1. JSX Optimizer
DeNA Co., Ltd.
Kazuho Oku

2. The Basic Idea of the JSX Optimizer
perform optimizations that JavaScript VMs
cannot
Jul 19 2012 JSX Optimizer 2

3. Benchmark Scores
Box2D.jsx
+13.5% (iOS 5.0, iPhone 4)
+11.8% (iOS 5.1, iPhone 4S)
+28.7% (Android 2.3, Xperia ray)
numbers are increase in fps
ToDo: post the details
Jul 19 2012 JSX Optimizer 3

4. The Nature of JavaScript
a dynamic language
primitives are untyped
a + 1 becomes "a1" if a is "a", 2 if a is 1
objects are untyped
a.p may point to whatever
solution: inline caching
V8, JavaScriptCore, SpiderMonkey
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5. Inline Caching
The idea:
most of the code will be called with the same
types of arguments
cache the expectation at the call site
Example: point.x
type of point will be { x: ?, y: ? }
thus if the expectation succeeded, point.x will be:
load point_addr[offset_of_x]
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6. Guards in Inline Caching
Example: a = b.p + 1
typeof(b) is as expected
load b.p
typeof(b.p) is number
add one
store the result
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7. Guards in Inline Caching (cont'd)
Example: f()
typeof(f) is function
setup stack frame
jump to the code of f
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8. Guards in Inline Caching (cont'd)
Example: f() // after inlined
typeof(f) is function
 f has not been altered
execute the code expanded in-line
Jul 19 2012 JSX Optimizer 8

9. Guards in Inline Caching (cont'd)
Example: o.f()
typeof(o) is as expected
load o.prototype
typeof (o.prototype) is as expected
load o.prototype.f
typeof(f) is function
setup stack frame
jump to the code of f
Jul 19 2012 JSX Optimizer 9

10. Guards in Inline Caching (cont'd)
Example: o.f() // after inlined
typeof(o) is as expected
load o.prototype
typeof (o.prototype) is as expected
load o.prototype.f
typeof(f) is function
 f has not been altered
execute the code expanded in-line
Jul 19 2012 JSX Optimizer 10

11. What interpreters do / do not
inter-statement optimization
V8 does
SpiderMonkey and JavaScriptCore seem not
inline-expand the JavaScript functions
at least V8 does
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12. JSX optimizer
minimizes the need for IC
caches property access
JSX does not have accessors (Object.defineProperty)
inline-expands the functions
converts member methods to static functions
generates IC-friendly code
always initializes the properties in same order
Note: { x: 0, y: 0 } and { y: 1, x: 1 } are different
types
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13. JSX optimizer
minimizes the pressure on GC
unboxing
folds constants
JavaScript does not have a true const
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14. Example: affine transformation
input:
new Matrix(1, 0, 0, 0, 2, 0).transform(new Point(x, y))
output:
{x: x + 0 * y, y: 0 * x + 2 * y}
Note: 0 * n cannot be optimized to 0, since it becomes NaN if n is NaN
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15. Current Optimization Subcommands
 lto
 no-assert
 no-log
 unclassify
 fold-const
 inline
 dce
 return-if
 lcse
 unbox
 array-length
Jul 19 2012 JSX Optimizer 15

16. Diffs. from an ordinary optimizing compiler
ordinary compiler
generate a low-level non-redundant code
JSX compiler
most code are left as is
while optimizing the code wherever possible
Jul 19 2012 JSX Optimizer 16

17. The Difficulties and Considerations
switch to SSA-based code?
might end-up in generating a very different code
from source
would be difficult to debug (by the users)
may become slow (than the same-looking code in
JavaScript); GCC has the problem
it is troublesome to optimize the code while
maintaining the original code structure
example. support various kind of loops: do-while, for,
for..in, while
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18. The Difficulties and Considerations (cont'd)
Other optimization techniques?
inline allocation?
more powerful dce / dse
reduce the number of locals?
Jul 19 2012 JSX Optimizer 18

19. Some Microbenchmarks
http://jsperf.com/local-vs-member/2
http://jsperf.com/construct-object-vs-array/3
http://jsperf.com/inherit-object-vs-array-set
http://jsperf.com/vars-in-nested-closure
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