Know your Javascript Engine

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  • \n
  • JavaScript now runs 10-100x faster than 5 years ago, fast on all major browsers\nDevelopers using it for new apps: interactive movies, games, photo editing, slides\nI’m going to explain how it works to help you get the most out of these engines\n
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  • Know your Javascript Engine

    1. 1. Know Your EnginesHow to Make Your JavaScript Fast Dave Mandelin June 15, 2011 O’Reilly Velocity
    2. 2. 5 years of progress... 10 JavaScript 7.5 Crun time vs. C 5 2.5 0 2006 2008 2011 one program on one popular browser: 10x faster!
    3. 3. ...lost in an instant!function f() { var sum = 0; for (var i = 0; i < N; ++i) { sum += i; }}function f() { eval(“”); var sum = 0; for (var i = 0; i < N; ++i) { sum += i; }}
    4. 4. ...lost in an instant!function f() { 80 var sum = 0; for (var i = 0; i < N; ++i) { sum += i; 60 }} 40 20function f() { eval(“”); 0 without eval with eval var sum = 0; for (var i = 0; i < N; ++i) { sum += i; with eval(“”) up to} } 10x slower!
    5. 5. Making JavaScript Fast Or, Not Making JavaScript SlowHow JITs make JavaScript not slowHow not to ruin animation with pausesHow to write JavaScript that’s not slow
    6. 6. The 2006 JavaScript Engine
    7. 7. Inside the 2006 JS Engine DOM StandardFront End Interpreter Library Garbage Collector
    8. 8. Inside the 2006 JS Engine// JavaScript sourcee.innerHTML = n + “ items”; DOM Standard Front End Interpreter Library Garbage Collector
    9. 9. Inside the 2006 JS Engine// JavaScript sourcee.innerHTML = n + “ items”; DOM Standard Front End Interpreter Library // bytecode (AST in some engines) Garbage tmp_0 = add var_1 str_3 Collector setprop var_0 ‘innerHTML’ tmp_0
    10. 10. Inside the 2006 JS Engine// JavaScript sourcee.innerHTML = n + “ items”; DOM Standard Front End Interpreter Library Run the bytecode // bytecode (AST in some engines) Garbage tmp_0 = add var_1 str_3 Collector setprop var_0 ‘innerHTML’ tmp_0
    11. 11. Inside the 2006 JS Engine// JavaScript sourcee.innerHTML = n + “ items”; DOM Standard Front End Interpreter Library Run the bytecode Reclaim memory // bytecode (AST in some engines) Garbage tmp_0 = add var_1 str_3 Collector setprop var_0 ‘innerHTML’ tmp_0
    12. 12. Inside the 2006 JS Engine Set innerHTML// JavaScript sourcee.innerHTML = n + “ items”; DOM Standard Front End Interpreter Library Run the bytecode Reclaim memory // bytecode (AST in some engines) Garbage tmp_0 = add var_1 str_3 Collector setprop var_0 ‘innerHTML’ tmp_0
    13. 13. Why it’s hard to make JS fast Because JavaScript is an untyped language. untyped = no type declarations
    14. 14. Operations in an untyped language x = y + z can mean many things • if y and z are numbers, numeric addition • if y and z are strings, concatenation • and many other cases; y and z can have different types
    15. 15. Engine-Internal TypesJS engines use finer-grained types internally. JavaScript type number object
    16. 16. Engine-Internal TypesJS engines use finer-grained types internally. JavaScript type Engine type number 32-bit* integer 64-bit floating-point object
    17. 17. Engine-Internal TypesJS engines use finer-grained types internally. JavaScript type Engine type number 32-bit* integer 64-bit floating-point { a: 1 } { a: 1, b: 2 } object { a: get ... } { a: 1, __proto__ = new C }
    18. 18. Engine-Internal TypesJS engines use finer-grained types internally. JavaScript type Engine type number 32-bit* integer 64-bit floating-point { a: 1 } { a: 1, b: 2 } Different object { a: get ... } shapes { a: 1, __proto__ = new C }
    19. 19. Values in an untyped languageBecause JavaScript is untyped, the interpreter needs boxed values. Boxed Unboxed Purpose Storage Computation Examples (INT, 55) 55 (STRING, “foo”) “foo” Definition (type tag, C++ value) C++ value only boxed values can be stored in variables, only unboxed values can be computed with (+, *, etc)
    20. 20. Running Code in the Interpreter Here’s what the interpreter must do to execute x = y + z:
    21. 21. Running Code in the Interpreter Here’s what the interpreter must do to execute x = y + z: ‣ read the operation x = y + z from memory
    22. 22. Running Code in the Interpreter Here’s what the interpreter must do to execute x = y + z: ‣ read the operation x = y + z from memory ‣ read the boxed inputs y and z from memory
    23. 23. Running Code in the Interpreter Here’s what the interpreter must do to execute x = y + z: ‣ read the operation x = y + z from memory ‣ read the boxed inputs y and z from memory ‣ check the types of y and z and choose the action
    24. 24. Running Code in the Interpreter Here’s what the interpreter must do to execute x = y + z: ‣ read the operation x = y + z from memory ‣ read the boxed inputs y and z from memory ‣ check the types of y and z and choose the action ‣ unbox y and z
    25. 25. Running Code in the Interpreter Here’s what the interpreter must do to execute x = y + z: ‣ read the operation x = y + z from memory ‣ read the boxed inputs y and z from memory ‣ check the types of y and z and choose the action ‣ unbox y and z ‣ execute the action
    26. 26. Running Code in the Interpreter Here’s what the interpreter must do to execute x = y + z: ‣ read the operation x = y + z from memory ‣ read the boxed inputs y and z from memory ‣ check the types of y and z and choose the action ‣ unbox y and z ‣ execute the action ‣ box the output x
    27. 27. Running Code in the Interpreter Here’s what the interpreter must do to execute x = y + z: ‣ read the operation x = y + z from memory ‣ read the boxed inputs y and z from memory ‣ check the types of y and z and choose the action ‣ unbox y and z ‣ execute the action ‣ box the output x ‣ write the boxed output x to memory
    28. 28. Running Code in the Interpreter Here’s what the interpreter must do to execute x = y + z: ‣ read the operation x = y + z from memory ‣ read the boxed inputs y and z from memory ‣ check the types of y and z and choose the action ‣ unbox y and z This is the only real work! ‣ execute the action ‣ box the output x ‣ write the boxed output x to memory
    29. 29. Running Code in the Interpreter Here’s what the interpreter must do to execute x = y + z: ‣ read the operation x = y + z from memory ‣ read the boxed inputs y and z from memory ‣ check the types of y and z and choose the action ‣ unbox y and z This is the only real work! ‣ execute the action ‣ box the output x Everything else is ‣ write the boxed output x to memory overhead.
