Can a conforming C# compiler optimize away a local (but unused) variable if it is the only strong reference to an object?

Question

or, in other words:

Can an object referenced by a local variable be reclaimed before the variable goes out of scope (eg. because the variable is assigned, but then not used again), or is that object guaranteed to be ineligible for garbage collection until the variable goes out of scope?

Let me explain:

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void Case_1()
{
    var weakRef = new WeakReference(new object());

    GC.Collect();  // <-- doesn't have to be an explicit call; just assume that
                   //     garbage collection would occur at this point.

    if (weakRef.IsAlive) ...
}

In this code example, I obviously have to plan for the possibility that the new'ed object is reclaimed by the garbage collector; therefore the if statement.

(Note that I'm using weakRef for the sole purpose of checking if the new'ed object is still around.)

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void Case_2()
{
    var unusedLocalVar = new object();
    var weakRef = new WeakReference(unusedLocalVar);

    GC.Collect();  // <-- doesn't have to be an explicit call; just assume that
                   //     garbage collection would occur at this point.

    Debug.Assert(weakRef.IsAlive);
}

The main change in this code example from the previous one is that the new'ed object is strongly referenced by a local variable (unusedLocalVar). However, this variable is never used again after the weak reference (weakRef) has been created.

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Question: Is a conforming C# compiler allowed to optimize the first two lines of Case_2 into those of Case_1 if it sees that unusedLocalVar is only used in one place, namely as an argument to the WeakReference constructor? i.e. is there any possibility that the assertion in Case_2 could ever fail?

Answer 1

It doesn't matter what the C# compiler does - the JITter/GC are allowed to clean up local references once they're no longer alive in a method body. Look at the docs for GC.KeepAlive

Also, this powerpoint presentation, especially from slide 30 onwards, helps to explain what the JIT/GC can get up to.

Answer 2

Good question: I think the best answer would be "it depends". If the reference was inlined, and if the GC ran and memory was in demand, then perhaps: this will depend on the implementation.

My opinion: since this can only come about with a WeakReference, you're explicitly acknowledging the possibility that your reference may be collected at any time and code accordingly. It may not happen now, but if the VM configuration is changed or the VM optimiser changes in a proceeding release it may start to fail.

@Damien_The_Unbeliever: unusedLocalVar is in scope and thus not available for collection until the end of the method body in this example, unless it is inlined, in which case it may be collected beforehand.