Should array length be stored into a local variable in C#?

I notice that people often use construction like this:

var length = array.Length;
for (int i = 0; i < length; i++) {
    //do smth
}

They think that having a call to the Array.Length on each iteration will make CLR to take more time to execute the code. To avoid it they store the length value in a local variable.
Let’s find out (once and for all !) if this is a viable thing or using a temporary variable is a waste of time.

To start, let’s examine these C# methods:

public int WithoutVariable() {
    int sum = 0;
    for (int i = 0; i < array.Length; i++) {
        sum += array[i];
    }
    return sum;
}
public int WithVariable() {
    int sum = 0;
    int length = array.Length;
    for (int i = 0; i < length; i++) {
        sum += array[i];
    }
    return sum;
}

Here is how it looks after been processed by the JIT compiler (for .NET Framework 4.7.2 under LegacyJIT-x86):


It’s trivial to notice that they have the exact same number of assembler instructions — 15. Even the logic of these instructions is almost the same. There’s a slight difference in the order of initializing variables and comparisons on whether the cycle should continue. We can note that in both cases the array length is registered two times before the cycle:

• To check for 0 (arrayLength)
  
• Into the temporary variable for checking the cycle condition (arrayLength2).
  

It turns out that both methods will compile into the exact same code, but the first one is written faster, even though there isn’t any benefit in terms of execution time.
The assembler code above led me to some thoughts and I decided to check a couple more methods:

public int WithoutVariable() {
    int sum = 0;
    for(int i = 0; i < array.Length; i++) {
        sum += array[i] + array.Length;
    }
    return sum;
}

public int WithVariable() {
    int sum = 0;
    int length = array.Length;
    for(int i = 0; i < length; i++) {
        sum += array[i] + length;
    }
    return sum;
}

Now the current element and array length are being added up, but in the first case the array length is being requested every time, and in the second case it’s saved once into a local variable. Let’s look at the assembler code of these methods:


Once again, the number of instructions are the same, as well as (almost) the instructions themselves. The only difference is the order of initializing variables and the check condition for continuation of the cycle. You can note that in the calculation of sum, only first length of array is taken into account. It’s obvious that this:

int arrayLength2 = localRefToArray.Length;
    mov     edi, dword ptr [edx+4]
if (i >=arrayLength2) throw new IndexOutOfRangeException();
    cmp     ecx, edi
    jae     04b12d39

in all four methods is an inlined array bounds checking and it’s executed for each element of the array.

We can already make the first conclusion: using an extra variable to try to speed up the cycle is a waste of time, since the compiler will do it for you anyway. The only reason to store a length array into a variable is to make the code more readable.

ForEach is another situation entirely. Consider the following three methods:

public int ForEachWithoutLength() {
    int sum = 0;
    foreach (int i in array) {
        sum += i;
    }
    return sum;
}

public int ForEachWithLengthWithoutLocalVariable() {
    int sum = 0;
    foreach (int i in array) {
        sum += i + array.Length;
    }
    return sum;
}

public int ForEachWithLengthWithLocalVariable() {
    int sum = 0;
    int length = array.Length;
    foreach (int i in array) {
        sum += i + length;
    }
    return sum;
}

And here’s the code after JIT:




The first thing that comes to mind is that it takes less assembler instructions than the for cycle (for example, for simple element summation it took 12 instructions in foreach, but 15 in for).

Overall, here are results of for vs foreach benchmark for 1 million-element arrays:

sum+=array[i];

And for

sum+=array[i] + array.Length;

ForEach walks through the array a lot quicker than for. Why? To find out, we need to compare the code after JIT:


Let’s look at ForEachWithoutLength. The array length is requested only once and there aren’t any checks for the array boundaries. That happens because the ForEach cycle first restricts changing the collection inside the cycle, and second one won’t ever go outside the collection. Due to that, JIT can afford to remove the checks array boundaries.

Now let’s look carefully at ForEachWithLengthWIthoutLocalVariable. There’s only one strange part, where sum+=length happens not to previously saved local variable arrayLength, but to a new one that the app requests from memory each time. That means, there will be N+1 memory requests for array length, where N is an array length.

And now we come to ForEachWithLengthWithLocalVariable. The code there is exactly the same as in the previous example, except the handling of the array length. The compiler once again generated a local variable arrayLength that’s used to check if the array is empty, but the compiler still honestly saved our stated local variable length, and that’s what’s used in the summation inside the cycle. It turns out that this method requests the array length from memory only twice. The difference is very hard to notice in the real world.

In all cases, assembler code turned out so simple because the methods themselves are simple. If the methods had more parameters, it would have to work with the stack, variables might get stores outside of registers, there would’ve been more checks, but the main logic would remains the same: introducing a local variable for array length is only useful for making more readable code. It also turned out that Foreach often walks through the array faster than For.