    30. 30. The 2011 JavaScript Engine
    31. 31. Inside the 2011 JS Engine Garbage Collector DOM InterpreterJavaScript source Standard Library Front End bytecode/AST
    32. 32. Inside the 2011 JS Engine Garbage Collector DOM InterpreterJavaScript source Standard Library Front End JIT Compiler Compile to x86/x64/ARM bytecode/AST
    33. 33. Inside the 2011 JS Engine Garbage Collector DOM InterpreterJavaScript source Standard Library Fast! x86/x64/ARM Front End JIT Compiler Compile to x86/x64/ARM CPU bytecode/AST
    34. 34. Inside the 2011 JS Engine Garbage Collector DOM InterpreterJavaScript source Standard Library Fast! x86/x64/ARM Front End JIT Compiler Compile to x86/x64/ARM CPU Type-Specializing bytecode/AST JIT Compiler Ultra Fast!
    35. 35. Inside the 2011 JS Engine Garbage Collector DOM InterpreterJavaScript source Standard Library Fast! x86/x64/ARM Front End JIT Compiler Compile to x86/x64/ARM CPU Type-Specializing bytecode/AST JIT Compiler Ultra Fast!
    36. 36. Inside the 2011 JS Engine THE Garbage DOM Collector SLOW ZONE InterpreterJavaScript source Standard Library Fast! x86/x64/ARM Front End JIT Compiler Compile to x86/x64/ARM CPU Type-Specializing bytecode/AST JIT Compiler Ultra Fast!
    37. 37. Running Code with the JIT All Major The basic JIT compiler on x = y + z: Browsers‣ read the operation x = y + z from memory‣ read the inputs y and z from memory‣ check the types of y and z and choose the action‣ unbox y and z‣ execute the action‣ box the output x‣ write the output x to memory
    38. 38. Running Code with the JIT All Major The basic JIT compiler on x = y + z: Browsers‣ read the operation x = y + z from memory CPU does it for us!‣ read the inputs y and z from memory‣ check the types of y and z and choose the action‣ unbox y and z‣ execute the action‣ box the output x‣ write the output x to memory
    39. 39. Running Code with the JIT All Major The basic JIT compiler on x = y + z: Browsers‣ read the operation x = y + z from memory CPU does it for us!‣ read the inputs y and z from memory‣ check the types of y and z and choose the action‣ unbox y and z‣ execute the action‣ box the output x‣ write the output x to memory JIT code can keep things in registers
    40. 40. Choosing the action in the JIT
    41. 41. Choosing the action in the JIT• Many cases for operators like +
    42. 42. Choosing the action in the JIT• Many cases for operators like +• Engines generate fast JIT code for “common cases” • number + number • string + string
    43. 43. Choosing the action in the JIT• Many cases for operators like +• Engines generate fast JIT code for “common cases” • number + number • string + string• “Rare cases” run in the slow zone • number + undefined
    44. 44. JITs for Regular Expressions All Major Browsers• There is a separate JIT for regular expressions• Regular expressions are generally faster than manual search• Still in the slow zone: • Some complex regexes (example: backreferences) • Building result arrays (test much faster than exec)
    45. 45. Object Properties function f(obj) { return obj.a + 1; }
    46. 46. Object Properties function f(obj) { return obj.a + 1; }• Need to search obj for a property named a slow
    47. 47. Object Properties function f(obj) { return obj.a + 1; }• Need to search obj for a property named a slow• May need to search prototype chain up several levels super-slow
    48. 48. Object Properties function f(obj) { return obj.a + 1; }• Need to search obj for a property named a slow• May need to search prototype chain up several levels super-slow• Finally, once we’ve found it, get the property value fast!
