This document discusses running JavaScript on microcontrollers. It evaluates two JavaScript engines: JavaScriptCore and Duktape. JavaScriptCore was too large, with initial attempts resulting in binaries over 2 MB. Duktape produced smaller binaries down to 108 KB and passed 96.5% of ECMAScript tests. While most SunSpider tests failed due to timeouts or memory limits, Duktape shows promise for running JavaScript on resource-constrained microcontroller devices.

1. JavaScript on Microcontrollers
Samsung Open Source Group 1
Tilmann Scheller
LLVM Compiler Engineer
t.scheller@samsung.com
Samsung Open Source Group
Samsung Research UK
2nd Samsung Web Tech Talk
San Jose, USA, October 30, 2014

2. Overview
● Introduction
● JavaScript engine code size
● JavaScript engine correctness/performance
● Summary
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3. Introduction
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4. 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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5. Target hardware
● Cortex-M3 microcontroller (32-bit ARM Thumb-2)
● 96KB of RAM
● 512KB of flash memory
● Clocked at 84MHz
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6. 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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7. 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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8. 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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9. 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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10. Code size
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11. 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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12. 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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13. 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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14. 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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15. Code size
Static binary
Engine
Libraries
108
161
100
83
70
59
212
187
127
318
655
790
1207
311
0 200 400 600 800 1000 1200 1400 1600 1800
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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

16. Correctness/Performance
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17. 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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18. test262
85 713
288957
Min Avg Max
350000
300000
250000
200000
150000
100000
50000
0
Execution time (ms)
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Min Avg Max
80
70
60
50
40
30
20
10
0
54
57
75
Memory usage (KB)

19. 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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20. SunSpider
access-nbody
160
140
120
100
80
60
40
20
access-fannkuch
bitops-bits-in-byte
bitops-3bit-bits-in-byte
controlflow-recursive
bitops-bitwise-and
math-partial-sums
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math-spectral-norm
string-fasta
0
149
41.5
64.3
74.8
140.2
25.9
62.3
21.8
119.9
54
64
50 49 50
70
51 53 57
Execution time (s)
Memory usage (KB)

21. Summary
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22. 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
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23. Thank you.
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24. We are hiring!
Contact Information:
Tilmann Scheller
t.scheller@samsung.com
Samsung Open Source Group
Samsung Research UK
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