Calling a non-void function without using its return value. What actually happens?

Question

So, I found a similar question here, but the answers are more about style and whether or not you are able to do it.

My question is, what actually happens when you call a non-void function that returns an object, but you never assign or use said returned object? So, less about whether or not you can, because I absolutely know you can and understand the other question linked above... what does the compiler/runtime environment do?

This is not a language specific question, but if you answer, please specify what language you are referring to, since behaviors will differ.

Answer 1

I believe that for both C# and Java, the result ends up on the stack, and the compiler then forces a pop instruction to ignore it. Eric Lippert's blog post on "The void is invariant" has some more information on this.

For example, consider the following C# code:

using System;

public class Test 
{
    static int Foo() { return 10; }
    
    public static void Main()
    {
        Foo();
        Foo();
    }
}

The IL generated (by the MS C# 4 compiler) for the Main method is:

.method public hidebysig static void Main() cil managed
{
    .entrypoint
    .maxstack 8
    L_0000: call int32 Test::Foo()
    L_0005: pop 
    L_0006: call int32 Test::Foo()
    L_000b: pop 
    L_000c: ret 
}

Note the calls to pop - which disappear if you make Foo a void method.