    49. 49. ICs: a mini-JIT for objects All Major Browsers
    50. 50. ICs: a mini-JIT for objects All Major Browsers• Properties become fast with inline caching (we prefer IC)
    51. 51. ICs: a mini-JIT for objects All Major Browsers• Properties become fast with inline caching (we prefer IC)• Basic plan:
    52. 52. ICs: a mini-JIT for objects All Major Browsers• Properties become fast with inline caching (we prefer IC)• Basic plan: 1. First time around, search for the property in the Slow Zone
    53. 53. ICs: a mini-JIT for objects All Major Browsers• Properties become fast with inline caching (we prefer IC)• Basic plan: 1. First time around, search for the property in the Slow Zone 2. But record the steps done to actually get the property
    54. 54. ICs: a mini-JIT for objects All Major Browsers• Properties become fast with inline caching (we prefer IC)• Basic plan: 1. First time around, search for the property in the Slow Zone 2. But record the steps done to actually get the property 3. Then JIT a little piece of code that does just that
    55. 55. ICs: Example Example Codevar obj1 = { a: 1, b: 2, c: 3 };var obj2 = { b: 2 };function f(obj) { return obj.b + 1;}
    56. 56. ICs: Example Example Codevar obj1 = { a: 1, b: 2, c: 3 };var obj2 = { b: 2 };function f(obj) { return obj.b + 1;} Generated JIT Code ... jump slowPropAccess slowPropAccess:continue_1: ... set up call ... call ICGetProp ; C++ Slow Zone jump continue_1
    57. 57. ICs: Example Example Code shape=12, in position 1var obj1 = { a: 1, b: 2, c: 3 };var obj2 = { b: 2 };function f(obj) { return obj.b + 1;} Generated JIT Code ... jump slowPropAccess slowPropAccess:continue_1: ... set up call ... call ICGetProp ; C++ Slow Zone jump continue_1
    58. 58. ICs: Example Example Code icStub_1: shape=12, in position 1 compare obj.shape, 12var obj1 = { a: 1, b: 2, c: 3 }; jumpIfFalse slowPropAccessvar obj2 = { b: 2 }; load obj.props[1] jump continue_1function f(obj) { return obj.b + 1;} Generated JIT Code ... jump slowPropAccess slowPropAccess:continue_1: ... set up call ... call ICGetProp ; C++ Slow Zone jump continue_1
    59. 59. ICs: Example Example Code icStub_1: shape=12, in position 1 compare obj.shape, 12var obj1 = { a: 1, b: 2, c: 3 }; jumpIfFalse slowPropAccessvar obj2 = { b: 2 }; load obj.props[1] jump continue_1function f(obj) { return obj.b + 1;} Generated JIT Code ... jump slowPropAccess slowPropAccess:continue_1: ... set up call ... call ICGetProp ; C++ Slow Zone jump continue_1
    60. 60. ICs: Example Example Code icStub_1: shape=12, in position 1 compare obj.shape, 12var obj1 = { a: 1, b: 2, c: 3 }; jumpIfFalse slowPropAccessvar obj2 = { b: 2 }; load obj.props[1] jump continue_1function f(obj) { return obj.b + 1;} Generated JIT Code ... jump slowPropAccess slowPropAccess:continue_1: ... set up call ... call ICGetProp ; C++ Slow Zone jump continue_1
    61. 61. ICs: Example Example Code icStub_1: shape=12, in position 1 compare obj.shape, 12var obj1 = { a: 1, b: 2, c: 3 }; jumpIfFalse slowPropAccessvar obj2 = { b: 2 }; load obj.props[1] shape=15, in position 0 jump continue_1function f(obj) { return obj.b + 1;} Generated JIT Code ... jump slowPropAccess slowPropAccess:continue_1: ... set up call ... call ICGetProp ; C++ Slow Zone jump continue_1
    62. 62. ICs: Example Example Code icStub_1: shape=12, in position 1 compare obj.shape, 12var obj1 = { a: 1, b: 2, c: 3 }; jumpIfFalse slowPropAccessvar obj2 = { b: 2 }; load obj.props[1] shape=15, in position 0 jump continue_1function f(obj) { return obj.b + 1;} icStub_2: compare obj.shape, 15 jumpIfFalse slowPropAccess Generated JIT Code load obj.props[0] jump continue_1 ... jump slowPropAccess slowPropAccess:continue_1: ... set up call ... call ICGetProp ; C++ Slow Zone jump continue_1
    63. 63. ICs: Example Example Code icStub_1: shape=12, in position 1 compare obj.shape, 12var obj1 = { a: 1, b: 2, c: 3 }; jumpIfFalse slowPropAccessvar obj2 = { b: 2 }; load obj.props[1] shape=15, in position 0 jump continue_1function f(obj) { return obj.b + 1;} icStub_2: compare obj.shape, 15 jumpIfFalse slowPropAccess Generated JIT Code load obj.props[0] jump continue_1 ... jump slowPropAccess slowPropAccess:continue_1: ... set up call ... call ICGetProp ; C++ Slow Zone jump continue_1
    64. 64. ICs: Example Example Code icStub_1: shape=12, in position 1 compare obj.shape, 12var obj1 = { a: 1, b: 2, c: 3 }; jumpIfFalse slowPropAccessvar obj2 = { b: 2 }; load obj.props[1] shape=15, in position 0 jump continue_1function f(obj) { return obj.b + 1;} icStub_2: compare obj.shape, 15 jumpIfFalse slowPropAccess Generated JIT Code load obj.props[0] jump continue_1 ... jump slowPropAccess slowPropAccess:continue_1: ... set up call ... call ICGetProp ; C++ Slow Zone jump continue_1
    65. 65. These are fast because of ICs Global Variable Accessvar q = 4;var r;function f(obj) { r = q;}
    66. 66. These are fast because of ICs Global Variable Accessvar q = 4;var r;function f(obj) { r = q;} Direct Property Accessvar obj1 = { a: 1, b: 2, c: 3 };var obj2 = { b: 2 };function f(obj) { obj2.b = obj1.c;}
    67. 67. These are fast because of ICs Global Variable Access Closure Variable Accessvar q = 4; var f = function() {var r; var x = 1; var g = function() {function f(obj) { var sum = 0; r = q; for (var i = 0; i < N; ++i) {} sum += x; } return sum; Direct Property Access } return g();var obj1 = { a: 1, b: 2, c: 3 }; }var obj2 = { b: 2 };function f(obj) { obj2.b = obj1.c;}
    68. 68. Prototypes don’t hurt muchfunction A(x) { this.x = x;}function B(y) { this.y = y;}B.prototype = new A;function C(z) { this.z = z;}C.prototype = new B;
    69. 69. Prototypes don’t hurt much new Afunction A(x) { this.x = x;} new Bfunction B(y) { proto this.y = y;} new C(1)B.prototype = new A;function C(z) { this.z = z;}C.prototype = new B;
