Appendix

Last updated 2 years ago

Appendix

Examples, explanations and suggestions for v8 bailout reasons. Help alinode users optimize according to CPU-Profiler prompts.

Index

Bailout reasons

Assignment to parameter in arguments object

Simple example

// sloppy mode only
function test(a) {
  if (arguments.length < 2) {
    a = 0;
  }
}

Why
- Only occurs when reassigning parameters in a function.
Advices
- You cannot reassign variable a.
- It is better to use strict mode.

Bad value context for arguments value

Simple example

// strict & sloppy modes
function test1() {
  arguments[0] = 0;
}

// strict & sloppy modes
function test2() {
  arguments.length = 0;
}

// strict & sloppy modes
function test3() {
  return arguments;
}

// strict & sloppy modes
function test4() {
  var args = [].slice.call(arguments);
}

// strict & sloppy modes
function test5() {
  var a = arguments;
  return function() {
    return a;
  };
}

Why
- Requires further specification of the arguments array.
Advices
- You can read: https://github.com/petkaantonov/bluebird/wiki/Optimization-killers#3-managing-arguments
External links
- https://github.com/bevry/taskgroup/issues/12

ForInStatement with non-local each variable

Simple example

// strict & sloppy modes
function test1() {
  var obj = {};
  for(key in obj);
}

// strict & sloppy modes
function key() {
  return 'a';
}
function test2() {
  var obj = {};
  for(key() in obj);
}

Why
- https://github.com/yjhjstz/v8-git-mirror/blob/master/src/hydrogen.cc#L5254
Advices
- Only pure local variables can be used in for...in.
- https://github.com/petkaantonov/bluebird/wiki/Optimization-killers#5-for-in
External links
- https://github.com/mbostock/d3/pull/2686

Object literal with complex property

Simple example

// strict & sloppy modes
function test() {
  return {
    __proto__: 3
  };
}

Why
Advices
- Simplify the Object.

ForInStatement is not fast case

Simple example

for (var prop in obj) {
  /* lots of code */
}

Why
- Too much code in the for loop.
Advices
- Extract the code in the for loop into a function.

Reference to a variable which requires dynamic lookup

Simple example

// sloppy mode only
function test() {
  with ({x:1}) {
    return x;
  }
}

Why
Advices
- TurboFan can optimize.

TryCatchStatement

Simple example

// strict & sloppy modes OR // sloppy mode only
function func() {
  return 3;
  try {} catch(e) {}
}

Why
- try/catch makes the control flow unstable and difficult to optimize at runtime.
Advices
- Do not use try/catch in heavily loaded functions.
- Can be refactored as try { func() } catch

TryFinallyStatement

Simple example

// strict & sloppy modes OR // sloppy mode only
function func() {
  return 3;
  try {} finally {}
}

Why
Advices

Unsupported phi use of arguments

简单例子

// strict & sloppy modes
function test1() {
  var _arguments = arguments;
  if (0 === 0) { // anything evaluating to true, except a number or `true`
    _arguments = [0]; // Unsupported phi use of arguments
  }
}

// strict & sloppy modes
function test2() {
  var _arguments = arguments;
  for (var i = 0; i < 1; i++) {
    _arguments = [0]; // Unsupported phi use of arguments
  }
}

// strict & sloppy modes
function test3() {
  var _arguments = arguments;
  var again = true;
  while (again) {
    _arguments = [0]; // Unsupported phi use of arguments
    again = false;
  }
}

Why
- Crankshaft cannot determine whether _arguments is an object or an array.
Advices
- It is best to operate on a copy of arguments.




### Yield

* Simple example

```js
// strict & sloppy modes
function* test() {
  yield 0;
}

Why
- Generator state preservation and restoration is achieved by copying function stack frames, which is not suitable for optimized compilers.
Advices
- Currently, there is no need to consider this issue. TurboFan can optimize.
External links:
- https://groups.google.com/forum/#!topic/v8-users/KnnUb-u4rA8

Resources

PreviousNode.js tuning

Last updated 2 years ago