[Vm-dev] problem with Interpreter>>#printNameOfClass:count:

[Javier Pimás]

Hi, this method of Interpreter is used for debugging, and should print the
name of the class passed in console:

printNameOfClass: classOop count: cnt
"Details: The count argument is used to avoid a possible infinite recursion
if classOop is a corrupted object."

cnt <= 0 ifTrue: [ ^ self print: 'bad class' ].
(self sizeBitsOf: classOop) = (7 * self bytesPerWord) "(Metaclass instSize+1
* 4)"
 ifTrue: [self printNameOfClass: (self fetchPointer: 5 "thisClass" ofObject:
classOop)
count: cnt - 1.
 self print: ' class']
ifFalse: [self printStringOf: (self fetchPointer: 6 "name" ofObject:
classOop)]

it's logic is this: if the classOop is of a normal Class instance, it should
use the instance var number six, which corresponds to the class' name. If
not, then the classOop represents a Metaclass instance, and then it will
fetch inst var number 5, which is the corresponding class, print its name,
and then print 'class' to let you distinguish it that it is a metaclass.

The problem of the method is that it determines if it's a class instance or
a metaclass instance by looking at the size of the oop, which seems to have
changed, at least in pharo (Metaclass instSize+1 * 4 = 36 =
9*bytesPerWord).

The method could be fixed by changing 7 to 9 then. But it will break if
Metaclass changes again. So I have this idea: make it check if the classOop
is a metaclass instance. Metaclass class is not a special object, but can be
obtained by accessing any special one, like with

Integer class class

or directly with

nil class class class (!!)

then the proposed code is:

printNameOfClass: classOop count: cnt "Details: The count argument is used
to avoid a possible infinite recursion if classOop is a corrupted object." |
metaclassClass | cnt <= 0 ifTrue: [ ^ self print: 'bad class' ].
metaclassClass := self fetchClassOf: (self fetchClassOf: (self fetchPointer:
ClassInteger ofObject: specialObjectsOop)). metaclassClass = (self
fetchClassOf: classOop) "Is it a metaclass?" ifTrue: [self printNameOfClass:
(self fetchPointer: 5 "thisClass" ofObject: classOop) count: cnt - 1. self
print: ' class'] ifFalse: [self printStringOf: (self fetchPointer: 6 "name"
ofObject: classOop)]


I'd also like to have this included too:

printClassOf: oop
 "Print the class of the oop in console"
self printNameOfClass: (self fetchClassOf: oop) count: 5.

which is simple but pretty much useful.

What do you think?

Regards,
           Javier.

-- 
Javier Pimás
Ciudad de Buenos Aires
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.squeakfoundation.org/pipermail/vm-dev/attachments/20110401/08427b2c/attachment.htm