    70. 70. Prototypes don’t hurt much new Afunction A(x) { this.x = x;} new Bfunction B(y) { proto this.y = y;} new C(1) new C(2)B.prototype = new A;function C(z) { this.z = z;}C.prototype = new B;
    71. 71. Prototypes don’t hurt much new Afunction A(x) { this.x = x;} new Bfunction B(y) { proto this.y = y;} new C(1) new C(2) new C(3)B.prototype = new A;function C(z) { this.z = z;}C.prototype = new B;
    72. 72. Prototypes don’t hurt much new Afunction A(x) { this.x = x;} new Bfunction B(y) { proto this.y = y;} new C(1) new C(2) new C(3)B.prototype = new A;function C(z) { this.z = z; Shape of new C objects determines prototype}C.prototype = new B;
    73. 73. Prototypes don’t hurt much new Afunction A(x) { this.x = x;} new Bfunction B(y) { proto this.y = y;} new C(1) new C(2) new C(3)B.prototype = new A;function C(z) { this.z = z; Shape of new C objects determines prototype}C.prototype = new B; -> IC can generate code that checks shape, then reads directly from prototype without walking
    74. 74. Many Shapes Slow Down ICsWhat happens if many shapes of obj are passed to f? function f(obj) { return obj.p; } ICs end up looking like this:
    75. 75. Many Shapes Slow Down ICs What happens if many shapes of obj are passed to f? function f(obj) { return obj.p; } ICs end up looking like this:jumpIf shape != 12read for shape 12
    76. 76. Many Shapes Slow Down ICs What happens if many shapes of obj are passed to f? function f(obj) { return obj.p; } ICs end up looking like this:jumpIf shape != 12read for shape 12 jumpIf shape != 15 read for shape 15
    77. 77. Many Shapes Slow Down ICs What happens if many shapes of obj are passed to f? function f(obj) { return obj.p; } ICs end up looking like this:jumpIf shape != 12read for shape 12 jumpIf shape != 15 read for shape 15 jumpIf shape != 6 read for shape 6
    78. 78. Many Shapes Slow Down ICs What happens if many shapes of obj are passed to f? function f(obj) { return obj.p; } ICs end up looking like this: ...jumpIf shape != 12 jumpIf shape != 16read for shape 12 read for shape 16 jumpIf shape != 15 jumpIf shape != 22 read for shape 15 read for shape 22 jumpIf shape != 6 jumpIf shape != 3 read for shape 6 read for shape 3
    79. 79. Many shapes in practice 100 IE IE Slow Zone for 2+ shapes Opera Chrome 75 Opera # of shapes doesn’t matter!nanoseconds/iteration Firefox Safari 50 Chrome more shapes -> slower Firefox 25 slower with more shapes, but levels off in Slow Zone Safari 0 1 2 8 16 32 100 200 # of shapes at property read site
    80. 80. Deeply Nested Closures are Slowervar f = function() { var x; var g = function() { var h = function() { var y; var i = function () { var j = function() { z = x + y;
    81. 81. Deeply Nested Closures are Slowervar f = function() { f call object var x; var g = function() { var h = function() { h call object var y; var i = function () { var j = function() { j call object z = x + y; First call to f
    82. 82. Deeply Nested Closures are Slowervar f = function() { f call object f call object var x; var g = function() { var h = function() { h call object h call object var y; var i = function () { var j = function() { j call object j call object z = x + y; First call to f Second call to f
    83. 83. Deeply Nested Closures are Slowervar f = function() { f call object f call object var x; var g = function() { var h = function() { h call object h call object var y; var i = function () { var j = function() { j call object j call object z = x + y; First call to f Second call to f• Prototype chains don’t slow us down, but deep closure nesting does. Why?
    84. 84. Deeply Nested Closures are Slowervar f = function() { f call object f call object var x; var g = function() { var h = function() { h call object h call object var y; var i = function () { var j = function() { j call object j call object z = x + y; First call to f Second call to f• Prototype chains don’t slow us down, but deep closure nesting does. Why?• Every call to f generates a unique closure object to hold x.
    85. 85. Deeply Nested Closures are Slowervar f = function() { f call object f call object var x; var g = function() { var h = function() { h call object h call object var y; var i = function () { var j = function() { j call object j call object z = x + y; First call to f Second call to f• Prototype chains don’t slow us down, but deep closure nesting does. Why?• Every call to f generates a unique closure object to hold x.• The engine must walk up to x each time
    86. 86. Properties in the Slow Zone
    87. 87. Properties in the Slow Zone Undefined Property (Fast on Firefox, Chrome)var a = {};a.x;
    88. 88. Properties in the Slow Zone Undefined Property (Fast on Firefox, Chrome) var a = {}; a.x; DOM Access(I only tested .id, so take with a grain of salt-- other properties may differ)var a = document.getByElementId(“foo”);a.id;
    89. 89. Properties in the Slow Zone Undefined Property Scripted Getter (Fast on Firefox, Chrome) (Fast on IE) var a = {}; var a = { x: get() { return 1; } }; a.x; a.x; DOM Access(I only tested .id, so take with a grain of salt-- other properties may differ)var a = document.getByElementId(“foo”);a.id;
    90. 90. Properties in the Slow Zone Undefined Property Scripted Getter (Fast on Firefox, Chrome) (Fast on IE) var a = {}; var a = { x: get() { return 1; } }; a.x; a.x; DOM Access Scripted Setter(I only tested .id, so take with a grain of salt-- other properties may differ) var a = { x: set(y) { this.x_ = y; } }; a.x = 1;var a = document.getByElementId(“foo”);a.id;
    91. 91. The Type-Specializing JIT Firefox 3.5+ (Tracemonkey) Chrome 11+ (Crankshaft)
    92. 92. Types FTW!If only JavaScript had type declarations...
