Allow StackItems to be deepcopied as immutable

[erikzhang]

No description provided.

[shargon]

If we add a readOnly, we should be able to know if the StackItem is in this mode. I don't like it too much.

[erikzhang]

It is useful if we send StackItems to a contract but don't want it to modify it.

[devhawk]

It is useful if we send StackItems to a contract but don't want it to modify it.

If we're making a copy, who cares if the other contract modifies it?

[erikzhang]

After becoming an immutable type, many copies can be avoided.

See neo-project/neo#2747 (comment)

With immutable types, it only needs to be copied once when the notification is sent.

[erikzhang]

And in the future, we can allow contracts to make an object immutable directly with a single instruction (e.g., OpCode.ASIMMUTABLE) without copying it.

[erikzhang]

@devhawk @shargon In a Notify and GetNotifications scenario, if we don't make objects immutable when Notify, then we have to make a deep copy of all notifications every time we GetNotifications afterwards. This greatly affects performance.

[devhawk]

I believe you, but I'd like to see benchmark, but I'd like to know exactly how much it impacts performance. Can we benchmark before and after?

[erikzhang]

@devhawk After this is merged, we can easily benchmark in neo-project/neo#2748.

[erikzhang]

@devhawk I simply ran the benchmark from neo-project/neo#2748.

In the case of doing a deep copy every time, we get the following results:

Benchmark: NeoIssue2725,        Time: 00:00:11.1730555

But if I turn off deep copying (because deep copying is not needed if it's an immutable type), we get the following result:

Benchmark: NeoIssue2725,        Time: 00:00:08.0009047

The improvement is not obvious, that's because the attack code used there focuses on the GC, not the deep copy.

[erikzhang]

Merge?