Why this JavaScript program is faster than C++ to fill a big array?

Question

On the development of a language that compiles to JavaScript, I'm considering targeting C++ too, in order to generate faster programs when needed. My plan was to use std::vectors to hold the dynamic arrays of my language. Filling a big array repeatedly will be a normal operation (double buffering), so I've tested that:

#include <iostream>
#include <vector>
std::vector<int> gen(int w,int h){
    std::vector<int> a;
    a.resize(w*h);
    for (int i=0; i<w*h; ++i)
        a[i]=i;
    return a;
};  
int main(){
    for (int i=0; i<100; ++i)
        std::vector<int> a = gen(1000,1000);
};

Curiously, this program is not faster than the JavaScript equivalent:

gen = function(w,h){
    var a = new Float32Array(w*h);
    for (var i=0; i<w*h; ++i)
        a[i]=i;
    return a;
};
for (var i=0; i<100; ++i)
    gen(1000,1000);

Surprisingly, the JS version runs 3x faster.

clang++ my_program.cpp -o my_program
time ./my_program
real    0m1.393s
user    0m1.379s
sys     0m0.005s

time node my_program.js
real    0m0.458s
user    0m0.320s
sys 0m0.132s

Why is this happening? Should I reconsider?

Have you compiled with optimizations on? Also, no need for the call to resize(). Just instantiate the vector like this: std::vector<int> a(w*h); — juanchopanza
– juanchopanza, Commented Nov 17, 2013 at 12:11
The problem with optimizations (I tried -O2) is that apparently it just generates a program that does nothing (because I guess it detects this program isn't doing anything). So it runs instantly. Problem is it obviously won't be able to apply those optimizations and run instantly when I'm actually using those arrays on the actual application. Ideas? — MaiaVictor
– MaiaVictor, Commented Nov 17, 2013 at 12:12
Then you must construct a realistic test case. Obviously, no code is infinitely faster than come code, so C++ is infinitely faster than Javascript :-) — juanchopanza
– juanchopanza, Commented Nov 17, 2013 at 12:14
@Viclib: so you are saying that JavaScript is faster but only if you tell C++ to be really, really slow? If you set up and unequal setting, e.g., chain Udain Bolt left foot to a huge metal ball and have him compete in 100m race then, you shouldn't surprised about unexpected results (although I would expect Udain to still win). — Dietmar Kühl
– Dietmar Kühl, Commented Nov 17, 2013 at 12:17

john · Accepted Answer · 2013-11-17 12:10:35Z

3

Try this version

void gen(int w,int h,std::vector<int>& a){
    a.resize(w*h);
    for (int i=0; i<w*h; ++i)
        a[i]=i;
}

int main(){
    for (int i=0; i<100; ++i) {
        std::vector<int> a;
        gen(1000,1000,a);
    }
};

The problem with your version is that it copies the vector when it returns the value. Therefore it's not a direct equivalent to the JS version.

answered Nov 17, 2013 at 12:10

john

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1 Comment

juanchopanza Over a year ago

I would have expected copy elision to kick in here. Of course, it is not guaranteed, but still...

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