object file generation may happen all together or in separate threads.