[Vm-dev] urgent info required on Slang's shift treatment...

[Igor Stasenko]

2009/3/3 Igor Stasenko <siguctua at gmail.com>:
> 2009/3/3 Eliot Miranda <eliot.miranda at gmail.com>:
>>
>>
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> - Show quoted text -
>> On Tue, Mar 3, 2009 at 12:09 PM, Igor Stasenko <siguctua at gmail.com> wrote:
>>>
>>> 2009/3/3 Eliot Miranda <eliot.miranda at gmail.com>:
>>> >
>>> > Hi All,
>>> >     I'm being bitten by Slang's treatment of bitShift: & >>.  In both cases (generateBitShift:on:indent: & generateShiftRight:on:indent:) Slang generates an unsigned shift by explicitly casting the shifted expression to usqInt.  I can understand the benefit of having an unsigned shift.  But there are times when one really needs a signed shift.  Further, the Smalltalk versions of both bitShift: and >> are signed shifts.
>>> > Dare I change e.g. generateShiftRight:on:indent: to leave the expression alone and generate either a signed or an unsigned shift based on the variable's declaration?  Or must I live with a maddening cCode: '(signed)' inSmalltalk: [] carbuncle?
>>> > E.
>>> >
>>>
>>> I think an easier way would be to add:
>>>
>>> #<<+    #generateSignedShiftLeft:on:indent:
>>> #>>+    #generateSignedShiftRight:on:indent:
>>>
>>> in #initializeCTranslationDictionary
>>> so you can use:
>>>
>>> a <<+ b
>>> or
>>> a >>+ b
>>>
>>> without writing horrible cCode:
>>
>> good work-around.  Thanks.
>>
>
> btw, you not alone noticed that :)
>
> I have an incomplete rewrite of code gen.. where i threw away numerous
> #generateXXX
> for common binary operators and replaced them with single:
> #generateBinaryOp:
>
> and when i came to shifts it is also makes me wonder, why an argument
> needs to be converted to unsigned one before shiftting..
>
> One thing, that i may guess, that shitfing is used extensively when
> dealing with headers..
>
> header := self longAt: oop.
>
> type := header bitAnd: TypeMask >> TypeShift.
>
> and since all 32 bits of header is used for different purposes, it is
> better to treat header as an unsigned word for shifting operations.
> I don't see any other uses of shifts in VM which could/should need
> casting to unsigned.
>
maybe also big integers?

>>>
>>> --
> - Show quoted text -
>>> Best regards,
>>> Igor Stasenko AKA sig.
>>
>>
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> --
> Best regards,
> Igor Stasenko AKA sig.
>



-- 
Best regards,
Igor Stasenko AKA sig.