V8 internals for JavaScript developers

Google Experts Summit 2017

Transcript

2. V8 internals for JavaScript developers @mathias
3. Elements kinds in V8
4. const array = [1, 2, 3];
5. const array = [1, 2, 3]; // elements kind: PACKED_SMI_ELEMENTS
6. const array = [1, 2, 3]; // elements kind: PACKED_SMI_ELEMENTS array.push(4.56);
7. const array = [1, 2, 3]; // elements kind: PACKED_SMI_ELEMENTS array.push(4.56); // elements kind: PACKED_DOUBLE_ELEMENTS
8. const array = [1, 2, 3]; // elements kind: PACKED_SMI_ELEMENTS array.push(4.56); // elements kind: PACKED_DOUBLE_ELEMENTS array.push('x');
9. const array = [1, 2, 3]; // elements kind: PACKED_SMI_ELEMENTS array.push(4.56); // elements kind: PACKED_DOUBLE_ELEMENTS array.push('x'); // elements kind: PACKED_ELEMENTS
10. Smi Doubles Regular elements Elements kinds
11. const array = [1, 2, 3]; // elements kind: PACKED_SMI_ELEMENTS array.push(4.56); // elements kind: PACKED_DOUBLE_ELEMENTS array.push('x'); // elements kind: PACKED_ELEMENTS
12. array.length; // 5 index 0 1 2 3 4 value 1 2 3 4.56 'x'
13. array.length; // 5 array[9] = 1; // array[5] until array[8] are now holes index 0 1 2 3 4 5 6 7 8 9 value 1 2 3 4.56 'x' 1
14. array.length; // 5 array[9] = 1; // array[5] until array[8] are now holes // elements kind: HOLEY_ELEMENTS index 0 1 2 3 4 5 6 7 8 9 value 1 2 3 4.56 'x' 1
15. array[8]; // → ??? index 0 1 2 3 4 5 6 7 8 9 value 1 2 3 4.56 'x' 1
16. array[8]; // → ??? ❌ index 0 1 2 3 4 5 6 7 8 9 value 1 2 3 4.56 'x' 1
17. array[8]; // → ??? ❌ 8 >= 0 && 8 < array.length; // bounds check // → true index 0 1 2 3 4 5 6 7 8 9 value 1 2 3 4.56 'x' 1
18. array[8]; // → ??? ❌ 8 >= 0 && 8 < array.length; // bounds check // → true ❌ index 0 1 2 3 4 5 6 7 8 9 value 1 2 3 4.56 'x' 1
19. array[8]; // → ??? ❌ 8 >= 0 && 8 < array.length; // bounds check // → true ❌ hasOwnProperty(array, '8'); // → false index 0 1 2 3 4 5 6 7 8 9 value 1 2 3 4.56 'x' 1
20. index 0 1 2 3 4 5 6 7 8 9 value 1 2 3 4.56 'x' 1 array[8]; // → ??? ❌ 8 >= 0 && 8 < array.length; // bounds check // → true ❌ hasOwnProperty(array, '8'); // → false ❌
21. array[8]; // → ??? ❌ 8 >= 0 && 8 < array.length; // bounds check // → true ❌ hasOwnProperty(array, '8'); // → false ❌ hasOwnProperty(Array.prototype, '8'); // → false
22. array[8]; // → ??? ❌ 8 >= 0 && 8 < array.length; // bounds check // → true ❌ hasOwnProperty(array, '8'); // → false ❌ hasOwnProperty(Array.prototype, '8'); // → false ❌
23. array[8]; // → ??? ❌ 8 >= 0 && 8 < array.length; // bounds check // → true ❌ hasOwnProperty(array, '8'); // → false ❌ hasOwnProperty(Array.prototype, '8'); // → false ❌ hasOwnProperty(Object.prototype, '8'); // → false
24. array[8]; // → ??? ❌ 8 >= 0 && 8 < array.length; // bounds check // → true ❌ hasOwnProperty(array, '8'); // → false ❌ hasOwnProperty(Array.prototype, '8'); // → false ❌ hasOwnProperty(Object.prototype, '8'); // → false ✅
25. array[8]; // → undefined ✅ 8 >= 0 && 8 < array.length; // bounds check // → true hasOwnProperty(array, '8'); // → false hasOwnProperty(Array.prototype, '8'); // → false hasOwnProperty(Object.prototype, '8'); // → false ✅
26. packedArray[8]; // → undefined ✅ 8 >= 0 && 8 < packedArray.length; // bounds check // → true ✅ hasOwnProperty(packedArray, '8'); // → true ✅ hasOwnProperty(Array.prototype, '8'); // → false ✅ hasOwnProperty(Object.prototype, '8'); // → false ✅
27. packedArray[8]; // → undefined ✅ 8 >= 0 && 8 < packedArray.length; // bounds check // → true ✅ hasOwnProperty(packedArray, '8'); // → true ✅ hasOwnProperty(Array.prototype, '8'); // → false ✅ hasOwnProperty(Object.prototype, '8'); // → false ✅
28. array[0]; // → ???
