C# optimizations and side effects

Question

Can optimizations done by the C# compiler or the JITter have visible side effects?

One example I've though off.

var x = new Something();
A(x);
B(x);

When calling A(x) x is guaranteed to be kept alive to the end of A - because B uses the same parameter. But if B is defined as

public void B(Something x) { }

Then the B(x) can be eliminated by the optimizer and then a GC.KeepAlive(x) call might be necessary instead.

Can this optimization actually be done by the JITter?

Are there other optimizations that might have visible side effects, except stack trace changes?

Answer 1

If your function B does not use the parameter x, then eliminating it and collecting x early does not have any visible side effects.

To be "visible side effects", they have to be visible to the program, not to an external tool like a debugger or object viewer.