On 12 October 2011 23:22, Levente Uzonyi <leves@elte.hu> wrote:
On Wed, 12 Oct 2011, Mariano Martinez Peck wrote:
On Wed, Oct 12, 2011 at 5:38 PM, Levente Uzonyi <leves@elte.hu> wrote:
On Wed, 12 Oct 2011, Clara Allende wrote:
 Hi guys,
I'm wondering, why?
ProtoObject>> ~~ anObject  "Answer whether the receiver and the argument are not the same object  (do not have the same object pointer)."
 self == anObject    ifTrue: [^ false]    ifFalse: [^ true]
Hi Carla. I can think about two things. The first one, is the one Levente said, performance. If you analyze the bycode of this method, you will see that it is extremely fast because:
1) #== has an special associated bytecode, that is, them VM maps such bytecode to an specific primitive and it is directly executed. It means that the method #== is really never sent. 2) ifTrue:ifFalse: is also optimized (inlined) by the compiler. Again, it method is never executed and instead the compiler replace a message send bytecode with jump ones.
Another possible reason (it may not be the case, but in another places it is), is to prevent VM interruption for check other processes. In summary, the VM checks whether it should execute another process of the queue after a method execution. As you know, some parts of the scheduling process is done at the image side. And from there we lack a way to say to the VM, "please execute this method without checking others processes". Hence, in a few yet very specific places of PRocess, Scheduler, Semaphore, etc, #== is used as a mean of executing something WITHOUT being interrupted. I can imagine that it may happen the same with #~~. So if you implement such method with a #not, you will indeed send a message, proving a possibilty to be interrupted.
I don't see how this would be a reason. When you send #~~, then you explicitly allow a suspension point, so not using #not won't avoid suspension points.
To my thinking, in a first place, i would ask: why instead of fixing the code which may be affected by having or not suspension points, we introducing new workarounds ?? I think it is bad excuse for introduction of new primitive. If we miss some atomicity for semaphores, lets add a primitive(s) which fixing the semaphore issue(s). But introducing new primitive to avoid suspension?!?! i cannot follow this line of thinking.
Another reasons, similar to the previous one, is that sometimes #== is also used as a way to avoid executing method. So..there are some methods (I don't remember if #allInstancesDo: or #allObjectsDo:) will loop forever because
#allInstancesDo: uses primitives to avoid the problem. #allObjectsDo: uses a custom marker object, so creating new objects won't cause infinite loop (because gc doesn't change the order of any two objects).
the loop condition would be creating objects (remember that method execution creates objects such as MethodContext).
Contexts object are not created until requested and these loops don't need contexts. Some old methods interating through all/some objects use 0 as the marker object. These may have problems with objects created during their loops.
Levente
-- Best regards, Igor Stasenko.