29. array[0]; // → ??? ❌
30. array[0]; // → ??? ❌ 0 >= 0 && 0 < array.length; // bounds check // → true
31. array[0]; // → ??? ❌ 0 >= 0 && 0 < array.length; // bounds check // → true ❌
32. array[0]; // → ??? ❌ 0 >= 0 && 0 < array.length; // bounds check // → true ❌ hasOwnProperty(array, '0'); // → true
33. array[0]; // → ??? ❌ 0 >= 0 && 0 < array.length; // bounds check // → true ❌ hasOwnProperty(array, '0'); // → true ✅
34. array[0]; // → 1 ✅ 0 >= 0 && 0 < array.length; // bounds check // → true hasOwnProperty(array, '0'); // → true ✅
35. PACKED > HOLEY
36. PACKED > HOLEY
37. Smi Doubles Regular elements Elements kinds
38. Smi, packed Doubles, packed Regular elements, packed Smi, holey Doubles, holey Regular elements, holey
39. lattice
40. PACKED_SMI_ELEMENTS HOLEY_SMI_ELEMENTS PACKED_DOUBLE_ELEMENTS HOLEY_DOUBLE_ELEMENTS PACKED_ELEMENTS HOLEY_ELEMENTS
41. const array = new Array(3);
42. const array = new Array(3); index 0 1 2 value
43. const array = new Array(3); // HOLEY_SMI_ELEMENTS index 0 1 2 value
44. const array = new Array(3); // HOLEY_SMI_ELEMENTS array[0] = 'a'; index 0 1 2 value 'a'
45. const array = new Array(3); // HOLEY_SMI_ELEMENTS array[0] = 'a'; // HOLEY_ELEMENTS index 0 1 2 value 'a'
46. const array = new Array(3); // HOLEY_SMI_ELEMENTS array[0] = 'a'; // HOLEY_ELEMENTS array[1] = 'b'; index 0 1 2 value 'a' 'b'
47. const array = new Array(3); // HOLEY_SMI_ELEMENTS array[0] = 'a'; // HOLEY_ELEMENTS array[1] = 'b'; array[2] = 'c'; index 0 1 2 value 'a' 'b' 'c' now packed!
48. const array = new Array(3); // HOLEY_SMI_ELEMENTS array[0] = 'a'; // HOLEY_ELEMENTS array[1] = 'b'; array[2] = 'c'; // HOLEY_ELEMENTS (still!) now packed! but it’s too late index 0 1 2 value 'a' 'b' 'c'
49. const array = ['a', 'b', 'c']; // elements kind: PACKED_ELEMENTS
50. const array = ['a', 'b', 'c']; // elements kind: PACKED_ELEMENTS // … array.push(someValue); array.push(someOtherValue);
51. Avoid holes! #ProTip Avoid holes
52. for (let i = 0, item; (item = items[i]) != null; i++) { doSomething(item); }
53. for (let i = 0, item; (item = items[i]) != null; i++) { doSomething(item); }
54. for (let i = 0, item; (item = items[i]) != null; i++) { doSomething(item); } for (let index = 0; index < items.length; index++) { doSomething(item); }
55. for (const item of items) { doSomething(item); }
56. Avoid holes! #ProTip Avoid out-of-bounds reads
57. [3, 2, 1, +0]; // PACKED_SMI_ELEMENTS
58. [3, 2, 1, +0]; // PACKED_SMI_ELEMENTS [3, 2, 1, -0]; // PACKED_DOUBLE_ELEMENTS
59. [3, 2, 1, +0]; // PACKED_SMI_ELEMENTS [3, 2, 1, -0]; // PACKED_DOUBLE_ELEMENTS [3, 2, 1, NaN, Infinity]; // PACKED_DOUBLE_ELEMENTS
60. Avoid holes! #ProTip Avoid elements kind transitions
61. const arrayLike = {}; arrayLike[0] = 'a'; arrayLike[1] = 'b'; arrayLike[2] = 'c'; arrayLike.length = 3;
62. Array.prototype.forEach.call(arrayLike, (value, index) => { console.log(`${ index }: ${ value }`); }); // This logs '0: a', then '1: b', and finally '2: c'.
