I've been through a few questions over the network about this subject but I didn't find any answer for my question, or it's for another language or it doesn't answer totally (dead code is not useless code) so here's my question:
Is (explicit or not) useless code ignored by the compiler?
For example, in this code:
double[] TestRunTime = SomeFunctionThatReturnDoubles;
// A bit of code skipped
int i = 0;
for (int j = 0; j < TestRunTime.Length; j++)
{
}
double prevSpec_OilCons = 0;
will the for loop be removed?
The background is that I maintain a lot of code (that I didn't write) and I was wondering if useless code should be a target or if I could let the compiler take care of that.
Well, your variables i and prevSpec_OilCons, if not used anywhere will be optimized away, but not your loop.
So if your code looks like:
static void Main(string[] args)
{
int[] TestRunTime = { 1, 2, 3 };
int i = 0;
for (int j = 0; j < TestRunTime.Length; j++)
{
}
double prevSpec_OilCons = 0;
Console.WriteLine("Code end");
}
under ILSpy it will be:
private static void Main(string[] args)
{
int[] TestRunTime = new int[]
{
1,
2,
3
};
for (int i = 0; i < TestRunTime.Length; i++)
{
}
Console.WriteLine("Code end");
}
Since the loop has couple of statements, like comparison and increment, it could be used for implementing somewhat short delay/wait period. (although not a good practice to do so).
Consider the following loop, which is an empty loop, but it will take a lot of time to get executed.
for (long j = 0; j < long.MaxValue; j++)
{
}
The loop in your code, is not a dead code, as far as dead code is concerned, the following is a dead code, and will be optimized away.
if (false)
{
Console.Write("Shouldn't be here");
}
The loop, will not even be removed by the .NET jitters. Based on this answer
The loop can't be removed, the code is not dead, for instance:
// Just some function, right?
private static Double[] SomeFunctionThatReturnDoubles() {
return null;
}
...
double[] TestRunTime = SomeFunctionThatReturnDoubles();
...
// You'll end up with exception since TestRunTime is null
for (int j = 0; j < TestRunTime.Length; j++)
{
}
...
Usually, compiler just can't predict all possible outcomes of SomeFunctionThatReturnDoubles and that's why it preserves the loop
In your loop there are two an operation implicit in each iteration. An increment:
j++;
and comparison
j<TestRunTime.Length;
So, the loop is not empty although it looks like it is. There is something being executed at the end and this is not ignored by the compiler of course.
This happens also in other loops.