Should I cache simple math?

Question

If I have something like this:

num1 = 10-10/20

num2 = 10/20+1

would there be a performance benefit to caching the "10/20" since I'm using it twice, or would the caching itself be more expensive than just doing the math again?

Answer 1

Answer by elenzil · May 07, 2020 at 06:31 PM

this is a great question.

I would strongly say that from a performance point of view there's no benefit unless this math is being executed more than a thousand times per second and even then it's extremely dubious. If this is literally the math at hand as well, the compiler will convert these expressions into constants at compile-time, so caching the constants in variables could actually decrease your performance.

However, from a code correctness and maintainability point of view, reducing duplication is generally good.

Computers are super fast, so I would definitely prioritize correctness and maintainability over performance.

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Bunny83 · May 07, 2020 at 10:40 PM 0

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Right. Also as you already said if an expression consists of only literal values the compiler will actually pre-calculate the expression and just store the result as a single value. In the case of the example code the two lines:

 num1 = 10-10/20
 num2 = 10/20+1

would actually simplify to

 num1 = 10
 num2 = 1

since "10/20" is an an integer division it will result in just "0".

edit
I forgot to mention that actual compile time constants that are used in an expression are also pre evaluated at compile time. For example this expression:

 public class MyClass
 {
     const int v1 = 5;
     const int v2 = 10;
     static int v3 = 10;
     public int resultValue = 3 + v1 * v2 * 2 + v3;

Becomes this after compilation (taken through ILSpy from the compiled assembly)

 public class MyClass
 {
     private const int v1 = 5;
     private const int v2 = 10;
     private static int v3 = 10;
     public int resultValue = 103 + MyClass.v3;

As you can see the expression "3 + v1 v2 2" has been reduced to "103". Static variables however are not precalculated since they can actually be changed ar runtime. Even static readonly variables are not necessarily pre evaluated.

Should I cache simple math?

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