[Vm-dev] primitiveBitShift

[Helmut Rohregger]

Am 15.04.2013 11:40, schrieb Igor Stasenko:
>   
> On 15 April 2013 11:31, Helmut Rohregger <helmut.rohregger at gmail.com> wrote:
>> Hi all,
>>
>> i am interested in the implementations of primitives in the VMMaker package
>> and found some code, which i think may be implemented incorrectly.
>> Maybe some of you can help me out and correct me if i am wrong.
>>
>> Its located in InterpreterPrimitives>>#primitiveBitShift (VMMaker-dtl.319):
>> the code: "self success: integerArgument <= 31" checks, if we lose bits by
>> shifting to the left. This is because of processing SmallIntegers here,
>> isn't it? But a "1 bitShift: 31" will produce a LargePositiveInteger instead
>> of a SmallInteger. In fact a left shift of 30 will produce a
>> LargePositiveInteger too.
>>
>> I know the code works, because the resulting object gets checked being a
>> SmallInteger and fails if not. But the correct check would be "self success:
>> integerArgument <= 29", isn't it?
>> What do you think?
>>
> hmm., how do you know if shifted result will fit into smallint, if you
> don't check both operands?
>
> "1 << 29 " fits
> "SmallInteger maxVal << 29 " doesnt.
> even
> "SmallInteger maxVal << 1 " doesnt.

but " x << 30 " does never fit, no matter which value 'x' has.