Dotty Overall Structure
The compiler code is found in package dotty.tools. It spans the following three sub-packages:
backend Compiler backends (currently for JVM and JS)
dotc The main compiler
io Helper modules for file access and classpath handling.
The dotc package contains some main classes that can be run as separate programs. The most important one is class Main. Main
inherits from Driver which contains the highest level functions for starting a compiler and processing some sources. Driver
in turn is based on two other high-level classes, Compiler and Run.
Package Structure
Most functionality of scalac
is implemented in subpackages of dotc
. Here's a list of sub-packages and their focus.
.
├── ast // Abstract syntax trees
├── config // Compiler configuration, settings, platform specific definitions.
├── core // Core data structures and operations, with specific subpackages for:
│ ├── classfile // Reading of Java classfiles into core data structures
│ ├── tasty // Reading and writing of TASTY files to/from core data structures
│ └── unpickleScala2 // Reading of Scala2 symbol information into core data structures
├── parsing // Scanner and parser
├── printing // Pretty-printing trees, types and other data
├── repl // The interactive REPL
├── reporting // Reporting of error messages, warnings and other info.
├── rewrites // Helpers for rewriting Scala 2's constructs into dotty's.
├── semanticdb // Helpers for exporting semanticdb from trees.
├── transform // Miniphases and helpers for tree transformations.
├── typer // Type-checking and other frontend phases
└── util // General purpose utility classes and modules.
Contexts
scalac
has almost no global state (the only significant bit of global state is the name table, which is used to hash strings into unique names). Instead, all essential bits of information that can vary over a compiler run are collected in a Context. Most methods in scalac
take a Context
value as an implicit parameter.
Contexts give a convenient way to customize values in some part of the call-graph. To run, e.g. some compiler function f
at a given phase phase
, we invoke f
with an explicit context parameter, like this
f(/*normal args*/)(using ctx.withPhase(phase))
This assumes that f
is defined in the way most compiler functions are:
def f(/*normal parameters*/)(implicit ctx: Context) ...
Compiler code follows the convention that all implicit Context
parameters are named ctx
. This is important to avoid implicit ambiguities in the case where nested methods contain each a Context parameters. The common name ensures then that the implicit parameters properly shadow each other.
Sometimes we want to make sure that implicit contexts are not captured in closures or other long-lived objects, be it because we want to enforce that nested methods each get their own implicit context, or because we want to avoid a space leak in the case where a closure can survive several compiler runs. A typical case is a completer for a symbol representing an external class, which produces the attributes of the symbol on demand, and which might never be invoked. In that case we follow the convention that any context parameter is explicit, not implicit, so we can track where it is used, and that it has a name different from ctx
. Commonly used is ictx
for "initialization context".
With these two conventions in place, it has turned out that implicit contexts work amazingly well as a device for dependency injection and bulk parameterization. There is of course always the danger that an unexpected implicit will be passed, but in practice this has not turned out to be much of a problem.
Compiler Phases
Seen from a temporal perspective, the scalac
compiler consists of a list of phases. The current list of phases is specified in class Compiler as follows:
def phases: List[List[Phase]] =
frontendPhases ::: picklerPhases ::: transformPhases ::: backendPhases
/** Phases dealing with the frontend up to trees ready for TASTY pickling */
protected def frontendPhases: List[List[Phase]] =
List(new Parser) :: // scanner, parser
List(new TyperPhase) :: // namer, typer
List(new YCheckPositions) :: // YCheck positions
List(new sbt.ExtractDependencies) :: // Sends information on classes' dependencies to sbt via callbacks
List(new semanticdb.ExtractSemanticDB) :: // Extract info into .semanticdb files
List(new PostTyper) :: // Additional checks and cleanups after type checking
List(new sjs.PrepJSInterop) :: // Additional checks and transformations for Scala.js (Scala.js only)
List(new sbt.ExtractAPI) :: // Sends a representation of the API of classes to sbt via callbacks
List(new SetRootTree) :: // Set the `rootTreeOrProvider` on class symbols
Nil
/** Phases dealing with TASTY tree pickling and unpickling */
protected def picklerPhases: List[List[Phase]] =
List(new Pickler) :: // Generate TASTY info
List(new Inlining) :: // Inline and execute macros
List(new PostInlining) :: // Add mirror support for inlined code
List(new Staging) :: // Check staging levels and heal staged types
List(new Splicing) :: // Replace level 1 splices with holes
List(new PickleQuotes) :: // Turn quoted trees into explicit run-time data structures
Nil
/** Phases dealing with the transformation from pickled trees to backend trees */
protected def transformPhases: List[List[Phase]] =
List(new FirstTransform, // Some transformations to put trees into a canonical form
new CheckReentrant, // Internal use only: Check that compiled program has no data races involving global vars
new ElimPackagePrefixes, // Eliminate references to package prefixes in Select nodes
new CookComments, // Cook the comments: expand variables, doc, etc.
new CheckStatic, // Check restrictions that apply to @static members
new BetaReduce, // Reduce closure applications
new init.Checker) :: // Check initialization of objects
List(new ElimRepeated, // Rewrite vararg parameters and arguments
new ExpandSAMs, // Expand single abstract method closures to anonymous classes
new ProtectedAccessors, // Add accessors for protected members
new ExtensionMethods, // Expand methods of value classes with extension methods
new UncacheGivenAliases, // Avoid caching RHS of simple parameterless given aliases
new ByNameClosures, // Expand arguments to by-name parameters to closures
new HoistSuperArgs, // Hoist complex arguments of supercalls to enclosing scope
new SpecializeApplyMethods, // Adds specialized methods to FunctionN
new RefChecks) :: // Various checks mostly related to abstract members and overriding
List(new ElimOpaque, // Turn opaque into normal aliases
new TryCatchPatterns, // Compile cases in try/catch
new PatternMatcher, // Compile pattern matches
new sjs.ExplicitJSClasses, // Make all JS classes explicit (Scala.js only)
new ExplicitOuter, // Add accessors to outer classes from nested ones.
