[Vm-dev] bitShift: and runtime sign discussion

[Nicolas Cellier]

Another candidate in the same category of refactoring..,


2012/12/23 Igor Stasenko <siguctua at gmail.com>:
>
> On 23 December 2012 18:05, David T. Lewis <lewis at mail.msen.com> wrote:
>>
>> On Sun, Dec 23, 2012 at 02:14:37PM +0100, Nicolas Cellier wrote:
>>>
>>> As you may know, the CCodeGenerator is clever enough to
>>> #generateBitShift:on:indent: with proper direction << or >> when the
>>> shift is literal.
>>> When the shift in (expr bitShift: shift) is a variable or another
>>> expression, it will produce a runtime test (shift <0) ? expr>>shift :
>>> expr <<shift;
>>> This is good for an arbitrary shift, but more than often we know the
>>> sign and direction of the shift in advance, and this runtime test is
>>> void.
>>> We must be aware that aggressive inlining will spread these useless
>>> code all around.
>>> Well, we might ignore impact on performance but we shall better not
>>> ignore warnings generated by C compiler (or XCode analyzer) if we want
>>> the code to be portable to other compilers (including future versions
>>> of gcc).
>>> ...and the eventual warnings will be spreaded too
>>>
>>> Maybe we could arrange for CCodeGenerator to recognize more cases when
>>> the shift is unsigned, but
>>> 1) the CCodeGenerator is yet unable to infer expression type (except
>>> some hack for declared variable)
>>> 2) we almost never use unsigned declaration in vm source anyway.
>>>
>>> In the mean time, I suggest we use << and >> directlty in smalltalk
>>> code (slang) when we know the direction in advance.
>>
>> IMO it is better to write these expressions directly, as you have done
>> in your examples. There are many places in the VM code where types are
>> not carefully declared, and I suspect that trying to make the code
>> generator be more clever would only lead to unintended side effects.
>> Better to just write these explicitly using << and >> when you know
>> that it is safe to do so.
>>
> +1
>
> --
> Best regards,
> Igor Stasenko.
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