swift-mirror/docs/Lexicon.md

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](http://llvm.org/docs/Lexicon.html).

Glossary
========

## 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:

```swift
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](#abstraction-pattern). 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](#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.)

## access path

Broadly, an "access path" is a list of "accesses" which must be chained together
to compute some output from an input. For instance, the generics system has a
type called a `ConformancePath` which explains how to, for example,
walk from `T: Collection` to `T: Sequence` to `T.Iterator: IteratorProtocol`.
There are several different kinds of "access path" in different parts of the compiler,
but they all follow this basic theme.

In the specific context of imports, an "access path" is the `Bar` portion of a scoped
import like `import class Foo.Bar`. Theoretically, it could have several identifiers
to designate a nested type, although the compiler doesn't currently support this. It can
also be empty, matching all top-level declarations in the module.

Note, however, that there has historically been some confusion about the meaning of
"access path" with regards to imports. You might see some code use "access path"
to include the `Foo` part or even to describe a chain of submodule names where a
declaration is not valid at all. (Strictly, the chain of module names is a "module path"
and the combination of module path + access path is an "import path".)

See `ImportPath` and the types nested inside it for more on this.

## access pattern

Defines how some particular storage (a property or a subscript) is accessed.
For example, when accessing a property `let y = a.x`, the compiler could potentially
use `get` accessor or the `_read` accessor. Similarly, for a modification like
`a.x += 1`, the compiler could use `get` + `set` or it could use `_modify`.

The access pattern can differ for call-sites which can/cannot see the underlying
implementation. Clients which cannot see the underlying implementation are said
to use the conservative access pattern.

## 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](#IR).

## build wrangler

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

## canonical SIL

[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 type with [sugar](#sugared-type) removed. These can be compared for
pointer equality to test whether two types are equivalent in the language
semantics.

Note that canonical types are independent of the generic signature, so
two [type parameters](#type-parameter) that are spelled differently but
related by a same-type requirement are not canonically equal. However,
they will have the same [reduced type](#reduced-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 `ProtocolConformanceRef` type`.
See also [witness table](#witness-table).

## contextual type

This has two unrelated meanings:

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. A type that does not contain [type parameters](#type-parameters), but may
   contain [archetypes](#archetype). Compare with
   [interface type](#interface-type).

## critical edge

An edge in a control flow graph where the destination has multiple predecessors
and the source has multiple successors.

## currency type

A type that's meant to be commonly passed around and stored, like `Array`, as
opposed to a type that's useful for temporary/internal purposes but which you
wouldn't normally use in an external interface, like `ArraySlice`. Having broad
agreement about the currency type you use for a particular kind of data (e.g.
using `Array` to pass around sequential collections) generally makes the whole
ecosystem better by reducing artificial barriers to passing data from one system 
to another, and it gives algorithm writers an obvious target to ensure they
optimize for. That's where the analogy to currency comes from: agreeing on a
currency type improves the flow of information in a program in some of the same
ways that agreeing on a currency improves the flow of trade in an economy.

## customization point

Informal term for a protocol requirement that has a default implementation,
i.e. one that conforming types don't *have* to implement but have the option
to "customize".

## dependency sink

Any request that uses a matching dependency source to write dependency
edges into the referenced name trackers. For example, a request that
performs direct lookup will write the name being looked up into the
name tracker associated with the file that issued the lookup request.
The request evaluator automatically determines the appropriate tracker
for a dependency sink to write into based on the current active
[dependency source](#dependency-source) request.

## dependency source

Any request that defines a scope under which reference dependencies may be
registered. For example, a request to type check an entire file is a
dependency source. Dependency sources are automatically managed by the
request evaluator as request evaluation proceeds. Dependency sources provide
one half of the necessary information to complete a full dependency edge.
The other half is provided by corresponding
[dependency sink](#dependency-sink) requests.

## dependent member type

One of the two kinds of [type parameters](#type-parameters). Consists of a
base type, together with the name of an associated type. The base type is
some other type parameter, and the associated type must be declared in some
protocol that the base type conforms to.

## DI (definite initialization)

The feature that no uninitialized variables, constants, or properties will
be read by a program, or the analysis pass that operates on [SIL](#sil) to
guarantee this. This was 
[discussed on Apple's Swift blog](https://developer.apple.com/swift/blog/?id=28).

## DNM

"Do not merge". Placed in PR titles where discussion or analysis is still
ongoing.

## DSO

Dynamic shared object, a shared library file (.so/.dylib/.dll, the extension of
which depends on the platform) to be used by multiple applications while they
are executing.

## 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".

## equivalence class

Given an [equivalence relation](#equivalence-relation) over some set, together
with an element of that set, the equivalence class of that element is the subset
of all elements equivalent to this element.

