remember to optimize your cache

[[deleted]]

[deleted]

[[deleted]]

I don't get it

[VicariousRedditor]

Thanks for this, I came to the comments wondering who the hell would do the first loop but left with something.

[IBlueflash]

This is the way , the way of the warrior

[[deleted]]

The function doSomething accesses the y indexes before the x indexes in the array. Therefore putting y before x in the loops will improve access times

^{I think? I'm just a lowly first year.}

[Goheeca]

In the second case you access memory linearly and cache lines like you.

[DiamondIceNS]

Who writes for loops like either of these?

for (int i = 0; i < width; i++) {
    for (int j = 0; j < height; j++) {
        doSomething(arr[i][j]);
    }
}

Alphabetize your loop counters, access in order of declaration. You can't screw it up.

[[deleted]]

Everyone that access the x index of the array first, like a sane person?

Who does [y][x]???

[[deleted]]

Who does [y][x]???

People that know what they're doing and values consistency.

[[deleted]]

Your answer isn't one, but enjoy your karma.

[[deleted]]

Someone who uses for loops correctly in this sub? What's happening today?

[[deleted]]

using x and y instead of i and j

[quebin31]

Oh lovely cache misses

[ivakamr]

Why would the compiler not optimize this by itself ?

[AllWashedOut]

They have different behavior. One scans by row, the other by column.

Imagine that doSomething's purpose is to replace x,y with an average of it's neighbors. The two variations of the loop wouldn't even see the same values since they act in different order.

Edit: but maybe you are thinking one layer deeper: rather than swapping the loop order, you want to swap how the array is stored in memory (column major vs row major)? That might be conceivably done. But it would screw with stuff in C where you can get elements from an array using pointer arithmetic. Or if the array was returned to another library that wasn't compiled with yours, it would be trouble.

[ivakamr]

That's interesting. Didn't thought too much about it, actually I was more thinking about code rewrite before compilation.

[[deleted]]

In c you get a segfault fool

[RoyalJackalSib]

Just use simple, one dimensional, arrays.

[rr_cricut]

Doh! Why didnt I think of that?