new ExplicitSelf, // Make references to non-trivial self types explicit as casts
new ElimByName, // Expand by-name parameter references
new StringInterpolatorOpt) :: // Optimizes raw and s string interpolators by rewriting them to string concatentations
List(new PruneErasedDefs, // Drop erased definitions from scopes and simplify erased expressions
new InlinePatterns, // Remove placeholders of inlined patterns
new VCInlineMethods, // Inlines calls to value class methods
new SeqLiterals, // Express vararg arguments as arrays
new InterceptedMethods, // Special handling of `==`, `|=`, `getClass` methods
new Getters, // Replace non-private vals and vars with getter defs (fields are added later)
new SpecializeFunctions, // Specialized Function{0,1,2} by replacing super with specialized super
new CollectNullableFields, // Collect fields that can be nulled out after use in lazy initialization
new ElimOuterSelect, // Expand outer selections
new ResolveSuper, // Implement super accessors
new FunctionXXLForwarders, // Add forwarders for FunctionXXL apply method
new ParamForwarding, // Add forwarders for aliases of superclass parameters
new TupleOptimizations, // Optimize generic operations on tuples
new LetOverApply, // Lift blocks from receivers of applications
new ArrayConstructors) :: // Intercept creation of (non-generic) arrays and intrinsify.
List(new Erasure) :: // Rewrite types to JVM model, erasing all type parameters, abstract types and refinements.
List(new ElimErasedValueType, // Expand erased value types to their underlying implmementation types
new PureStats, // Remove pure stats from blocks
new VCElideAllocations, // Peep-hole optimization to eliminate unnecessary value class allocations
new ArrayApply, // Optimize `scala.Array.apply([....])` and `scala.Array.apply(..., [....])` into `[...]`
new sjs.AddLocalJSFakeNews, // Adds fake new invocations to local JS classes in calls to `createLocalJSClass`
new ElimPolyFunction, // Rewrite PolyFunction subclasses to FunctionN subclasses
new TailRec, // Rewrite tail recursion to loops
new CompleteJavaEnums, // Fill in constructors for Java enums
new Mixin, // Expand trait fields and trait initializers
new LazyVals, // Expand lazy vals
new Memoize, // Add private fields to getters and setters
new NonLocalReturns, // Expand non-local returns
new CapturedVars) :: // Represent vars captured by closures as heap objects
List(new Constructors, // Collect initialization code in primary constructors
// Note: constructors changes decls in transformTemplate, no InfoTransformers should be added after it
new Instrumentation) :: // Count calls and allocations under -Yinstrument
List(new LambdaLift, // Lifts out nested functions to class scope, storing free variables in environments
// Note: in this mini-phase block scopes are incorrect. No phases that rely on scopes should be here
new ElimStaticThis, // Replace `this` references to static objects by global identifiers
new CountOuterAccesses) :: // Identify outer accessors that can be dropped
List(new DropOuterAccessors, // Drop unused outer accessors
new Flatten, // Lift all inner classes to package scope
new RenameLifted, // Renames lifted classes to local numbering scheme
new TransformWildcards, // Replace wildcards with default values
new MoveStatics, // Move static methods from companion to the class itself
new ExpandPrivate, // Widen private definitions accessed from nested classes
new RestoreScopes, // Repair scopes rendered invalid by moving definitions in prior phases of the group
new SelectStatic, // get rid of selects that would be compiled into GetStatic
new sjs.JUnitBootstrappers, // Generate JUnit-specific bootstrapper classes for Scala.js (not enabled by default)
new CollectSuperCalls) :: // Find classes that are called with super
Nil
/** Generate the output of the compilation */
protected def backendPhases: List[List[Phase]] =
List(new backend.sjs.GenSJSIR) :: // Generate .sjsir files for Scala.js (not enabled by default)
List(new GenBCode) :: // Generate JVM bytecode
Nil
Note that phases are grouped, so the phases
method is of type List[List[Phase]]
. The idea is that all phases in a group are fused into a single tree traversal. That way, phases can be kept small (most phases perform a single function) without requiring an excessive number of tree traversals (which are costly, because they have generally bad cache locality).
Phases fall into four categories:
-
Frontend phases:
Frontend
,PostTyper
andPickler
.FrontEnd
parses the source programs and generates untyped abstract syntax trees, which are then typechecked and transformed into typed abstract syntax trees.PostTyper
performs checks and cleanups that require a fully typed program. In particular, it- creates super accessors representing
super
calls in traits - creates implementations of synthetic (compiler-implemented) methods
- avoids storing parameters passed unchanged from subclass to superclass in duplicate fields.
Finally
Pickler
serializes the typed syntax trees produced by the frontend as TASTY data structures. - creates super accessors representing
-
High-level transformations: All phases from
FirstTransform
toErasure
. Most of these phases transform syntax trees, expanding high-level constructs to more primitive ones. The last phase in the group,Erasure
translates all types into types supported directly by the JVM. To do this, it performs another type checking pass, but using the rules of the JVM's type system instead of Scala's. -
Low-level transformations: All phases from
ElimErasedValueType
toCollectSuperCalls
. These further transform trees until they are essentially a structured version of Java bytecode. -
Code generators: These map the transformed trees to Java classfiles or .sjsir files.