RE: nb_peek

For both nb_peek and nb_poke, you can get the same functionality
by simply querying the current # items in the fifo and then doing
peek/poke using a non-negative number less than the current size.

If the user is passing in negative numbers then it seems to me that
there is a fair chance there is a bug in his code, in which case we
should disallow it in the LRM and put a check in the fifo implementation
to catch it.

Stuart

>-----Original Message-----
>From: Philipp A. Hartmann [mailto:philipp.hartmann@offis.de]
>Sent: Monday, March 29, 2010 12:06 PM
>To: Stuart Swan
>Cc: john.aynsley@doulos.com; systemc-p1666-technical@eda.org
>Subject: Re: nb_peek
>
>John, Stuart,
>
>On 29/03/10 20:29, Stuart Swan wrote:
>>
>> I think that is some hack that Adam Rose probably put in - I don't
>> think it should be included in the LRM since the same functionality
>> can be achieved using other (and cleaner) TLM fifo apis.
>
>I think nb_peek and nb_poke are both clean and useful for non-consuming
>fifo accesses. Doing this from "both ends" of the current queue is a
>valid use case (not only for debugging).
>
> Consequently, I would even extend the functionality for both functions
>to accept negative indices smaller than -1 to access inner elements from
>the end. See below.
>
>> From: john.aynsley@doulos.com
>> Sent: Monday, March 29, 2010 9:29 AM To:
>> Subject: nb_peek
>>
>> The usual behavior of method
>>
>> tlm_fifo<T>::nb_peek( T &t , int n ) const
>>
>> is to return a reference to the item at location n, where 0 <= n <
>> size()-1
>
>This should be used(), not size(), right?
>
>Also note, that the current documentation in tlm_fifo_ifs.h of these
>functions does not match their implementation. It currently says:
>
> // non blocking peek and poke - no notification
> //
> // n is index of data :
> // 0 <= n < size(), where 0 is most recently written, and size() - 1
> // is oldest ie the one about to be read.
>
> Whereas in the implementation n=0 returns the one about to be read and
>used()-1 would return the last written element. used() == size() is
>only a special case, if the FIFO is currently completely filled.
>
>> However, the code includes a hack to accept n == -1, and have it
>> equivalent to n == size()-1, i.e. peek the oldest item in the FIFO.
>> Strangely, nb_poke does not have the equivalent special case.
>>
>> Do we want to document this special case of nb_peek in the LRM, in
>> which case, should we add the same special case n == -1 to nb_poke?
>
>Peeking/poking from the writing end of the FIFO currently requires to
>pass "used() - n" (except for the current hack).
>
>I would find it useful and consistent to allow arguments
> -used() <= n < used()
>where negative arguments are equivalent to "used() + n".
>
> This would both reduce the amount of typing and the risk of
>programming errors (e.g. to use size() instead of used() in case of
>partly filled FIFOs).
>
>Just my 2¢. :-)
>
>Greetings from Oldenburg,
>Philipp
>
>--
>Philipp A. Hartmann
>Hardware/Software Design Methodology Group
>
>OFFIS
>R&D Division Transportation | FuE-Bereich Verkehr
>Escherweg 2 · 26121 Oldenburg · Germany
>Phone/Fax: +49-441-9722-420/282 · PGP: 0x9161A5C0 · http://www.offis.de/

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