Zero-Overhead Metaprogramming

Using Self-Optimizing Interpreters to Remove the Runtime Cost of Reflective Programming

Stefan Marr, INRIA LilleResearch collaboration with Chris Seaton, Oracle Labs

and Stéphane Ducasse, INRIA Lille___PLDI, June 17, 2015

Runtime Metaprogramming==

Just Another Form of Late Binding

Optimized as Such

2

Zero-Overhead Metaprogramming

Using Self-Optimizing Interpreters to Remove the Runtime Cost of Reflective Programming

Stefan Marr, INRIA LilleResearch collaboration with Chris Seaton, Oracle Labs

and Stéphane Ducasse, INRIA Lille___PLDI, June 17, 2015

4

Runtime Metaprogramming

class Proxy def method_missing(name, *args, &block) target.send(name, *args, &block) end

end

obj.invoke('foo', [])obj.getField(idx)obj.setField(idx, val)

Powerful and UsefulFrameworks, Domain-Specific Languages, …

For Late Binding Languages!

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Metaobject Protocol ExampleBuilding a Safe Actor Framework

class ActorDomain : Domain { fn writeToField(obj, fieldIdx, value) { if (Domain.current() == this) { obj.setField(fieldIdx, value); } else { throw new IsolationError(obj); } } /* ... */}

Overhead for every field write!

http://stefan-marr.de/research/omop/

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Metaprogramming is slooow!

meth.invoke() 0.7x OverheadDynamic Proxies 6.5x Overhead

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Everybody Knows:

Runtime Metaprogramming is slooow!

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OPTIMIZING REFLECTIVE OPERATIONS

obj.invoke('foo', [])obj.getField(idx)obj.setField(idx, val)

Method Invocation, Field Accesses, …

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Reflective Method Invocation

cnt.invoke('+', [1])

How to optimize this?

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Optimize Direct Invocation

cnt + 1

Hölzle, Chambers, Ungar, ’91.

cnt.+(1)

is polymorphic-> Cache at Send Site

Solution: Polymorphic Inline Cache

• Avoids lookup• Enables JIT to

inline

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Generalize Polymorphic Inline Cachesto Dispatch Chains

cnt 1

invocation

read var literal

+dispatch chain

dispatch

method

cnt.+(1)

Common Place in Self-Optimizing Interpreters:Würthinger et al. [2012], Humer et al. [2014], Wöß et al. [2014]

Chain Nodes can have arbitrary behavior

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Dispatch Chain for Method Invocation

UnInit

cnt: 0 (Integer object)

+ int ++

check class==int

true

falseUnInit

CacheNode

cnt.+(1)

• Avoids lookup• Enables inlining

int

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Reflective Method Invocation

cnt.invoke('+', [1])

How to optimize this?

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Optimizing Reflective Method Invocationcnt.invoke('+', [1])

'+

UnIn

InvokeNode

cnt

[1]

dispatch chainon method name

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NameNode

UnIn

methoddispatch chain

dispatch

method

dispatch chainon method name

Optimizing Reflective Method Invocationcnt.invoke('+', [1])

'+'

InvokeNode

cnt

[1]

'+'

check name=='+'true

false

nesting of dispatch chainsresolves variability

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Simple Metaprogramming Solved!

class Proxy def method_missing(name, *args, &block) target.send(name, *args, &block) end

end

obj.invoke('foo', [])obj.getField(idx)obj.setField(idx, val)✔

✔ ?

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Metaobject Protocol ExampleBuilding a Safe Actor Framework

class ActorDomain : Domain { fn writeToField(obj, fieldIdx, value) { if (Domain.current() == this) { obj.setField(fieldIdx, value); } else { throw new IsolationError(obj); } } /* ... */}

http://stefan-marr.de/research/omop/

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An Actor Exampleactor.fieldA := 1

actor

.fieldA :=

1

temp variableread

literal

field write semantic dependson metaobject

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Dispatch Chains to Resolve Variability

field writeUnIn

dispatch chainon metaobjectCacheMOb

j.fieldA := ActorDomain.

writeToField()

Intercessionhandler

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Dispatch Chains to Resolve Variability

field write

UnIn

dispatch chainon metaobjectCacheMOb

j.fieldA :=

Shortcut for standard semantics

ActorDomain.writeToField()

Intercessionhandler

StdDomain

StdWrite

Standarddirect write

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DOES IT WORK?Is it fast?

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Evaluation: Self-Optimizing Interpreters

Academic “Simplicity”Verify idea independent of compilation technique• Meta-Tracing (RPython)• Partial Evaluation (Truffle)

Industrial “Sophistication”High-Performance Truffle Backend for JRuby• Case Study with Production

Code

http://som-st.github.io

+Trufflehttps://github.com/jruby/jruby/wiki/Truffle

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Simple Metaprogramming: Zero Overhead

http://stefan-marr.de/papers/pldi-marr-et-al-zero-overhead-metaprogramming-artifacts/

Reflective & Direct:

Identical Machine Code!

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Production Code: Image Processing

+Truffle

Optimize Simple

Metaprogramming!

Spee

dup

over

uno

ptim

ized

(hig

her i

s be

tter

)

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OMOP Overhead

meta-tracing partial evaluation

Overhead: 4% (min. -1%, max. 19%) Overhead: 9% (min. -7%, max. 38%)Metaobject Protocols:

Fast without Compromises!

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Open Research Questions

• Do programs with MOPs have classic trimodal distribution of send-site polymorphism?– i.e. does basic PIC hypothesis apply?– Same polymorphism degree, inlining limits, …

• How to implement dispatch chains efficiently in classic tier JIT compilers?

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Dispatch Chains: A Generalization of PICs

• Complete removal of reflective overhead– Simple and sufficient– For meta-tracing and partial evaluation

• Enables – Zero-Overhead Metaprogramming–efficient MOPs for smarter DSLs

dispatch chaindispatch

method

Runtime Metaprogramming==

Just Another Form of Late Binding

Optimized as Such

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+Trufflehttp://stefan-marr.de/papers/pldi-marr-et-al-zero-overhead-metaprogramming/