    93. 93. Types FTW! If only JavaScript had type declarations...➡ The JIT would know the type of every local variable
    94. 94. Types FTW! If only JavaScript had type declarations...➡ The JIT would know the type of every local variable ➡ Know exactly what action to use (no type checks)
    95. 95. Types FTW! If only JavaScript had type declarations...➡ The JIT would know the type of every local variable ➡ Know exactly what action to use (no type checks) ➡ Local variables don’t need to be boxed (or unboxed)
    96. 96. Types FTW! If only JavaScript had type declarations...➡ The JIT would know the type of every local variable ➡ Know exactly what action to use (no type checks) ➡ Local variables don’t need to be boxed (or unboxed) We call this kind of JIT a type-specializing JIT
    97. 97. But JS doesn’t have types
    98. 98. But JS doesn’t have types• Problem: JS doesn’t have type declarations • won’t have them any time soon • we don’t want to wait
    99. 99. But JS doesn’t have types• Problem: JS doesn’t have type declarations • won’t have them any time soon • we don’t want to wait• Solution: run the program for a bit, monitor types
    100. 100. But JS doesn’t have types• Problem: JS doesn’t have type declarations • won’t have them any time soon • we don’t want to wait• Solution: run the program for a bit, monitor types• Then recompile optimized for those types
    101. 101. Running with the Type-Specializing JIT Firefox 3.5+ On x = y + z: Chrome 11+ ‣ read the operation x = y + z from memory ‣ read the inputs y and z from memory ‣ check the types of y and z and choose the action ‣ unbox y and z ‣ execute the action ‣ box the output x ‣ write the output x to memory
    102. 102. Running with the Type-Specializing JIT Firefox 3.5+ On x = y + z: Chrome 11+ ‣ read the operation x = y + z from memory ‣ read the inputs y and z from memory ‣ check the types of y and z and choose the action ‣ unbox y and z ‣ execute the action ‣ box the output x ‣ write the output x to memory
    103. 103. Running with the Type-Specializing JIT Firefox 3.5+ On x = y + z: Chrome 11+ ‣ read the operation x = y + z from memory ‣ read the inputs y and z from memory ‣ check the types of y and z and choose the action ‣ unbox y and z ‣ execute the action ‣ box the output x ‣ write the output x to memory
    104. 104. Further Optimization 1 Automatic Inlining original codefunction getPop(city) { return popdata[city.id];}for (var i = 0; i < N; ++i) { total += getPop(city);}
    105. 105. Further Optimization 1 Automatic Inlining original code JIT compiles as iffunction getPop(city) { you wrote this return popdata[city.id];} for (var i = 0; i < N; ++i) { total += popdata[city.id];for (var i = 0; i < N; ++i) { } total += getPop(city);}
    106. 106. Further Optimization 2 Loop Invariant Code Motion (LICM, “hoisting”) original codefor (var i = 0; i < N; ++i) { total += a[i] * (1 + options.tax);}
    107. 107. Further Optimization 2 Loop Invariant Code Motion (LICM, “hoisting”) original code JIT compiles as if you wrote thisfor (var i = 0; i < N; ++i) { var f = 1 + options.tax; total += a[i] * for (var i = 0; i < N; ++i) { (1 + options.tax); total += a[i] * f;} }
    108. 108. Optimize Only Hot Code
    109. 109. Optimize Only Hot Code• Type-specializing JITs can have a hefty startup cost • Need to collect the type information • Advanced compiler optimizations take longer to run
    110. 110. Optimize Only Hot Code• Type-specializing JITs can have a hefty startup cost • Need to collect the type information • Advanced compiler optimizations take longer to run• Therefore, type specialization is applied selectively • Only on hot code • Tracemonkey: hot = 70 iterations • Crankshaft: hot = according to a profiler • Only if judged to be worthwhile (incomprehensible heuristics)
    111. 111. Current Limitations
    112. 112. Current Limitations• What happens if the types change after compiling? • Just a few changes -> recompile, slight slowdown • Many changes -> give up and deoptimize to basic JIT
    113. 113. Current Limitations• What happens if the types change after compiling? • Just a few changes -> recompile, slight slowdown • Many changes -> give up and deoptimize to basic JIT• Array elements, object properties, and closed-over variables • Usually still boxed • Still need to check type and unbox on get, box on set • Typed arrays might help, but support is not always there yet
    114. 114. Current Limitations• What happens if the types change after compiling? • Just a few changes -> recompile, slight slowdown • Many changes -> give up and deoptimize to basic JIT• Array elements, object properties, and closed-over variables • Usually still boxed • Still need to check type and unbox on get, box on set • Typed arrays might help, but support is not always there yet• JS semantics require overflow checks for integer math
    115. 115. Type Inference for JITs Current Research @Mozilla
    116. 116. Type Inference
    117. 117. Type Inference• Trying to get rid of the last few instances of boxing (from before: array and object properties)
    118. 118. Type Inference• Trying to get rid of the last few instances of boxing (from before: array and object properties)• Idea: use static program analysis to prove types • of object props, array elements, called functions • or, almost prove types, and also prove minimal checks needed
    119. 119. Type Inference Example var a = []; for (var i = 0; i < N; ++i) { a[i] = i * i; ] var sum = 0; for (var i = 0; i < N; ++i) { sum += a[i]; }Type inference gets this...
