Storing an array of references to arrays

Question

I want to create an array of references to my arrays. The reason for this is because i want to optimise my fast Fourier transform algorithm to be branchless - or rather, less branchy.

The idea behind it is i have two arrays:

Array1 and Array2

I need to ping pong between the two in a for loop so i want to store the reference to the arrays like this:

[0] = Array1Ref
[1] = Array2Ref
[2] = Array1Ref
. . .

Is it possible to do this in C#? If so how would you define such an array - would i need to use unsafe ?

Why not just do a swap of the two array variables at the end of the loop? — Nico Schertler
– Nico Schertler, Commented Jul 24, 2020 at 5:40
Because that involves using if statements such as if(i%2 == 0) arr1 : arr2 I am trying to see if i can make it faster by aligning the for loop iteration with an array of references. This is a micro optimisation attempt. — WDUK
– WDUK, Commented Jul 24, 2020 at 5:41
var someArray = new WhatEverTypeItIs[][]; which is an array of array of WhatEverTypeItIs — TheGeneral
– TheGeneral, Commented Jul 24, 2020 at 5:42

Sweeper · Accepted Answer · 2020-07-24 05:54:54Z

1

If you just want to access a different array in each iteration of the for loop without using a conditional, you can keep swapping two variables and use one of them.

var arrayRef = Array1;
var theOtherArrayRef = Array2;
for (...) {
    // use arrayRef in places where you would have accessed the array of array references
    
    ...

    // C# 7 tuple syntax
    (arrayRef, theOtherArrayRef) = (theOtherArrayRef, arrayRef);

    // pre-C# 7:
    /*
    var temp = arrayRef;
    arrayRef = theOtherArrayRef;
    theOtherArrayRef = arrayRef;
    */
}

answered Jul 24, 2020 at 5:54

Sweeper

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3 Comments

WDUK Over a year ago

Does the C#7 tuple compile to the same code or different ?

Sweeper Over a year ago

@WDUK My simple investigation on sharplab.io showed that they compile to different IL, but are JIT'ed to the same ASM code.

Sweeper Over a year ago

@WDUK Wait, no, if I use arrays the IL is the same as well. (I used ints at first).

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