js on-microcontrollers

1Samsung Open Source Group

JavaScript on Microcontrollers

Tilmann SchellerLLVM Compiler Engineert.scheller@samsung.com

Samsung Open Source GroupSamsung Research UK

2nd Samsung Web Tech TalkSan Jose, USA, October 30, 2014

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Overview

● Introduction

● JavaScript engine code size

● JavaScript engine correctness/performance

● Summary

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Introduction

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Introduction

● Wearable devices becoming more and more widespread

● Weight and energy efficiency a big concern

● JavaScript very popular, easy to learn

● Scales from embedded to servers

● Execute JavaScript on a microcontroller!

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Target hardware

● Cortex-M3 microcontroller (32-bit ARM Thumb-2)

● 96KB of RAM

● 512KB of flash memory

● Clocked at 84MHz

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JS Engines

● Goal: JS engine which fits in the microcontroller constraints

● Option one: Pick an established JS engine and strip it down to fit on a microcontroller

● Option two: Pick a lightweight engine and make sure that it meets all the requirements

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JS Engines

● Two approaches pursued in parallel:

– Try to minimize JavaScriptCore so that it fits on a microcontroller

– Get the lightweight Duktape engine running on a microcontroller

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JavaScriptCore

● Well-established JS engine running on millions of devices

● High-performance (four distinct levels of optimization)

● Never intended to run in resource-constrained environments, primary focus is maximum performance

● Written in C++

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Duktape

● Lightweight JS engine

● Focus on portability and a low footprint

● Written in C and about 40k lines of code (excluding comments)

● ECMAScript E5/E5.1 compliant

● Liberal license

● Active community, has been around for several years

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Code size

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JavaScriptCore minimization

● Challenge: JavaScriptCore was never intended for environments like this

● First attempt:

– Took r170522 (June 26, 2014) of WebKit and built the EFL configuration for Thumb-2

● Original (3.63 MB)● Reduced (2.39 MB)● Static (21.21 MB)

– Several MB of heap memory consumed during runtime

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JavaScriptCore minimization

● Current snapshot: way too big!

● Second try: Use a snapshot from 2008 just before SquirrelFish was merged

● Performed minimizations:

– Qt dependency was removed

– Reduced the size of ICU data by disabling extra features

– Rebuilt ICU with the –Os compiler option

– Pthread was removed

– Built older ICU libs

– Stripped static JSC binary is now 1,647 KB (was: 19,578 KB)

– The engine part of is 311 KB

– The Unicode libraries are 562 Kbyte and the remaining size comes from libc, libm and libstdc++ libraries.

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JavaScriptCore minimization

● Engine size close to target size

● However, libraries rely on too many system calls to make it feasible to run on a baremetal system

● Too much effort to get JSC working on a baremetal system

● Aborting JSC investigation and directing all efforts towards Duktape

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Duktape

● Compiled for Thumb-2

● Using the newlib C standard library

● No OS running on the microcontroller

● Statically linked binary

● C library has a big impact on the size of the executable

● Disabling certain optimizations (through preprocessor macros), helps to reduce code size even further

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Code size

Libraries

Engine

Static binary

0 200 400 600 800 1000 1200 1400 1600 1800

59

83

187

70

100

212

127

161

318

655

108

790

1207

311

1647

Duktape Thumb-2 baremetal noopt

Duktape Thumb-2 baremetal

Duktape ARM baremetal

Duktape ARM Linux glibc

JSC Thumb-2 Linux glibc

KB

Compiled at -Os ..., noopt = Duktape optimizations not compiled in,baremetal builds link against Newlib

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Correctness/Performance

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Correctness - test262

● Duktape 0.12 and Arduino library 1.5.8, without DUK_OPT defines

● test262 (ECMAScript conformance test suite) results:

– Ran 11824 tests● Passed 11411 tests (96.5%)● Failed 413 tests (3.5%)

● Failure details:

– 144 tests ran out of memory

– 10 tests timed out (300 secs)

– 90 tests failed because of an undefined gettimeofday() function

– 169 tests fail because of other reasons (have to be investigated)

– Summary: 167 failures specific to the Arduino Due and 246 tests which fail on x86-64 as well

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test262

Min Avg Max0

50000

100000

150000

200000

250000

300000

350000

85 713

288957

Execution time (ms)

Min Avg Max0

10

20

30

40

50

60

70

80

5457

75

Memory usage (KB)

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SunSpider

● Duktape 0.12 and Arduino library 1.5.8, without DUK_OPT defines

● Ran 26 tests

– Passed 9 tests (34.6%)

– Failed 17 tests (65.4%)

● Failure details:

– 3 tests were too big (string-unpack-code.js (164K), string-tagcloud.js (172K), regexp-dna.js (400K))

– 13 tests run out of memory (3d-cube.js, 3d-morph.js, 3d-raytrace.js, access-binary-trees.js, access-nsieve.js, bitops-nsieve-bits.js, crypto-aes.js, crypto-md5.js, crypto-sha1.js, date-format-tofte.js, date-format-xparb.js, string-base64.js, string-validate-input.js)

– 1 test failed because of undefined gettimeofday() function (math-cordic.js)

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SunSpider

access-fannkuchaccess-nbody

bitops-3bit-bits-in-bytebitops-bits-in-byte

bitops-bitwise-andcontrolflow-recursive

math-partial-sumsmath-spectral-norm

string-fasta

0

20

40

60

80

100

120

140

160149

41.5

64.3

74.8

140.2

25.9

62.3

21.8

119.9

54

64

50 49 50

70

51 5357

Execution time (s)

Memory usage (KB)

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Summary

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Summary

● JavaScriptCore proved to be unsuitable for microcontrollers

● Duktape fits well into flash and main memory

● Duktape has promising correctness results

● Ongoing effort to evaluate Duktape performance

Thank you.

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Contact Information:

Tilmann Schellert.scheller@samsung.com

Samsung Open Source GroupSamsung Research UK