    120. 120. Type Inference Example var a = []; for (var i = 0; i < N; ++i) { a[i] = i * i; ] var sum = 0; for (var i = 0; i < N; ++i) { sum += a[i]; }Type inference gets this... “i is always a number, so i * i is always a number, so a[_] is always a number!”
    121. 121. Type Inference Example var a = []; var a = []; for (var i = 0; i < N; ++i) { for (var i = 0; i < N; ++i) { a[i] = i * i; if (i % 2) ] a[i] = i * i; else var sum = 0; a[i] = “foo”; for (var i = 0; i < N; ++i) { ] sum += a[i]; } var sum = 0; for (var i = 0; i < N; ++i) { if (i % 2)Type inference gets this... sum += a[i]; } “i is always a number, so i * i is always a number, ...but not this. so a[_] is always a number!”
    122. 122. Type-stable JavaScript The key to running faster in future JITs is type-stable JavaScript. This means JavaScript where you coulddeclare a single engine-internal type for each variable.
    123. 123. Type-stable JS: examples Type-stablevar g = 34;var o1 = { a: 56 };var o2 = { a: 99 };for (var i = 0; i < 10; ++i) { var o = i % 2 ? o1 : o2; g += o.a;}g = 0;
    124. 124. Type-stable JS: examples Type-stable NOT type-stablevar g = 34; var g = 34;var o1 = { a: 56 }; var o1 = { a: 56 };var o2 = { a: 99 }; var o2 = { z: 22, a: 56 };for (var i = 0; i < 10; ++i) { for (var i = 0; i < 10; ++i) { var o = i % 2 ? o1 : o2; var o = i % 2 ? o1 : o2; g += o.a; g += o.a;} }g = 0; g = “hello”;
    125. 125. Type-stable JS: examples Type-stable NOT type-stablevar g = 34; var g = 34;var o1 = { a: 56 }; var o1 = { a: 56 };var o2 = { a: 99 }; var o2 = { z: 22, a: 56 };for (var i = 0; i < 10; ++i) { for (var i = 0; i < 10; ++i) { var o = i % 2 ? o1 : o2; var o = i % 2 ? o1 : o2; g += o.a; g += o.a; Different shapes} }g = 0; g = “hello”;
    126. 126. Type-stable JS: examples Type-stable NOT type-stablevar g = 34; var g = 34;var o1 = { a: 56 }; var o1 = { a: 56 };var o2 = { a: 99 }; var o2 = { z: 22, a: 56 };for (var i = 0; i < 10; ++i) { for (var i = 0; i < 10; ++i) { var o = i % 2 ? o1 : o2; var o = i % 2 ? o1 : o2; g += o.a; g += o.a; Different shapes} }g = 0; g = “hello”; Type change
    127. 127. Garbage Collection
    128. 128. What Allocates Memory? Objectsnew Object();new MyConstructor();{ a: 4, b: 5 }Object.create(); Arraysnew Array();[ 1, 2, 3, 4 ]; Stringsnew String(“hello”);“<p>” + e.innerHTML + “</p>”
    129. 129. What Allocates Memory? Objects Function Objectsnew Object(); var x = function () { ... }new MyConstructor(); new Function(code);{ a: 4, b: 5 }Object.create(); Arraysnew Array();[ 1, 2, 3, 4 ]; Stringsnew String(“hello”);“<p>” + e.innerHTML + “</p>”
    130. 130. What Allocates Memory? Objects Function Objectsnew Object(); var x = function () { ... }new MyConstructor(); new Function(code);{ a: 4, b: 5 }Object.create(); Arrays Closure Environmentsnew Array(); function outer(name) {[ 1, 2, 3, 4 ]; var x = name; return function inner() { return “Hi, “ + name; Strings } }new String(“hello”);“<p>” + e.innerHTML + “</p>”
    131. 131. What Allocates Memory? Objects Function Objectsnew Object(); var x = function () { ... }new MyConstructor(); new Function(code);{ a: 4, b: 5 }Object.create(); Arrays Closure Environmentsnew Array(); function outer(name) {[ 1, 2, 3, 4 ]; var x = name; return function inner() { return “Hi, “ + name; Strings } }new String(“hello”); name is stored in an“<p>” + e.innerHTML + “</p>” implicitly created object!
