[Vm-dev] Interpreter>>signed32BitValueOf: and signed64BitValueOf: broken

[Igor Stasenko]

On 20/03/2008, Andreas Raab <andreas.raab at gmx.de> wrote:
>
>  Hi -
>
>  I just noticed hat Interpreter>>signed32BitValueOf: and
>  signed64BitValueOf: are broken for edge cases. The following example
>  will illustrate the problem:
>
>  array := IntegerArray new: 1.
>  array at: 1 put: 16rFFFFFFFF. "should fail but doesn't"
>  array at: 1. "answers -1 incorrectly"
>
>  array := IntegerArray new: 1.
>  array at: 1 put: -16rFFFFFFFF. "should fail but doesn't"
>  array at: 1. "answers 1 incorrectly"
>
>  The problem is that both signed32BitValueOf: as well as
>  signed64BitValueOf: do not test whether the high bit of the magnitude is
>  set (which it mustn't to fit into a signed integer). The fix is trivial
>  in both cases - basically all that's needed at the end of both functions
>  is this:
>
>    "Filter out values out of range for the signed interpretation such as
>    16rFFFFFFFF (positive w/ bit 32 set) and -16rFFFFFFFF (negative w/ bit
>    32 set). Since the sign is implicit in the class we require that the
>    high bit of the magnitude is not set which is a simple test here"
>    value < 0 ifTrue:[^self primitiveFail].
>    negative
>      ifTrue:[^0 - value]
>      ifFalse:[^value]
>

This C weak types always a place of uncertainty for me.
What i fear of, that if you change implementation of
#signed32BitValueOf: it can break more things than it was before. Due
to same reasons, of course.

>  Cheers,
>
>    - Andreas
>


-- 
Best regards,
Igor Stasenko AKA sig.