yi lin, steve blackburn the australian national university bypassing portability pitfalls of...

Yi Lin, Steve BlackburnThe Australian National University

Bypassing Portability Pitfallsof High-level Low-level Programming

IntroductionPortability issues of high-level low-level programming

2Lin & Blackburn | Bypassing Portability Pitfalls | VMIL’12

Program Portability

• Portability across platforms• Portability across runtimes

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low-level code high-level languages

Introduction

Lin & Blackburn | Bypassing Portability Pitfalls | VMIL’12

High-level Low-level Programming

• High-level languages (HLL)✔ Correctness (memory-safety, type-safety)✔ Software engineering tools: a(abstraction, encapsulation)

• Applied to low-level systems programming

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Robustness Productivity

Introduction


The Portability Pitfalls of HLLL

✘Hardware portability– Low-level code is hardware-sensitive

✘Portability between runtimes– High-level languages need to be extended, and cannot run on a stock

runtime

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Code reusability Development Cost

Introduction


Example: Portable MMTk

Started in JikesRVM, ported to VMKit, Rotor

Shortcomings of previous porting– Inconvenient/slow integration with legacy code: FFI– Repeated work for each porting.

Language extensions need to be implemented for different targets

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Introduction


Possibility of Translation Into LLL

✔ Portable low-level languages (LLL)✗Difficult to map full HLL into LLL

– Dynamic dispatch, exceptions, dynamic dispatch, GC, etc.

✔ High-level low-level lang. is restricted– Restrictions may facilitate translation to LLL

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Introduction


Presentation Outline

Two topics, key to our approach:• HLL restrictions in systems programming• Translation into LLL

Outcome: RJava (restricted subset of Java)– Application to VM implementation– Explicit restriction definition– High-level benefits– Improved portability


Introduction

HLL Restrictionin Systems Programming

Observations, Design Concerns and Concrete RJava,


Observations

MMTk B/L Compiler JikesRVM Eclipse

Lines of Code 29933 17762 113359 n/a

new / LOC 0.59% 0.86% 2.40% 4.47%

throws / LOC - - 0.21% 1.33%

import java.* / LOC - 0.03% 0.40% 0.82%

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most restricted unrestricted

HLL Restriction

Studied use of language features in four non-trivial contexts


Observations (cont.)

Restrictions exist for:• Correctness

– No GC/allocation in Memory manager g use raw memory– No threading/scheduling in Scheduler g use primitive locks

• Performance– Avoid dynamic dispatch in performance-critical areas– Avoid array bound checks in uninterruptible area


HLL Restriction

Observations (cont.)

Benefits of restricted HLL• Benefits reduced, principally static

– Type/memory-safety at source code level– Software engineering tools

• Restrictions are applied to limited scope– Maximizing benefits

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Restricted HLL has advantages over LLL


HLL Restriction

Design Concerns of RJava

• Expressiveness vs. Restriction– Mappability to LL– We do not introduce more restrictions than necessary

• Fixed vs. Flexible Restrictions for scope– Restrictions are different among different VM components– Restrictions may be still different within one VM component


HLL Restriction

Concrete RJava: Extensions

vmmagic extensions [1]• Type-system extensions

– Raw storage– Unboxed types

• Semantic extensions– Intrinsic functions– Semantic regimes

[1] Frampton et al., VEE09


HLL Restriction

Concrete RJava: Restrictions

• Restriction Rules– Forbid language features– Allow static checking

• Restriction Rulesets– 1-to-1 mapping with scopes

• @RJavaCore Ruleset– Predefined ruleset– Defines RJava – Minimum restriction

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@RestrictionRule

public@interface NoDynamicLoading {}

compiler intrinsics checking rule

@RestrictionRuleset

@NoDynamicLoading@NoReflection@NoException@NoCastOnMagicTypepublic@interface RJavaCore {}

@RJavaCorepublic class SomeRJavaClass {}


HLL Restriction

Example: MMTk with RJava

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MMTk codebase

@RestrictedRuleset

@RJavaCore@NoRunTimeAllocation@NoClassLibrary@Uninterruptiblepublic @interface MMTk {}

disallow infinite regress

disallow thread switching


HLL Restriction

Example: MMTk with RJava (cont.)

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@RestrictedRuleset

@MMTk@NoVirtualMethodpublic @interface MMTkFastpath {}

avoid dynamic dispatch

fast path

MMTk codebase


HLL Restriction

Example: MMTk with RJava (cont.)

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@RestrictedRuleset

@MMTkFastpath@NoPutfield@NoPutstaticpublic @interface MMTkWriteBarrier {}

disallow infinite regress

MMTk codebase

fast path

write barrier


HLL Restriction

Translation into LLLCurrent Work, RJava Toolchain, RJava frontend


Bypass Approach

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RJava Toolchain:1. Static constraint checking tool2. Frontend: RJava into LLL3. Backend: LLL into executable


Translating R-HLL to LL

RJava Frontend

• Generating plain LL code, with no dependency on Java runtime.

• Besides syntax mapping, the frontend needs to: – Implement compiler intrinsics– Unbox magic types– Remove dependencies on class library


Translating R-HLL to LL

Summary and Future WorkRJava Toolchain, frontend


Summary

• Portability pitfalls of HL-LL programming– Affecting code reusability and development cost– Possibility of low-level language bypass

• HLL restriction and RJava– Observations, design concerns, concrete RJava

• LLL bypass and RJava toolchain


Summary & Future

Future Work - Bootstrapping Java VM

• VMKit– RJava frontend to LLVM, use to compile MMTk

• Bootstrapping Java VM– Interpreter/baseline compiler in RJava– Similar to PyPy/RPython approach– Better VM portability


Summary & Future

yi lin, steve blackburn the australian national university bypassing portability pitfalls of...

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