Will C# compiler optimize this operation inside a loop?

Question

What is the best approach between the code below where verticesInnerSides.Length / 2 is written in it's complete form each time :

for (int i = 0; i < verticesRoof.Length; i++) {
    verticesInnerSides[i * NB_VERTICES + verticesInnerSides.Length / 2] = startInside;
    verticesInnerSides[i * NB_VERTICES + verticesInnerSides.Length / 2 + 1] = startInside + Vector3.up * HEIGHT_ROOF;
    verticesInnerSides[i * NB_VERTICES + verticesInnerSides.Length / 2 + 2] = endInside;
    verticesInnerSides[i * NB_VERTICES + verticesInnerSides.Length / 2 + 3] = endInside + Vector3.up * HEIGHT_ROOF;
}

Or the following where it's written outside the loop once :

myCalculatedVar = verticesInnerSides.Length / 2;
for (int i = 0; i < verticesRoof.Length; i++) {
    verticesInnerSides[i * NB_VERTICES + myCalculatedVar] = startInside;
    verticesInnerSides[i * NB_VERTICES + myCalculatedVar + 1] = startInside + Vector3.up * HEIGHT_ROOF;
    verticesInnerSides[i * NB_VERTICES + myCalculatedVar + 2] = endInside;
    verticesInnerSides[i * NB_VERTICES + myCalculatedVar + 3] = endInside + Vector3.up * HEIGHT_ROOF;
}

Will C# compiler optimize this operation in the first case ?

Why hope the compiler optimizes it for you instead off writing it properly in the first place? — Ash Burlaczenko
– Ash Burlaczenko, Commented Feb 6, 2015 at 8:22
open ILDSM and check for yourself <br/> msdn.microsoft.com/en-us/library/aa309387(v=vs.71).aspx — Vignesh.N
– Vignesh.N, Commented Feb 6, 2015 at 8:25
It appears you are using the parallel array anti pattern. Please consider using objects and OOP. — Aron
– Aron, Commented Feb 6, 2015 at 9:29
@Vignesh.N, most optimizations applied at CLR layer, not at C# to IL compilation (method inlining for example). — Ivan Kochurkin
– Ivan Kochurkin, Commented Feb 6, 2015 at 9:30

Noctis · Accepted Answer · 2015-02-06 10:28:40Z

1

In my opinion the best way to reach readability and performance is:

myCalculatedVar = verticesInnerSides.Length / 2;
for (int i = 0; i < verticesRoof.Length; i++) {
    int ind = i * NB_VERTICES + myCalculatedVar;
    verticesInnerSides[ind] = startInside;
    verticesInnerSides[ind + 1] = startInside + Vector3.up * HEIGHT_ROOF;
    verticesInnerSides[ind + 2] = endInside;
    verticesInnerSides[ind + 3] = endInside + Vector3.up * HEIGHT_ROOF;
}

But you can waste your time and write some benchmarks test for testing this piece. I am sure that if a bottleneck will exist it will be in another place in the code.

edited Feb 6, 2015 at 10:28

Noctis

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answered Feb 6, 2015 at 9:25

Ivan Kochurkin

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Comments

Community · Accepted Answer · 2017-05-23 12:12:51Z

1

Why would you do it like the first code example to begin with ? The second one (without looking at the compiler) seems more efficient.
As per Code Complete, and many other suggestions: Don't optimize if you don't need to.
If you need to, simply benchmark. (There's plenty of threads, blogs and answers about what to look out for when benchmarking, but I'm sure you'll be able to find them :)

As an extra bonus, here's another answer with all the links you'll need :)

edited May 23, 2017 at 12:12

CommunityBot

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answered Feb 6, 2015 at 8:25

Noctis

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2 Comments

Tim Schmelter Over a year ago

I don't agree with the conventional wisdom that pre-mature optimization "is the root of all evil". Most optimizations come with no cost (like this). They are obvious and also can increase readability, maintainability and safeness. You only have one variable to check or change instead of multiple places in the code. If you don't optimize now, you'll probably never optimize it. It's much more difficult to understand if you can change something without causing problems if you look at the code later.

Noctis Over a year ago

@TimSchmelter I absolutely agree about readability. Code should be written to be read. If when you're done, you have a performance issue, find out where, and then optimize that bit. Otherwise, you might be wasting time and making your code harder to read, and at the same time, maybe even slower.

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