63. const actualArray = Array.prototype.slice.call(arrayLike, 0); actualArray.forEach((value, index) => { console.log(`${ index }: ${ value }`); }); // This logs '0: a', then '1: b', and finally '2: c'.
64. const logArgs = function() { Array.prototype.forEach.call(arguments, (value, index) => { console.log(`${ index }: ${ value }`); }); }; logArgs('a', 'b', 'c'); // This logs '0: a', then '1: b', and finally '2: c'.
65. const logArgs = (...args) => { args.forEach((value, index) => { console.log(`${ index }: ${ value }`); }); }; logArgs('a', 'b', 'c'); // This logs '0: a', then '1: b', and finally '2: c'.
66. Avoid holes! #ProTip Prefer arrays over array-like objects
67. $
68. $ rlwrap ~/projects/v8/out.gn/x64.debug/d8
69. $ rlwrap ~/projects/v8/out.gn/x64.debug/d8 --allow-natives-syntax
70. $ rlwrap ~/projects/v8/out.gn/x64.debug/d8 --allow-natives-syntax V8 version 6.2.0 (candidate) d8>
71. $ rlwrap ~/projects/v8/out.gn/x64.debug/d8 --allow-natives-syntax V8 version 6.2.0 (candidate) d8> const array = [1, 2, 3];
72. $ rlwrap ~/projects/v8/out.gn/x64.debug/d8 --allow-natives-syntax V8 version 6.2.0 (candidate) d8> const array = [1, 2, 3]; %DebugPrint(array);
73. $ rlwrap ~/projects/v8/out.gn/x64.debug/d8 --allow-natives-syntax V8 version 6.2.0 (candidate) d8> const array = [1, 2, 3]; %DebugPrint(array); DebugPrint: 0x313389e0e551: [JSArray] - map = 0x3133e0582889 [FastProperties] - prototype = 0x313360387f81 - elements = 0x313389e0e4c9 <FixedArray[3]> [PACKED_SMI_ELEMENTS (COW)] - length = 3 - properties = 0x3133dae02241 <FixedArray[0]> { #length: 0x31336c242839 <AccessorInfo> (const accessor descriptor) } …
74. $ rlwrap ~/projects/v8/out.gn/x64.debug/d8 --allow-natives-syntax V8 version 6.2.0 (candidate) d8> const array = [1, 2, 3]; %DebugPrint(array); DebugPrint: 0x313389e0e551: [JSArray] - map = 0x3133e0582889 [FastProperties] - prototype = 0x313360387f81 - elements = 0x313389e0e4c9 <FixedArray[3]> [PACKED_SMI_ELEMENTS (COW)] - length = 3 - properties = 0x3133dae02241 <FixedArray[0]> { #length: 0x31336c242839 <AccessorInfo> (const accessor descriptor) } …
76. Avoid holes. — J.K. Rowling
77. Avoid holes. Avoid out-of-bounds reads. — ancient Chinese proverb
78. Avoid holes. Avoid out-of-bounds reads. Avoid elements kind transitions. — Justin Bieber
79. Avoid holes. Avoid out-of-bounds reads. Avoid elements kind transitions. Prefer arrays over array-like objects. — Albert Einstein
80. Avoid holes. Avoid out-of-bounds reads. Avoid elements kind transitions. Prefer arrays over array-like objects. Eat your vegetables. — this slide, just now
81. Thank you! @mathias