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Subject: Re: Setting wording for bit manipulation for non-binary hardware
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2021-03-07 09:52:26


On March 7, 2021 6:26:41 PM Jason McKesson via Std-Discussion
<std-discussion_at_[hidden]> wrote:

> On Sun, Mar 7, 2021 at 4:14 AM Andrey Semashev via Std-Discussion
> <std-discussion_at_[hidden]> wrote:
>>
>> On 3/6/21 10:34 PM, Jason McKesson via Std-Discussion wrote:
>>> On Sat, Mar 6, 2021 at 12:55 PM Vishal Oza via Std-Discussion
>>> <std-discussion_at_[hidden]> wrote:
>>>>
>>>> I was thinking of making the bit and bitset libraries as well bit
>>>> manipulation operators with the exception of endian and possibly left and
>>>> right bitshift to undefined behavior if the base is not 2 in hardware.
>>>
>>> Um, what exactly does that mean?
>>>
>>> Here's what I mean. If you have the number 15, and you do a
>>> binary-left-shift it by 2, the number you get is 30. It *does not
>>> matter* how the hardware implements this. So long as `30 == 15 << 2`
>>> is true, the implementation is doing its job.
>>>
>>> Similarly, if you have the number 15, and you do a bitwise-and with 2,
>>> you get the number 13. Again, how the hardware does this is
>>> irrelevant; the implementation's job is to make the math work out.
>>
>> Bit manipulation operators affect bit representation of a number, the
>> resulting value is a consequence of that. In that sense, before we
>> mandated two's complement representation, applying bit operators to
>> signed integers would yield implementation-dependent numbers, although
>> the effect on bit representation (with some restrictions) was well defined.
>
> Bitwise operations on signed integers, pre-twos complement, were
> defined so long as the source integers were positive and the resulting
> value could be stored in a signed integer of that type.
>
> Also, don't forget that a specification describes behavior, not
> implementation.

Sure, my point is that the specification describes behavior in terms of bit
representation. The effect on the value is the consequence.

You could rewrite the spec to define behavior in terms of integer values,
but that would be useless for the binary world, and it's not clear how
useful that would be for platforms of different bases.


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