On Wed, 12 Oct 2011, Mariano Martinez Peck wrote:
On Wed, Oct 12, 2011 at 5:38 PM, Levente Uzonyi <[email protected]> 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.
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
So...again, I think it may happen the same with #~~.
That being said, I agree that the method deserve a GOOD comment explaining
the reasons of such optimization.
Cheers
Instead of:
ProtoObject>> ~~ anObject
"Answer whether the receiver and the argument are not the same object
(do not have the same object pointer)."
^(self == anObject) not
And why?
Object >> ~= anObject
"Answer whether the receiver and the argument do not represent the
same object."
^self = anObject == false
Instead of
Object>> ~= anObject
"Answer whether the receiver and the argument do not represent the
same object."
^(self = anObject) not.
Is there any particular reason for this that I'm missing?
Performance.
Levente
Thanks in advance!
--
"*Most good programmers do programming not because they expect to get paid
or get adulation by the public, but because it is fun to program.*"
Linus Torvalds
--
Mariano
http://marianopeck.wordpress.com
