JavaScript Optimization and Deoptimization

JavaScript is a weakly typed language, unlike strongly typed languages that require specifying parameter data types for function calls. It allows for very flexible parameter passing of various types for processing, as shown below:

function add(x, y) {
  // The + operator is a very complex operation in JavaScript
  return x + y;
}

add(1, 2);
add('1', 2);
add(null, 2);
add(undefined, 2);
add([], 2);
add({}, 2);
add([], {});

To perform the + operator operation, many APIs need to be called at the underlying execution level, such as ToPrimitive (to check if it's an object), ToString, ToNumber, etc., to convert the input parameters into data types suitable for the + operator.

Here, V8 will make assumptions about the parameters x and y like a strongly typed language, which allows it to eliminate some side-effect branches during execution. It also predicts that the code won't throw exceptions, thus enabling code optimization for maximum runtime performance. In Ignition, feedback information (Feedback Vector) is collected through bytecode, as shown below:

To view the runtime feedback information of the add function, we can use V8's Native API to print the runtime information of the add function, as shown below:

function add(x, y) {
  return x + y;
}

// Note: By default, ClosureFeedbackCellArray is used. To see the effect, we force enable FeedbackVector
// For more information, see: A lighter V8：https://v8.dev/blog/v8-lite
EnsureFeedbackVectorForFunction(add);
add(1, 2);
// Print detailed runtime information of add
DebugPrint(add);

Using the --allow-natives-syntax parameter allows calling the %DebugPrint underlying Native API in JavaScript (more APIs can be found in V8's runtime.h header file):

v8-debug --allow-natives-syntax  ./index.js

DebugPrint: 0x1d22082935b9: [Function] in OldSpace
 - map: 0x1d22082c2281 <Map(HOLEY_ELEMENTS)> [FastProperties]
 - prototype: 0x1d2208283b79 <JSFunction (sfi = 0x1d220820abbd)>
 - elements: 0x1d220800222d <FixedArray[0]> [HOLEY_ELEMENTS]
 - function prototype:
 - initial_map:
 - shared_info: 0x1d2208293491 <SharedFunctionInfo add>
 - name: 0x1d2208003f09 <String[3]: #add>
 // Contains the Ignition interpreter's trampoline pointer
 - builtin: InterpreterEntryTrampoline
 - formal_parameter_count: 2
 - kind: NormalFunction
 - context: 0x1d2208283649 <NativeContext[263]>
 - code: 0x1d2200005181 <Code BUILTIN InterpreterEntryTrampoline>
 - interpreted
 - bytecode: 0x1d2208293649 <BytecodeArray[6]>
 - source code: (x, y) {
    return x + y
}
 - properties: 0x1d220800222d <FixedArray[0]>
 - All own properties (excluding elements): {
    0x1d2208004bb5: [String] in ReadOnlySpace: #length: 0x1d2208204431 <AccessorInfo> (const accessor descriptor), location: descriptor
    0x1d2208004dfd: [String] in ReadOnlySpace: #name: 0x1d22082043ed <AccessorInfo> (const accessor descriptor), location: descriptor
    0x1d2208003fad: [String] in ReadOnlySpace: #arguments: 0x1d2208204365 <AccessorInfo> (const accessor descriptor), location: descriptor
    0x1d22080041f1: [String] in ReadOnlySpace: #caller: 0x1d22082043a9 <AccessorInfo> (const accessor descriptor), location: descriptor
    0x1d22080050b1: [String] in ReadOnlySpace: #prototype: 0x1d2208204475 <AccessorInfo> (const accessor descriptor), location: descriptor
 }

 // Below is the detailed feedback information
 - feedback vector: 0x1d2208293691: [FeedbackVector] in OldSpace
 - map: 0x1d2208002711 <Map>
 - length: 1
 - shared function info: 0x1d2208293491 <SharedFunctionInfo add>
 - no optimized code
 - optimization marker: OptimizationMarker::kNone
 - optimization tier: OptimizationTier::kNone
 - invocation count: 0
 - profiler ticks: 0
 - closure feedback cell array: 0x1d22080032b5: [ClosureFeedbackCellArray] in ReadOnlySpace
 - map: 0x1d2208002955 <Map>
 - length: 0

 - slot #0 BinaryOp BinaryOp:None {
     [0]: 0
  }
0x1d22082c2281: [Map]
 - type: JS_FUNCTION_TYPE
 - instance size: 32
 - inobject properties: 0
 - elements kind: HOLEY_ELEMENTS
 - unused property fields: 0
 - enum length: invalid
 - stable_map
 - callable
 - constructor
 - has_prototype_slot
 - back pointer: 0x1d22080023b5 <undefined>
 - prototype_validity cell: 0x1d22082044fd <Cell value= 1>
 - instance descriptors (own) #5: 0x1d2208283c29 <DescriptorArray[5]>
 - prototype: 0x1d2208283b79 <JSFunction (sfi = 0x1d220820abbd)>
 - constructor: 0x1d2208283bf5 <JSFunction Function (sfi = 0x1d220820acb9)>
 - dependent code: 0x1d22080021b9 <Other heap object (WEAK_FIXED_ARRAY_TYPE)>
 - construction counter: 0