    132. 132. GC Pauses Your Program!Time JavaScript GC Running Running JS Paused
    133. 133. GC Pauses Your Program!Time JavaScript GC Running Running JS Paused • Basic GC algorithm (mark and sweep) • Traverse all reachable objects (from locals, window, DOM) • Recycle objects that are not reachable
    134. 134. GC Pauses Your Program!Time JavaScript GC Running Running JS Paused • Basic GC algorithm (mark and sweep) • Traverse all reachable objects (from locals, window, DOM) • Recycle objects that are not reachable • The JS program is paused during GC for safe traversal
    135. 135. GC Pauses Your Program!Time JavaScript GC Running Running JS Paused • Basic GC algorithm (mark and sweep) • Traverse all reachable objects (from locals, window, DOM) • Recycle objects that are not reachable • The JS program is paused during GC for safe traversal • Pauses may be long: 100 ms or more • Serious problem for animation • Can also be a drag on general performance
    136. 136. Reducing Pauses with Science 1 Generational GC Chrome
    137. 137. Reducing Pauses with Science 1 Generational GC Chrome Idea: Optimize for creating many short-lived objects
    138. 138. Reducing Pauses with Science 1 Generational GC Chrome Idea: Optimize for creating many short-lived objects Create objects in a frequently collected nursery area
    139. 139. Reducing Pauses with Science 1 Generational GC Chrome Idea: Optimize for creating many short-lived objects Create objects in a frequently collected nursery area Promote long-lived objects to a rarely collected tenured area
    140. 140. Reducing Pauses with Science 1 Generational GC Chrome Idea: Optimize for creating many short-lived objects Create objects in a frequently collected nursery area Promote long-lived objects to a rarely collected tenured area JavaScript GC RunningSimple GC Running JS Paused
    141. 141. Reducing Pauses with Science 1 Generational GC Chrome Idea: Optimize for creating many short-lived objects Create objects in a frequently collected nursery area Promote long-lived objects to a rarely collected tenured area JavaScript GC Running Simple GC Running JS Paused JavaScriptGenerational GC Running
    142. 142. Reducing Pauses with Science 1 Generational GC Chrome Idea: Optimize for creating many short-lived objects Create objects in a frequently collected nursery area Promote long-lived objects to a rarely collected tenured area JavaScript GC Running Simple GC Running JS Paused JavaScriptGenerational GC Running nursery collection (<100 us)
    143. 143. Reducing Pauses with Science 1 Generational GC Chrome Idea: Optimize for creating many short-lived objects Create objects in a frequently collected nursery area Promote long-lived objects to a rarely collected tenured area JavaScript GC Running Simple GC Running JS Paused JavaScriptGenerational GC Running tenured collection nursery collection (<100 us)
    144. 144. Reducing Pauses with Science 1 Generational GC Chrome Idea: Optimize for creating many short-lived objects Create objects in a frequently collected nursery area Promote long-lived objects to a rarely collected tenured area JavaScript GC Running Simple GC Running JS Paused JavaScriptGenerational GC fewer pauses! Running tenured collection nursery collection (<100 us)
    145. 145. Generational GC by Examplescavenging young generation (aka nursery)mark-and-sweep tenured generation Message Message Array
    146. 146. Generational GC by Examplescavenging young generation (aka nursery) Pointmark-and-sweep tenured generation Message Message Array
    147. 147. Generational GC by Examplescavenging young generation (aka nursery) Point Pointmark-and-sweep tenured generation Message Message Array
    148. 148. Generational GC by Examplescavenging young generation (aka nursery) Point Point Linemark-and-sweep tenured generation Message Message Array
    149. 149. Generational GC by Examplescavenging young generation (aka nursery) Point Point Line a bmark-and-sweep tenured generation Message Message Array
    150. 150. Generational GC by Examplescavenging young generation (aka nursery) Point Point Line Point a bmark-and-sweep tenured generation Message Message Array
    151. 151. Generational GC by Examplescavenging young generation (aka nursery) Point Point Line Point Message a bmark-and-sweep tenured generation Message Message Array
    152. 152. Generational GC by Examplescavenging young generation (aka nursery) Point Point Line Point Message a bmark-and-sweep tenured generation Message Message Array
    153. 153. Generational GC by Examplescavenging young generation (aka nursery) Point Point Line Point Message Point a bmark-and-sweep tenured generation Message Message Array
    154. 154. Generational GC by Examplescavenging young generation (aka nursery) Point Point Line Point Point a bmark-and-sweep tenured generation Message Message Message Array
    155. 155. Generational GC by Examplescavenging young generation (aka nursery)mark-and-sweep tenured generation Message Message Message Array
    156. 156. Reducing Pauses with Science 1I Current Incremental GC Research @Mozilla
    157. 157. Reducing Pauses with Science 1I Current Incremental GC Research @Mozilla Idea: Do a little bit of GC traversal at a time
    158. 158. Reducing Pauses with Science 1I Current Incremental GC Research @Mozilla Idea: Do a little bit of GC traversal at a time JavaScript GC RunningSimple GC Running JS Paused
    159. 159. Reducing Pauses with Science 1I Current Incremental GC Research @Mozilla Idea: Do a little bit of GC traversal at a time JavaScript GC Running Simple GC Running JS PausedIncremental GC
    160. 160. Reducing Pauses with Science 1I Current Incremental GC Research @Mozilla Idea: Do a little bit of GC traversal at a time JavaScript GC Running Simple GC Running JS PausedIncremental GC shorter pauses!
    161. 161. Reducing Pauses in Practice
    162. 162. Reducing Pauses in Practice• For all GCs • Fewer live objects -> shorter pauses (if not incremental), less time spent in GC
    163. 163. Reducing Pauses in Practice• For all GCs • Fewer live objects -> shorter pauses (if not incremental), less time spent in GC• For simple GCs • Lower allocation rate (objects/second) -> less frequent pauses
    164. 164. Reducing Pauses in Practice• For all GCs • Fewer live objects -> shorter pauses (if not incremental), less time spent in GC• For simple GCs • Lower allocation rate (objects/second) -> less frequent pauses• For generational GCs • Short-lived objects don’t affect pause frequency • Long-lived objects cost extra (promotion = copying)
    165. 165. JavaScript Engines in Practice
    166. 166. Performance Faults• Performance fault: when a tiny change hurts performance • Sometimes, just makes one statement slower • Other times, deoptimizes the entire function!• Reasons we have performance faults • bug, tends to get quickly • “rare” case, will get fixed if not rare • hard to optimize, RSN...