A fact of mathematics is that every element of the set belongs to exactly one
equivalence class, so the equivalence classes form a disjoint partition of the set.

## equivalence relation

A relation `R` over some set `S` is an _equivalence relation_ if:

1. It is _reflexive_, which means that for all `x` in `S`, we have `x R x`.
2. It is _symmetric_, which means that any `x` and `y` in `S` that satisfy `x R y`
   also satisfy `y R x`.
3. It is _transitive_, which means that for any `x`, `y`, and `z` in `S` that
   satisfy `x R y` and `y R z`, we have `x R z`.

Most of the time, the notion of value equality in programming defines an
equivalence relation over the values of some type. A notable exception is
floating point equality, which fails to be reflexive, because under the
rules of IEEE floating point arithmetic, a NaN value is not equal to itself.

## existential type

A type that is a protocol composition (including a single protocol and *zero* protocols; the latter is the `Any` type).

## existential value

A value of [existential type](#existential-type), commonly referred to simply as an "existential".

## explicit module build

A module build where all dependency modules (including Clang modules) are
passed to the compiler explicitly by an external build system, including
any modules in caches. See also: [implicit module build](#implicit-module-build)
and [fast dependency scanner](#fast-dependency-scanner).

## generic parameter type

One of the two kinds of [type parameters](#type-parameters). Declared within a
generic parameter list, like `<T, U>`, attached to a generic declaration like
`struct Pair<T, U> {...}`. Uniquely identified within its lexical scope by two
integers, called the _depth_ and the _index_ of the generic parameter type.

## fast dependency scanner

A Swift compiler mode that scans a Swift module for import declarations and
resolves which modules will be loaded. It is based on the
[clang-scan-deps](https://llvm.org/devmtg/2019-04/slides/TechTalk-Lorenz-clang-scan-deps_Fast_dependency_scanning_for_explicit_modules.pdf)
library within Clang, for (Objective-)C modules, but is extended to also
understand textual Swift modules (.swiftinterface files).

The fast dependency scanner outputs a graph of compilation steps which can be
used by a build system to schedule
[explicit module builds](#explicit-module-builds).

## fragile

Describes a type or function where making changes will break binary
compatibility. See [LibraryEvolution.rst](LibraryEvolution.rst).

## gardening

Describes contributions which fix code that is not executed
(such as in a manifesto or README) and written text
(correcting typos and grammatical errors).

## generic environment

A mapping from [type parameters](#type-parameter) to [archetypes](#archetypes).
Every generic signature has a _primary_ generic environment associated with it.

Other kinds of generic environments exist for instantiating archetypes that
represent opaque return types, and opened existentials.

## generic signature

A representation of all generic parameters and the requirements imposed upon
them within the lexical scope of a declaration.

## GOT

[Global offset table](https://en.wikipedia.org/wiki/Global_Offset_Table),
a section in an executable or a shared library, which enables code to run
independently of the memory address where it is loaded at runtime. Entries
in GOT directly point to absolute addresses of symbols. The format of GOT
is platform-dependent. Loaders update GOT relocations either on
startup or on symbol access.

Additionally, IRGen makes heavy usage of "GOT" as in "GOT-equivalent variable"
to describe the way it forms references to global objects that may or may not be external references. 

## iff

["if and only if"](https://en.wikipedia.org/wiki/If_and_only_if).
This term comes from mathematics.

## implicit module build

A module build where the compiler is free to transparently build dependent
modules (including Clang modules), and access modules in different caches as
necessary. For example, if a textual Swift module (.swiftinterface file) for
a dependency does not have a corresponding binary Swift module (.swiftmodulea
file), the compiler may transparently build a binary Swift module from the
textual one as a cache for future compiler jobs, without involving any external
build system that invoked the compiler. See also:
[explicit module build](#explicit-module-build).

## 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 (_, _):`.

Contrast with [refutable pattern](#refutable-pattern)

## 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](#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](#rvalue).

## main module

The module for the file or files currently being compiled.

## mandatory passes / mandatory optimizations

Transformations over [SIL](#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"](http://sealiesoftware.com/blog/archive/2009/04/14/objc_explain_Classes_and_metaclasses.html); 
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.

## 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](#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](http://clang.llvm.org/docs/Modules.html).
7. Shorthand for a "precompiled module file"; effectively "precompiled
   headers" for an entire Clang module. Never used directly by Swift.
   See also [module cache](#module-cache).


## 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](#existential) value with its dynamic type pulled out, so that the
compiler can do something with it.

## overlay

A wrapper library that is implicitly imported "on top of" another library.
It contains an `@_exported` import of the underlying library, but it augments
it with additional APIs which, for one reason or another, are not included
in the underlying library directly.

