[Vm-dev] tempVectors use case and current issues

[Denis Kudriashov]

Hi.

I found interesting case where tempVectors can be used in remote scenarios.
The store into remote temp can be really remote (not just about outer
context).
I played with following example:

| temp |
temp := 10.
remote evaluate: [temp := temp + 1].
temp.


For the moment forget about remote thing and look into it as a normal local
case:
temp var here is managed indirectly through tempVector. You can see it
using expression after first assignment:

thisContext at: 1 "=>#(10)"


So the value in fact is stored in the array instance and read from it.
But because of optimization it happens out of the array control. No #at:
and #at:put: messages are sent during this code. VM magically changes the
state of this array (there are special bytecodes for this).

Now my remote use case. Imagine that vm actually sends #at: and #at:put:
messages to tempVector. Then remoting engine can transfer temp vector (as
part of context) as a proxy. So on remote side the block [temp := temp + 1]
will actually ask the sender (client) for the value and for the storage. So
all block semantics will be supported. Temp in remote outer context will be
modified. I think it would be super cool if such transparency would be
possible.

I played with this example using Seamless in Pharo. It already works in the
way I described but due to VM optimization it does not provide expected
behavior. And worse than that it actually corrupts transferred proxy
because in place of array the proxy instance is materialized.

This leads us to the issue with safety of tempVector operations. Following
example shows how we can affect the state of tempVector using reflection:

| temp |
temp := 10.
(thisContext at: 1) at: 1 put: 50.
[temp := temp + 1] value.
temp. "==>51"

It is cool that we can do it. But there is no any safety check in the VM
level over tempVector object:

| temp |
temp := 10.
thisContext at: 1 put: Object new.
[temp := temp + 1] value.
temp.


It breaks with DNU: #+ is sent to nil. Temp became nil.


| temp |
temp := 10.
thisContext at: 1 put: #() copy.
[temp := temp + 1] value.
temp.


Sometimes it breaks with same error. Sometimes it returns random number.
I guess in these cases VM breaks memory boundary of tempVector.

And two exotic cases:


| temp |
temp := 10.
(thisContext at: 1) beReadOnlyObject.
[temp := temp + 1] value.
temp.


It silently return 11. It does not break read only protection. But no error
is signalled.

| temp |
temp := 10.
(thisContext at: 1) become: #() copy.
[temp := temp + 1] value.
temp.


It returns #().  (In Pharo  #() + 1 = #()  ).
I use become to check how forwarding is working in that case. (it works
fine when array has correct size)

How we can improve this behavior? How it would effect performance?
My proposal is to send real messages to tempVector when it is not an array
instance. Then image will decide what to do.

Best regards,
Denis
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