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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Transcript

  • 1. Know Your EnginesHow to Make Your JavaScript Fast Dave Mandelin June 15, 2011 O’Reilly Velocity
  • 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. ...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. ...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. 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. The 2006 JavaScript Engine
  • 7. Inside the 2006 JS Engine DOM StandardFront End Interpreter Library Garbage Collector
  • 8. Inside the 2006 JS Engine// JavaScript sourcee.innerHTML = n + “ items”; DOM Standard Front End Interpreter Library Garbage Collector
  • 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. 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. 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. 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. Why it’s hard to make JS fast Because JavaScript is an untyped language. untyped = no type declarations
  • 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. Engine-Internal TypesJS engines use finer-grained types internally. JavaScript type number object
  • 16. Engine-Internal TypesJS engines use finer-grained types internally. JavaScript type Engine type number 32-bit* integer 64-bit floating-point object
  • 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. 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. 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. Running Code in the Interpreter Here’s what the interpreter must do to execute x = y + z:
  • 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. 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. 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. 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. 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. 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. 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. 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. 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. The 2011 JavaScript Engine
  • 31. Inside the 2011 JS Engine Garbage Collector DOM InterpreterJavaScript source Standard Library Front End bytecode/AST
  • 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. 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. 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. 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. 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. 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. 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. 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. Choosing the action in the JIT
  • 41. Choosing the action in the JIT• Many cases for operators like +
  • 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. 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. 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. Object Properties function f(obj) { return obj.a + 1; }
  • 46. Object Properties function f(obj) { return obj.a + 1; }• Need to search obj for a property named a slow
  • 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. 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. ICs: a mini-JIT for objects All Major Browsers
  • 50. ICs: a mini-JIT for objects All Major Browsers• Properties become fast with inline caching (we prefer IC)
  • 51. ICs: a mini-JIT for objects All Major Browsers• Properties become fast with inline caching (we prefer IC)• Basic plan:
  • 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. 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. 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. ICs: Example Example Codevar obj1 = { a: 1, b: 2, c: 3 };var obj2 = { b: 2 };function f(obj) { return obj.b + 1;}
  • 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. 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. 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. 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. 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. 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. 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. 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. 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. These are fast because of ICs Global Variable Accessvar q = 4;var r;function f(obj) { r = q;}
  • 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. Properties in the Slow Zone
  • 87. Properties in the Slow Zone Undefined Property (Fast on Firefox, Chrome)var a = {};a.x;
  • 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. 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. 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. The Type-Specializing JIT Firefox 3.5+ (Tracemonkey) Chrome 11+ (Crankshaft)
  • 92. Types FTW!If only JavaScript had type declarations...
  • 93. Types FTW! If only JavaScript had type declarations...➡ The JIT would know the type of every local variable
  • 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. 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. 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. But JS doesn’t have types
  • 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. 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. 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. 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. 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. 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. Further Optimization 1 Automatic Inlining original codefunction getPop(city) { return popdata[city.id];}for (var i = 0; i < N; ++i) { total += getPop(city);}
  • 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. Further Optimization 2 Loop Invariant Code Motion (LICM, “hoisting”) original codefor (var i = 0; i < N; ++i) { total += a[i] * (1 + options.tax);}
  • 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. Optimize Only Hot Code
  • 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. 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. Current Limitations
  • 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. 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. 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. Type Inference for JITs Current Research @Mozilla
  • 116. Type Inference
  • 117. Type Inference• Trying to get rid of the last few instances of boxing (from before: array and object properties)
  • 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. 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. 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. 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. 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. 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. 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. 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. 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. Garbage Collection
  • 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. 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. 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. 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. GC Pauses Your Program!Time JavaScript GC Running Running JS Paused
  • 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. 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. 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. Reducing Pauses with Science 1 Generational GC Chrome
  • 137. Reducing Pauses with Science 1 Generational GC Chrome Idea: Optimize for creating many short-lived objects
  • 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. 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. 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. 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. 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. 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. 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. Generational GC by Examplescavenging young generation (aka nursery)mark-and-sweep tenured generation Message Message Array
  • 146. Generational GC by Examplescavenging young generation (aka nursery) Pointmark-and-sweep tenured generation Message Message Array
  • 147. Generational GC by Examplescavenging young generation (aka nursery) Point Pointmark-and-sweep tenured generation Message Message Array
  • 148. Generational GC by Examplescavenging young generation (aka nursery) Point Point Linemark-and-sweep tenured generation Message Message Array
  • 149. Generational GC by Examplescavenging young generation (aka nursery) Point Point Line a bmark-and-sweep tenured generation Message Message Array
  • 150. Generational GC by Examplescavenging young generation (aka nursery) Point Point Line Point a bmark-and-sweep tenured generation Message Message Array
  • 151. Generational GC by Examplescavenging young generation (aka nursery) Point Point Line Point Message a bmark-and-sweep tenured generation Message Message Array
  • 152. Generational GC by Examplescavenging young generation (aka nursery) Point Point Line Point Message a bmark-and-sweep tenured generation Message Message Array
  • 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. Generational GC by Examplescavenging young generation (aka nursery) Point Point Line Point Point a bmark-and-sweep tenured generation Message Message Message Array
  • 155. Generational GC by Examplescavenging young generation (aka nursery)mark-and-sweep tenured generation Message Message Message Array
  • 156. Reducing Pauses with Science 1I Current Incremental GC Research @Mozilla
  • 157. Reducing Pauses with Science 1I Current Incremental GC Research @Mozilla Idea: Do a little bit of GC traversal at a time
  • 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. 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. 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. Reducing Pauses in Practice
  • 162. Reducing Pauses in Practice• For all GCs • Fewer live objects -> shorter pauses (if not incremental), less time spent in GC
  • 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. 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. JavaScript Engines in Practice
  • 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. Strings
  • 168. Strings• In the Slow Zone, but some things are faster than you might think
  • 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. 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. 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. 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. 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. 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. 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. Creating Objects Creating objects is slowDoesn’t matter too much how you create or populate
  • 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. OOP Styling
  • 179. OOP Styling Prototypefunction Point(x, y) { this.x = x; this.y = y;}Point.prototype = { distance: function(pt2) ...
  • 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. 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. 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. 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. 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. 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. Top 5 Things to Know
  • 187. Top 5 Things to Know5. Avoid eval, with, exceptions near perf-senstive code
  • 188. Top 5 Things to Know5. Avoid eval, with, exceptions near perf-senstive code4. Avoid creating objects in hot loops
  • 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. 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. 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. 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