Re: nb_peek

Philipp,

I agree with your description of the actual behavior versus the comments.
Thanks for pointing that out.

Given that we have managed without negative indices to date I do not see a
strong reason to introduce them now, so I agree with Stuart that we should
simply remove the -1 hack.

Re sc_prim_channel::wait, here's the def

    void wait( const sc_event& e )
        { sc_core::wait( e, simcontext() ); }

So I guess the hack in tlm_fifo<T>::peek is out-of-date. I will
investigate further.

John A

From:
"Philipp A. Hartmann" <philipp.hartmann@offis.de>
To:
Stuart Swan <stuart@cadence.com>
Cc:
"john.aynsley@doulos.com" <john.aynsley@doulos.com>,
"systemc-p1666-technical@eda.org" <systemc-p1666-technical@eda.org>
Date:
29/03/2010 20:07
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
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