    167. 167. Strings
    168. 168. Strings• In the Slow Zone, but some things are faster than you might think
    169. 169. Strings• In the Slow Zone, but some things are faster than you might think• .substring() is fast, O(1) • Don’t need to copy characters, just point within original
    170. 170. Strings• In the Slow Zone, but some things are faster than you might think• .substring() is fast, O(1) • Don’t need to copy characters, just point within original• Concatenation is also optimized • Batch up inputs in a rope or concat tree, concat all at once • Performance fault: prepending (Chrome, Opera)
    171. 171. Strings• In the Slow Zone, but some things are faster than you might think• .substring() is fast, O(1) // Prepending example var s = “”; •Don’t need to copy characters, just point iwithin<original { for (var = 0; i 100; ++i) s = i + s;• Concatenation is also optimized } • Batch up inputs in a rope or concat tree, concat all at once • Performance fault: prepending (Chrome, Opera)
    172. 172. Arrays fast: dense arrayvar a = []; Want a fast array?for (var i = 0; i < 100; ++i) { a[i] = 0; ‣ Make sure it’s dense} ‣ 0..N fill or push fill is always dense 3-15x slower: sparse array ‣ Huge gaps are always sparsevar a = []; ‣ N..0 fill is sparse on Firefoxa[10000] = 0;for (var i = 0; i < 100; ++i) { a[i] = 0; ‣ adding a named property is sparse on Firefox, IE}a.x = 7; // Fx, IE only
    173. 173. Iteration over Arraysfastest: index iteration// This runs in all in JIT code,// so it’s really fast.for (var i = 0; i < a.length; ++i) { sum += a[i];}
    174. 174. Iteration over Arrays 3-15x slower: functional style // This makes N function calls,fastest: index iteration // and most JITs don’t optimize // through C++ reduce(). sum = a.reduce(function(a, b) {// This runs in all in JIT code, return a + b; });// so it’s really fast.for (var i = 0; i < a.length; ++i) { sum += a[i];} 20-80x slower: for-in // This calls a C++ function to // navigate the property list. for (var i in a) { sum += a[i]; }
    175. 175. Functions• Function calls use ICs, so they are fast • Manual inlining can still help sometimes• Key performance faults: • f.call() - 1.3-35x slower than f() • f.apply() - 5-50x slower than f() • arguments - often very slow, but varies
    176. 176. Creating Objects Creating objects is slowDoesn’t matter too much how you create or populate
    177. 177. Creating Objects Creating objects is slowDoesn’t matter too much how you create or populate Exception: Constructors on Chrome are fast function Cons(x, y, z) { this.x = x; this.y = y; this.z = z; } for (var i = 0; i < N; ++i) new Cons(i, i + 1, i * 2);
    178. 178. OOP Styling
    179. 179. OOP Styling Prototypefunction Point(x, y) { this.x = x; this.y = y;}Point.prototype = { distance: function(pt2) ...
    180. 180. OOP Styling Prototype Information-Hidingfunction Point(x, y) { this.x = x; function Point(x, y) { this.y = y; return {} distance: function(pt2) ...Point.prototype = { } distance: function(pt2) ... }
    181. 181. OOP Styling Prototype Information-Hidingfunction Point(x, y) { this.x = x; function Point(x, y) { this.y = y; return {} distance: function(pt2) ...Point.prototype = { } distance: function(pt2) ... } Instance Methods function Point(x, y) { this.x = x; this.y = y; this.distance = function(pt2) ... }
    182. 182. OOP Styling Prototype Information-Hidingfunction Point(x, y) { this.x = x; function Point(x, y) { this.y = y; return {} distance: function(pt2) ...Point.prototype = { } distance: function(pt2) ... }Prototype style is much faster to create Instance Methods(each closure creates a function object) function Point(x, y) { this.x = x; this.y = y; this.distance = function(pt2) ... }
    183. 183. OOP Styling Prototype Information-Hidingfunction Point(x, y) { this.x = x; function Point(x, y) { this.y = y; return {} distance: function(pt2) ...Point.prototype = { } distance: function(pt2) ... }Prototype style is much faster to create Instance Methods(each closure creates a function object) function Point(x, y) { this.x = x; this.y = y; this.distance = function(pt2) ... }Using the objects is about the same
    184. 184. Exceptions• Exceptions assumed to be rare in perf-sensitive code • running a try statement is free on most browers • throw/catch is really slow• There are many performance faults around exceptions • just having a try statement deoptimizes on some browers • try-finally is perf fault on some
    185. 185. eval and with Short version: Do not use anywhere near performance sensitive code! Mind-Bogglingly Awful Still Terrible5-100x slower than using a function call 2-10x slower than without evalvar sum = 0; var sum = 0;for (var i = 0; i < N; ++i) { eval(“”); sum = eval(“sum + i”); for (var i = 0; i < N; ++i) {} sum = eval(“sum + i”); }
    186. 186. Top 5 Things to Know
    187. 187. Top 5 Things to Know5. Avoid eval, with, exceptions near perf-senstive code
    188. 188. Top 5 Things to Know5. Avoid eval, with, exceptions near perf-senstive code4. Avoid creating objects in hot loops
    189. 189. Top 5 Things to Know5. Avoid eval, with, exceptions near perf-senstive code4. Avoid creating objects in hot loops3. Use dense arrays (know what causes sparseness)
    190. 190. Top 5 Things to Know5. Avoid eval, with, exceptions near perf-senstive code4. Avoid creating objects in hot loops3. Use dense arrays (know what causes sparseness)2. Write type-stable code
    191. 191. Top 5 Things to Know5. Avoid eval, with, exceptions near perf-senstive code4. Avoid creating objects in hot loops3. Use dense arrays (know what causes sparseness)2. Write type-stable code1. ...
    192. 192. Talk To UsJS engine developers want to help you. Tell us about: • Performance faults you run into • Exciting apps that require fast JS • Anything interesting you discover about JS performance

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