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

[John M McIntosh]

Just to clarify with the current VMMaker source code and the FFI  
plugin if I do

| foo |
foo := ByteArray new: 4.
foo signedLongAt:1 put:  -2147483648

that fails, should it?


Begin forwarded message:

> From: John M McIntosh <johnmci at smalltalkconsulting.com>
> Date: November 23, 2008 11:27:14 PM PST (CA)
> To: Squeak Virtual Machine Development Discussion <vm-dev at lists.squeakfoundation.org 
> >
> Cc: Andreas Raab <andreas.raab at qwaq.com>, "David T. Lewis" <lewis at mail.msen.com 
> >
> Subject: Re: [Vm-dev] Interpreter>>signed32BitValueOf: and  
> signed64BitValueOf: broken
> Reply-To: johnmci at smalltalkconsulting.com
>
> Er is this still broken?
>
> I was writing some Sunits for the Alien stuff and found
>
> alien signedLongAt: 1 put: -1*16r80000000.  " -2147483648"
> failed.
> triggered by   value < 0 ifTrue:[^self primitiveFail].
> as coded below
> since value is -2147483648
>
> range is Signed: −2,147,483,648 to +2,147,483,647
> http://en.wikipedia.org/wiki/Integer_(computer_science)
>
>
>
> On 19-Mar-08, at 8:44 PM, Andreas Raab 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]
>>
>> Cheers,
>> - Andreas
>
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> John M. McIntosh <johnmci at smalltalkconsulting.com>
> Corporate Smalltalk Consulting Ltd.  http:// 
> www.smalltalkconsulting.com
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--
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John M. McIntosh <johnmci at smalltalkconsulting.com>
Corporate Smalltalk Consulting Ltd.  http://www.smalltalkconsulting.com
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