To make the add function optimized like HotSpot code, we force a function optimization here:

function add(x, y) {
  return x + y;
}

add(1, 2);
// Force enable function optimization
OptimizeFunctionOnNextCall(add);
EnsureFeedbackVectorForFunction(add);
add(1, 2);
// Print detailed runtime information of add
DebugPrint(add);

Using the --trace-opt parameter, we can track the compilation optimization information of the add function:

 v8-debug --allow-natives-syntax --trace-opt  ./index.js

[manually marking 0x3872082935bd <JSFunction add (sfi = 0x3872082934b9)> for non-concurrent optimization]
// Here, the TurboFan optimizing compiler is used to compile and optimize the add function
[compiling method 0x3872082935bd <JSFunction add (sfi = 0x3872082934b9)> (target TURBOFAN) using TurboFan]
[optimizing 0x3872082935bd <JSFunction add (sfi = 0x3872082934b9)> (target TURBOFAN) - took 0.097, 2.003, 0.273 ms]
DebugPrint: 0x3872082935bd: [Function] in OldSpace
 - map: 0x3872082c2281 <Map(HOLEY_ELEMENTS)> [FastProperties]
 - prototype: 0x387208283b79 <JSFunction (sfi = 0x38720820abbd)>
 - elements: 0x38720800222d <FixedArray[0]> [HOLEY_ELEMENTS]
 - function prototype:
 - initial_map:
 - shared_info: 0x3872082934b9 <SharedFunctionInfo add>
 - name: 0x387208003f09 <String[3]: #add>
 - formal_parameter_count: 2
 - kind: NormalFunction
 - context: 0x387208283649 <NativeContext[263]>
 - code: 0x387200044001 <Code TURBOFAN>
 - source code: (x, y) {
    return x + y
}
 - properties: 0x38720800222d <FixedArray[0]>
 - All own properties (excluding elements): {
    0x387208004bb5: [String] in ReadOnlySpace: #length: 0x387208204431 <AccessorInfo> (const accessor descriptor), location: descriptor
    0x387208004dfd: [String] in ReadOnlySpace: #name: 0x3872082043ed <AccessorInfo> (const accessor descriptor), location: descriptor
    0x387208003fad: [String] in ReadOnlySpace: #arguments: 0x387208204365 <AccessorInfo> (const accessor descriptor), location: descriptor
    0x3872080041f1: [String] in ReadOnlySpace: #caller: 0x3872082043a9 <AccessorInfo> (const accessor descriptor), location: descriptor
    0x3872080050b1: [String] in ReadOnlySpace: #prototype: 0x387208204475 <AccessorInfo> (const accessor descriptor), location: descriptor
 }
 - feedback vector: 0x387208293685: [FeedbackVector] in OldSpace
 - map: 0x387208002711 <Map>
 - length: 1
 - shared function info: 0x3872082934b9 <SharedFunctionInfo add>
 - no optimized code
 - optimization marker: OptimizationMarker::kNone
 - optimization tier: OptimizationTier::kNone
 // Invocation count increased by 1
 - invocation count: 1
 - profiler ticks: 0
 - closure feedback cell array: 0x3872080032b5: [ClosureFeedbackCellArray] in ReadOnlySpace
 - map: 0x387208002955 <Map>
 - length: 0

 - slot #0 BinaryOp BinaryOp:SignedSmall {
     [0]: 1
  }
0x3872082c2281: [Map]
 - type: JS_FUNCTION_TYPE
 - instance size: 32
 - inobject properties: 0
 - elements kind: HOLEY_ELEMENTS
 - unused property fields: 0
 - enum length: invalid
 - stable_map
 - callable
 - constructor
 - has_prototype_slot
 - back pointer: 0x3872080023b5 <undefined>
 - prototype_validity cell: 0x3872082044fd <Cell value= 1>
 - instance descriptors (own) #5: 0x387208283c29 <DescriptorArray[5]>
 - prototype: 0x387208283b79 <JSFunction (sfi = 0x38720820abbd)>
 - constructor: 0x387208283bf5 <JSFunction Function (sfi = 0x38720820acb9)>
 - dependent code: 0x3872080021b9 <Other heap object (WEAK_FIXED_ARRAY_TYPE)>
 - construction counter: 0

