post increment vs pre increment - Javascript Optimization

Question

I was browsing Google Code when I chanced upon this project called JSpeed - optimization for Javascript.

I noticed one of the optimization was to change i++ to ++i in for loop statements.

Before Optimization

for (i=0;i<1;i++) {}

for (var i = 0, j = 0; i < 1000000; i++, j++) {
    if (i == 4) {
        var tmp = i / 2;
    }

    if ((i % 2) == 0) {
        var tmp = i / 2;
        i++;
    }
}
var arr = new Array(1000000);
for (i = 0; i < arr.length; i++) {}

After optimization

for(var i=0;i<1;++i){}
for(var i=0,j=0;i<1000000;++i,++j){if(i==4){var tmp=i>>1;}
if((i&1)==0){var tmp=i>>1;i++;}}
var arr=new Array(1000000);for(var i=0,arr_len=arr.length;i<arr_len;++i){}

I know what pre and post increments do, but any idea how does this speeds the code up?

Answer 1

This is what I read and could answer your question: "preincrement (++i) adds one to the value of i, then returns i; in contrast, i++ returns i then adds one to it, which in theory results in the creation of a temporary variable storing the value of i before the increment operation was applied".

Answer 2

This is a faux optimization. As far as I understand it, you're saving 1 op code. If you're looking to optimize your code with this technique, then you've gone the wrong way. Also, most compilers/interpreters will optimize this for you anyway (reference 1). In short I wouldn't worry about. But, if you're really worried, you should use i+=1.

Here's the quick-and-dirty benchmark I just did

var MAX = 1000000, t=0,i=0;

t = (new Date()).getTime();
for ( i=0; i<MAX;i++ ) {}
t = (new Date()).getTime() - t;

console.log(t);

t = (new Date()).getTime();
for ( i=0; i<MAX;++i ) {}
t = (new Date()).getTime() - t;

console.log(t);

t = (new Date()).getTime();
for ( i=0; i<MAX;i+=1 ) {}
t = (new Date()).getTime() - t;

console.log(t);

Raw results

Post    Pre     +=
1071    1073    1060
1065    1048    1051
1070    1065    1060
1090    1070    1060
1070    1063    1068
1066    1060    1064
1053    1063    1054

Removed lowest and highest

Post    Pre     +=
1071    ----    1060
1065    ----    ----
1070    1065    1060
----    1070    1060
1070    1063    ----
1066    1060    1064
----    1063    1054

Averages

1068.4  1064.2  1059.6

Notice that this is over one million iterations and the results are within 9 milliseconds on average. Not really much of an optimization considering that most iterative processing in JavaScript is done over much smaller sets (DOM containers for example).

Answer 3

In theory, using a post-increment operator may produce a temporary. In practice, JavaScript compilers are smart enough to avoid that, especially in such trivial case.

For example, let's consider that sample code:

sh$ cat test.js 
function preInc(){
  for(i=0; i < 10; ++i)
    console.log(i);
}

function postInc(){
  for(i=0; i < 10; i++)
    console.log(i);
}

// force lazy compilation
preInc();
postInc();

In that case, the V8 compiler in NodeJS produces exactly the same bytecode (look esp. at opcodes 39-44 for the increment):

