Implementing Method Type Specialisation In Dotty T Outline What - - PowerPoint PPT Presentation

Method Type Specialisation in Dotty

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Implementing Method Type Specialisation In Dotty

Author Alexandre Sikiaridis Masters Student - EPFL Supervised by Dmitry Petrashko Martin Odersky

Outline

• What is specialisation?
• Method Specialisation - Implementation in Dotty
• Future Developments

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Method Type Specialisation in Dotty

What is Specialisation?

2/15

• Scala compiles parametric polymorphism away through

boxing

• This impacts runtime performance negatively

Method Type Specialisation in Dotty

What is Specialisation?

2/15

• Scala compiles parametric polymorphism away through

boxing

• This impacts runtime performance negatively

Method Type Specialisation in Dotty

What is Specialisation?

Method Type Specialisation in Dotty

2/15

Compiled with scalac

What is Specialisation?

3/15

• Specialisation is an optimisation option
• It avoids boxing by generating ’’specialised’’ copies of methods

and classes

Method Type Specialisation in Dotty

What is Specialisation?

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• Specialisation is an optimisation option
• It avoids boxing by generating ’’specialised’’ copies of methods

and classes

Method Type Specialisation in Dotty

@specialised(Int)

What is Specialisation?

Method Type Specialisation in Dotty

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Compiled with scalac

What is Specialisation?

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• Beware! Specialisation may result in significant code-size increase
• Creates 10 copies per type parameter (9 primitive types) – so

1’000 clones of methods like Function3

• “The greatest shortcoming of the human race is our inability to

understand the exponential function.” ~ Albert A. Bartlett

Method Type Specialisation in Dotty

What is Specialisation?

4/15

• Beware! Specialisation may result in significant code-size increase
• Creates 10 copies per type parameter (9 primitive types) – so

1’000 clones of methods like Function3

• “The greatest shortcoming of the human race is our inability to

understand the exponential function.” ~ Albert A. Bartlett

Method Type Specialisation in Dotty

Implementation in Dotty

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Method Type Specialisation in Dotty

Implementation in Dotty

• Can be divided into four steps
• Annotations retrieval
• Symbols generation
• Trees translation
• Method calls dispatching

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Method Type Specialisation in Dotty

Implementation in Dotty

• Can be divided into four steps
• Annotations retrieval
• Symbols generation
• Trees translation
• Method calls dispatching

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Method Type Specialisation in Dotty

PreSpecializer TypeSpecializer DenotTransformer

Annotations Retrieval

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Method Type Specialisation in Dotty

• Specialisation is done on-

demand

• Either user-provided,

requested by an earlier phase (e.g. Linker) or triggered with ‘’-Yspecialize:all’’

Annotations Retrieval

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Method Type Specialisation in Dotty

• Specialisation is done on-

demand

• Either user-provided,

requested by an earlier phase (e.g. Linker) or triggered with ‘’-Yspecialize:all’’

• @specialized annotations

trigger the optimisation

• Parameters to the annotation

can be Types or

Specializable Groups

Annotations Retrieval

7/15

Method Type Specialisation in Dotty

• Specialisation is done on-

demand

• Either user-provided,

requested by an earlier phase (e.g. Linker) or triggered with ‘’-Yspecialize:all’’

• @specialized annotations

trigger the optimisation

• Parameters to the annotation

can be Types or

Specializable Groups

• Those have to be retrieved

upfront

Annotations Retrieval

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Method Type Specialisation in Dotty

• Specialisation types are passed along to TypeSpecializer with the

concerned method’s symbol, through a PhaseCache

• Requests are later stored by TypeSpecializer in a map from symbol to

types list

Symbols Generation

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Method Type Specialisation in Dotty

• TypeSpecializer triggers the generation of specialised

symbols when appropriate

• Produces the combinatations of primitive types indicated by

PreSpecializer

• Calls DenotTransformer for concrete generation of Symbols

Symbols Generation

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Method Type Specialisation in Dotty

• Dotty performs caching for subtypes
• It holds ’’Frozen’’ Flags to ensure transformations are transferred

to subtypes

• Symbols must be generated during transformation of the

denotation of the owner

• They can then be entered in their owner’s scope
• Method declarations outside of classes can be specialised

without those precautions

Trees Translation

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Method Type Specialisation in Dotty

• TypeSpecializer goes

through all DefDef’s

• If it finds one that has been

requested for specialisation:

• It creates a PolyDefDef

remembering TypeBounds

• It instantiates a

TreeTypeMap handling

recursive translation of the RHS

Incomplete Type Information

12/15

Method Type Specialisation in Dotty

• In some cases, relying on type inference fails and creates

TypeMismatch errors

• From A<:B and B<:Int, typechecker does not infer A<:Int
• Returning Foo[A] does not fit the expected Foo[Int]
• Elected solution: type casts

(Refers to issue #592 | www.github.com/lampepfl/dotty/issues/592)

Incomplete Type Information

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Method Type Specialisation in Dotty

Method Calls Dispatching

• Method calls are compared to generated specialised variants
• If several variants are available, best fitting one is determined

by comparing argument types at call site and those of the specialised variant

• If not all types fit, method call remains generic

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Method Type Specialisation in Dotty

Future Developments

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Method Type Specialisation in Dotty

• Partial Specialisation
• ’’What if I only want to specialise T?’’
• A prototype exists, but needs some debugging still
• Class Specialisation
• More testing

A ’’Special’’ Thank You

• I. Dragos

Type Specialization in Scala 2.8

[http://www.scala-lang.org/old/sites/default/files/sids/dragos/Thu,%202010-05- 06,%2017:56/sid-spec.pdf]

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Method Type Specialisation in Dotty