There are two kinds of overlays:

A "clang overlay" (the older kind, so it's often just called an "overlay")
is a Swift library that adds Swift-specific functionality to a C-family
library or framework. Clang overlays are used with system libraries that
cannot be modified to add Swift features. A clang overlay has the same
module name as the underlying library and can do a few special things that
normal modules can't, like adding required initializers to classes. If a
module has a clang overlay, the Clang Importer will always load it unless it
is actually compiling the overlay itself. Apple has a number of clang
overlays for its own SDKs in `stdlib/public/Darwin/`.

A "separately-imported overlay" is a separate module with its own name.
Unlike a clang overlay, it can be imported in some SourceFiles and not
others. When the compiler processes import declarations, it determines which
separately-imported overlays need to be imported and then adds them to the
list of imports for that file; name lookup also knows to look through the
overlay when it looks for declarations in the underlying module.
Separately-imported overlays are used to implement the "cross-import
overlays" feature, which is used to augment a module with additional
functionality when it is imported alongside another module.

## parent type

The type in which a given declaration is nested. For example:

```swift
struct Outer {
  struct Inner {
  }
}
```

`Outer` is the parent type of `Inner`.

Note that the terms "parent type" and "superclass" refer to completely
different concepts.

## 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.

## PLT

Procedure linkage table, which is used to call external functions that don't
have their addresses known at link time. These addresses are then resolved
by a loader at run time.

## PR

1. "Problem Report": An issue reported in [LLVM's bug tracker](https://llvm.org/bugs/).
See also [SR](#SR).
2. "pull request"

## 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](https://bugreport.apple.com), or an issue reported 
on that system.

## raw SIL

[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](#abstraction-pattern) from its current representation.

## realization

The process of initializing an ObjC class for use by the ObjC runtime.
This consists of allocating runtime tracking data, fixing up method lists
and attaching categories.

This is distinct from the initialization performed by `+initialize`, which
happens only when the first message (other than `+load`) is sent to the class.

The order of operations is: realization, followed by `+load` (if present),
followed by `+initialize`. There are few cases where these can happen
at different times.

- Common case (no `+load` or special attributes): Realization is lazy and
  happens when the first message is sent to a class. After that, `+initialize`
  is run.
- If the class has a `+load` method: `+load`, as the name suggests, runs at
  load time; it is the ObjC equivalent of a static initializer in C++. For
  such a class, realization eagerly happens at load time before `+load` runs.
  (Fun aside: C++ static initializers run after `+load`.) `+initialize` still
  runs lazily on the first message.
- If the class is marked [`@_objc_non_lazy_realization`](/docs/ReferenceGuides/UnderscoredAttributes.md#_objc_non_lazy_realization):
  Realization happens at load time. `+initialize` still runs lazily on the first
  message.

It's possible to create a class that is realized but not initialized by
using a runtime function like `objc_getClass` before the class has been used.

See also: Mike Ash's blog post on
[Objective-C Class Loading and Initialization](https://www.mikeash.com/pyblog/friday-qa-2009-05-22-objective-c-class-loading-and-initialization.html),
which covers `+load` and `+initialize` in more detail.

## reduced type

A [canonical type](#canonical-type) which has been further simplified with
respect to a [generic signature](#generic-signature), by performing two
transformations until fixed point:
1. Replacing type parameters fixed to concrete types with those concrete type.s
2. Replacing all other type parameters with the smallest member of their
   [equivalence class](#equivalence-class), chosen according to a well-founded
   linear order.

See also [sugared type](#sugared-type).

## 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():`.

Contrast with [irrefutable pattern](#irrefutable-pattern)

## resilient

Describes a type or function where making certain changes will not break
binary compatibility. See [LibraryEvolution.rst](LibraryEvolution.rst).

## 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](#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 that was originally reported on the now-retired Jira instance that used
to be located at [bugs.swift.org](https://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).

See also [canonical type](#canonical-type), [reduced type](#reduced-type).

## TBD

Text-based dynamic library files (TBDs) are a textual representation of
the information in a dynamic library / shared library that is required
by the static linker.

Apple’s SDKs originally used Mach-O Dynamic Library Stubs. Mach-O Dynamic
Library Stubs are dynamic library files, but with all the text and data
stripped out.

## 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.

## type parameter

A type that can be used within the body of a generic declaration, that
abstracts over a concrete replacement type provided by the caller of a
generic declaration.

More specifically, a type parameter is either a
[generic parameter type](#generic-parameter-type), or a
[dependent member type](#dependent-member-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.

## VWT (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](#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.

## WIP

"Work-in-progress". Placed in PR titles to indicate that the PR is not ready
for review or merging.

## witness

The value or type that satisfies a protocol requirement.

## witness table

The [SIL](#sil) (and runtime) representation of a [conformance](#conformance); essentially a
[vtable](#vtable-virtual-dispatch-table) but for a protocol instead of
a class.

Only conceptually related to a [value witness table](#vwt-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.