sh$ node --version
v8.9.4
sh$ node --print-bytecode test.js | sed -nEe '/(pre|post)Inc/,/^\[/p'
[generating bytecode for function: preInc]
Parameter count 1
Frame size 24
   77 E> 0x1d4ea44cdad6 @    0 : 91                StackCheck 
   87 S> 0x1d4ea44cdad7 @    1 : 02                LdaZero 
   88 E> 0x1d4ea44cdad8 @    2 : 0c 00 03          StaGlobalSloppy [0], [3]
   94 S> 0x1d4ea44cdadb @    5 : 0a 00 05          LdaGlobal [0], [5]
         0x1d4ea44cdade @    8 : 1e fa             Star r0
         0x1d4ea44cdae0 @   10 : 03 0a             LdaSmi [10]
   94 E> 0x1d4ea44cdae2 @   12 : 5b fa 07          TestLessThan r0, [7]
         0x1d4ea44cdae5 @   15 : 86 23             JumpIfFalse [35] (0x1d4ea44cdb08 @ 50)
   83 E> 0x1d4ea44cdae7 @   17 : 91                StackCheck 
  109 S> 0x1d4ea44cdae8 @   18 : 0a 01 0d          LdaGlobal [1], [13]
         0x1d4ea44cdaeb @   21 : 1e f9             Star r1
  117 E> 0x1d4ea44cdaed @   23 : 20 f9 02 0f       LdaNamedProperty r1, [2], [15]
         0x1d4ea44cdaf1 @   27 : 1e fa             Star r0
  121 E> 0x1d4ea44cdaf3 @   29 : 0a 00 05          LdaGlobal [0], [5]
         0x1d4ea44cdaf6 @   32 : 1e f8             Star r2
  117 E> 0x1d4ea44cdaf8 @   34 : 4c fa f9 f8 0b    CallProperty1 r0, r1, r2, [11]
  102 S> 0x1d4ea44cdafd @   39 : 0a 00 05          LdaGlobal [0], [5]
         0x1d4ea44cdb00 @   42 : 41 0a             Inc [10]
  102 E> 0x1d4ea44cdb02 @   44 : 0c 00 08          StaGlobalSloppy [0], [8]
         0x1d4ea44cdb05 @   47 : 77 2a 00          JumpLoop [42], [0] (0x1d4ea44cdadb @ 5)
         0x1d4ea44cdb08 @   50 : 04                LdaUndefined 
  125 S> 0x1d4ea44cdb09 @   51 : 95                Return 
Constant pool (size = 3)
Handler Table (size = 16)
[generating bytecode for function: get]
[generating bytecode for function: postInc]
Parameter count 1
Frame size 24
  144 E> 0x1d4ea44d821e @    0 : 91                StackCheck 
  154 S> 0x1d4ea44d821f @    1 : 02                LdaZero 
  155 E> 0x1d4ea44d8220 @    2 : 0c 00 03          StaGlobalSloppy [0], [3]
  161 S> 0x1d4ea44d8223 @    5 : 0a 00 05          LdaGlobal [0], [5]
         0x1d4ea44d8226 @    8 : 1e fa             Star r0
         0x1d4ea44d8228 @   10 : 03 0a             LdaSmi [10]
  161 E> 0x1d4ea44d822a @   12 : 5b fa 07          TestLessThan r0, [7]
         0x1d4ea44d822d @   15 : 86 23             JumpIfFalse [35] (0x1d4ea44d8250 @ 50)
  150 E> 0x1d4ea44d822f @   17 : 91                StackCheck 
  176 S> 0x1d4ea44d8230 @   18 : 0a 01 0d          LdaGlobal [1], [13]
         0x1d4ea44d8233 @   21 : 1e f9             Star r1
  184 E> 0x1d4ea44d8235 @   23 : 20 f9 02 0f       LdaNamedProperty r1, [2], [15]
         0x1d4ea44d8239 @   27 : 1e fa             Star r0
  188 E> 0x1d4ea44d823b @   29 : 0a 00 05          LdaGlobal [0], [5]
         0x1d4ea44d823e @   32 : 1e f8             Star r2
  184 E> 0x1d4ea44d8240 @   34 : 4c fa f9 f8 0b    CallProperty1 r0, r1, r2, [11]
  168 S> 0x1d4ea44d8245 @   39 : 0a 00 05          LdaGlobal [0], [5]
         0x1d4ea44d8248 @   42 : 41 0a             Inc [10]
  168 E> 0x1d4ea44d824a @   44 : 0c 00 08          StaGlobalSloppy [0], [8]
         0x1d4ea44d824d @   47 : 77 2a 00          JumpLoop [42], [0] (0x1d4ea44d8223 @ 5)
         0x1d4ea44d8250 @   50 : 04                LdaUndefined 
  192 S> 0x1d4ea44d8251 @   51 : 95                Return 
Constant pool (size = 3)
Handler Table (size = 16)

Of course, other JavaScript compilers/interpreters may do otherwise, but this is doubtful.

