Re: How do you bitwise operations in Ada '83 and '95

[Simon Wright <simon@pushface.org>]

Stephen Leake <stephen_leake@acm.org> writes:

> Simon Wright <simon@pushface.org> writes:
>
>> Stephen Leake <stephen_leake@acm.org> writes, responding to me:
>>
>>>> I have a feeling that shift&mask ends up more understandable 
>>>
>>> Never.
>>>
>>>> and reviewable.
>>>
>>> In Ada 95, maybe. In Ada 2005, not.
>>
>> Really, understandable and reviewable are very similar, so I don't
>> see how something could be (possibly) more reviewable while never
>> more understandable.
>
> Hmm. To me, "understandable" means "expressed in terms I am familiar
> with, using idioms I am familiar with, and unambiguous". Or
> something like that.
>
> While "reviewable" means "traceable to source documentation".
>
> So if I am familiar with the Ada 95 idiom for endianness-independent
> rep clauses, they are "understandable". But because they are
> complex, they may be less reviewable. I've never done a _formal_
> review (trace to source docs) of any software, so I'm on shaky
> ground here.

To me, code review is a process exceuted by people whose intention is
to ensure that the code does the right thing. Part of that is to check
that it's meeting its requirements, some of the requirements are
customer-imposed functional ones. So the people have to be able to
understand the code, and it helps if it's obviously doing the right
thing.

There are ways of writing Ada 95 rep clauses that were
endian-indepednent, I found them very clever but perhaps too clever.

>> Seriously, the C
>>
>>   field = (longword >> 15) & 0x3f;
>>
>> is pretty clear.
>
> At the bit level, yes. But if the 7 bits are supposed to represent
> some object, actual names would be far preferable.

I hope no one thought I was proposing that this bit twiddling (C or
Ada) should get out of package bodies!