It's important to note that V8 automatically monitors code structure changes and performs deoptimization. For example, in the following code:

function add(x, y) {
  return x + y;
}

EnsureFeedbackVectorForFunction(add);

add(1, 2);
OptimizeFunctionOnNextCall(add);
add(1, 2);
// Change the parameter type of add function, previously all were number type, here we pass a string type
add(1, '2');
DebugPrint(add);

We can track the deoptimization information of the add function using the --trace-deopt parameter:

v8-debug --allow-natives-syntax --trace-deopt  ./index.js

// Perform deoptimization, reason: not a Smi (Smi will be explained in subsequent series of articles, here it means the input is not a small integer type)
[bailout (kind: deopt-eager, reason: not a Smi: begin. deoptimizing 0x08f70829363d <JSFunction add (sfi = 0x8f7082934c9)>, opt id 0, node id 58, bytecode offset 2, deopt exit 1, FP to SP delta 32, caller SP 0x7ffee9ce7d70, pc 0x08f700044162]
DebugPrint: 0x8f70829363d: [Function] in OldSpace
 - map: 0x08f7082c2281 <Map(HOLEY_ELEMENTS)> [FastProperties]
 - prototype: 0x08f708283b79 <JSFunction (sfi = 0x8f70820abbd)>
 - elements: 0x08f70800222d <FixedArray[0]> [HOLEY_ELEMENTS]
 - function prototype:
 - initial_map:
 - shared_info: 0x08f7082934c9 <SharedFunctionInfo add>
 - name: 0x08f708003f09 <String[3]: #add>
 - formal_parameter_count: 2
 - kind: NormalFunction
 - context: 0x08f708283649 <NativeContext[263]>
 - code: 0x08f700044001 <Code TURBOFAN>
 - interpreted
 - bytecode: 0x08f7082936cd <BytecodeArray[6]>
 - source code: (x, y) {
    return x + y
}
 - properties: 0x08f70800222d <FixedArray[0]>
 - All own properties (excluding elements): {
    0x8f708004bb5: [String] in ReadOnlySpace: #length: 0x08f708204431 <AccessorInfo> (const accessor descriptor), location: descriptor
    0x8f708004dfd: [String] in ReadOnlySpace: #name: 0x08f7082043ed <AccessorInfo> (const accessor descriptor), location: descriptor
    0x8f708003fad: [String] in ReadOnlySpace: #arguments: 0x08f708204365 <AccessorInfo> (const accessor descriptor), location: descriptor
    0x8f7080041f1: [String] in ReadOnlySpace: #caller: 0x08f7082043a9 <AccessorInfo> (const accessor descriptor), location: descriptor
    0x8f7080050b1: [String] in ReadOnlySpace: #prototype: 0x08f708204475 <AccessorInfo> (const accessor descriptor), location: descriptor
 }
 - feedback vector: 0x8f708293715: [FeedbackVector] in OldSpace
 - map: 0x08f708002711 <Map>
 - length: 1
 - shared function info: 0x08f7082934c9 <SharedFunctionInfo add>
 - no optimized code
 - optimization marker: OptimizationMarker::kNone
 - optimization tier: OptimizationTier::kNone
 - invocation count: 1
 - profiler ticks: 0
 - closure feedback cell array: 0x8f7080032b5: [ClosureFeedbackCellArray] in ReadOnlySpace
 - map: 0x08f708002955 <Map>
 - length: 0

 - slot #0 BinaryOp BinaryOp:Any {
     [0]: 127
  }
0x8f7082c2281: [Map]
 - type: JS_FUNCTION_TYPE
 - instance size: 32
 - inobject properties: 0
 - elements kind: HOLEY_ELEMENTS
 - unused property fields: 0
 - enum length: invalid
 - stable_map
 - callable
 - constructor
 - has_prototype_slot
 - back pointer: 0x08f7080023b5 <undefined>
 - prototype_validity cell: 0x08f7082044fd <Cell value= 1>
 - instance descriptors (own) #5: 0x08f708283c29 <DescriptorArray[5]>
 - prototype: 0x08f708283b79 <JSFunction (sfi = 0x8f70820abbd)>
 - constructor: 0x08f708283bf5 <JSFunction Function (sfi = 0x8f70820acb9)>
 - dependent code: 0x08f7080021b9 <Other heap object (WEAK_FIXED_ARRAY_TYPE)>
 - construction counter: 0

It's important to note that the process of deoptimization generates performance overhead. Therefore, in daily development, it's recommended to use TypeScript for type declarations, which can improve code performance to some extent.

Contributors

XiSenao SenaoXi

Changelog

Last edited 21 days ago

View full history

9ba4ed7-feat: i18n

64d2bfd-feat: reorganize document directory & change toolbox item to material item

c825862-doc(others-esbuild): esbuild transpiles commonjs behavior

eaafd87-chore: reformatting global code

548e2cc-chore: adjust project structure && optimize sidebar

cf57b03-chore: eslint code

61904a6-chore: init project