As the last word, for what it worth, I nevertheless consider as a best practice to use pre-increment when possible: since I frequently switch languages, I prefer using the syntax with the correct semantic for what I want, instead of relying on compiler smartness. For example, modern C compilers won't make any difference either. But in C++, this can have a significant impact with overloaded operator++.

Answer 4

Anatoliy's test included a post-increment inside the pre-increment test function :(

Here are the results without this side effect...

function test_post() {
    console.time('postIncrement');
    var i = 1000000, x = 0;
    do x++; while(i--);
    console.timeEnd('postIncrement');
}

function test_pre() {
    console.time('preIncrement');
    var i = 1000000, x = 0;
    do ++x; while(--i);
    console.timeEnd('preIncrement');
}

test_post();
test_pre();
test_post();
test_pre();
test_post();
test_pre();
test_post();
test_pre();

Output

postIncrement: 3.21ms
preIncrement: 2.4ms
postIncrement: 3.03ms
preIncrement: 2.3ms
postIncrement: 2.53ms
preIncrement: 1.93ms
postIncrement: 2.54ms
preIncrement: 1.9ms

That's a big difference.

Answer 5

Sounds like premature optimization. When you're nearly done your app, check where the bottlenecks are and optimize those as needed. But if you want a thorough guide to loop performance, check this out:

http://blogs.oracle.com/greimer/entry/best_way_to_code_a

But you never know when this will become obsolete because of JS engine improvements and variations between browsers. Best choice is to not worry about it until it's a problem. Make your code clear to read.

Edit: According to this guy the pre vs. post is statistically insignificant. (with pre possibly being worse)

Answer 6

The optimization isn't the pre versus post increment. It's the use of bitwise 'shift' and 'and' operators rather than divide and mod.

There is also the optimization of minifying the javascript to decrease the total size (but this is not a runtime optimization).

Answer 7

This is probably cargo-cult programming. It shouldn't make a difference when you're using a decent compilers/interpreters for languages that don't have arbitrary operator overloading.

This optimization made sense for C++ where

T x = ...;
++x

could modify a value in place whereas

T x = ...;
x++

would have to create a copy by doing something under-the-hood like

T x = ...;
T copy;
(copy = T(x), ++x, copy)

which could be expensive for large struct types or for types that do lots of computation in their `copy constructor.

Answer 8

Just tested it in firebug and found no difference between post- and preincrements. Maybe this optimization other platforms? Here is my code for firebug testing:

function test_post() {
    console.time('postIncrement');
    var i = 1000000, x = 0;
    do x++; while(i--);
    console.timeEnd('postIncrement');
}

function test_pre() {
    console.time('preIncrement');
    var i = 1000000, x = 0;
    do ++x; while(i--);
    console.timeEnd('preIncrement');
}

test_post();
test_pre();
test_post();
test_pre();
test_post();
test_pre();
test_post();
test_pre();

Output is:

postIncrement: 140ms
preIncrement: 160ms
postIncrement: 136ms
preIncrement: 157ms
postIncrement: 148ms
preIncrement: 137ms
postIncrement: 136ms
preIncrement: 148ms

Answer 9

Using post increment causes stack overflow. Why? start and end would always return the same value without first incrementing

function reverseString(string = [],start = 0,end = string.length - 1) {  
  if(start >= end) return
  let temp = string[start]
  string[start] = string[end]
  string[end] = temp
  //dont't do this
  //reverseString(string,start++,end--)
  reverseString(string,++start,--end)
  return array
}

let array = ["H","a","n","n","a","h"]
console.log(reverseString(array))