Re: Interfaces.Shift_Left

["Randy Brukardt" <randy@rrsoftware.com>]

"Niklas Holsti" <niklas.holsti@tidorum.invalid> wrote in message 
news:9ktu5mFn31U1@mid.individual.net...
> On 11-12-15 00:49 , Randy Brukardt wrote:
>> "awdorrin"<awdorrin@gmail.com>  wrote in message
>> news:a76daa61-49f7-4235-af65-11973a012f35@v29g2000yqv.googlegroups.com...
>>> I understand what you are saying and I do agree in theory.
>>>
>>> The situation is that bit shifting is being used to retrieve data of
>>> various bit lengths and positions from compound records.
>>>
>>> For instance, retrieving a 6-bit signed value from a 32-bit field.
>>
>> Right. This definitely should be done using record representation clauses 
>> if
>> there is a choice.
>
> I'm sorry to say that I disagree, and now always use the shift-and-mask 
> method.

Might as well write in C then, this sort of code is complete, unmaintainable 
junk.

> I used to be charmed by record representation clauses, until I wanted to 
> write Ada 95 code that was portable between machines with different bit 
> numbering orders.

It's true, that's a problem. Luckily, it's not a real problem in practice 
for the vast majority of users, since pretty much all popular machines these 
days use the same bit numbering.

> I know that it is claimed that record representation clauses can be 
> written to be portable between different bit numberings. But in my 
> opinion, the schemes that do not use the Bit_Order attribute make the 
> representation clauses ugly and hard to write and read, while the RM 
> language that describes the Bit_Order attribute and its effects has 
> surpassed my understanding whenever I've tried to read it -- I simply 
> could not be sure that it would have the effect that I wanted.

It will have the right effect or be illegal. (At least that was my 
understanding.) The problem is that the non-default bit ordering makes it 
trivial to declare discontiguous (from the persepective of the default 
bit-ordering) components, and we didn't want to force compilers to support 
that sort of stuff. There is little customer demand for that sort of thing 
(most of the ones that exist are like you and gave up completely wanting 
more control than the language could possibly provide), so it didn't make 
sense to require the effort.

> I still would like to define the bit-level structure of data in a 
> declarative way, rather than with field-extraction and field-insertion 
> operations, but I have reluctantly concluded that the operational method 
> is better for portable Ada programs, at least in my applications.
>
> Another factor in this decision is that in my applications, the raw data 
> (machine instructions for various processors) can actually have several 
> different structures (instruction formats), so an approach using record 
> representation clauses needs as many different record types and unchecked 
> conversions from the raw data (bit-strings or words) to the appropriate 
> record type. The operational approach entirely avoids these conversions.

Of course, the vast majority of Unchecked_Conversions generate no code at 
all, so these conversions have no run-time impact and IMHO they're still 
more readable than a bunch of shifts or multiplies.

Anyway, I understand that everyone's mileage may vary. I doubt that many 
applications have the problem of decoding data that are both in the wrong 
bit-order and are unknown at run-time.

(Truth-in-advertising provision: most of the Janus/Ada compiler uses 
multiplies and divides to create machine instructions, because early 
versions of Janus/Ada had no support for record representation clauses or 
any other bit operations at all. We used assembler code whenever those 
operations were unavoidable -- hardly an ideal approach for portability 
reasons. Later versions (like the never-finished 68K and SPARC codegens) 
used record rep. clauses; I recall we had some issues with non-default 
bit-ordering but don't remember anymore how we solved them.)

                                          Randy.