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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: CRC in Ada?
Date: 1997/03/13
Message-ID: <dewar.858263011@merv>#1/1
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References: <1997Mar2.220652@nova.wright.edu> <1997Mar5.131846.1@eisner>
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 <5g4k31$g54$1@A-abe.resnet.ucsb.edu> <JSA.97Mar12193731@alexandria>
Organization: New York University
Newsgroups: comp.lang.ada
Date: 1997-03-13T00:00:00+00:00
List-Id: <comp.lang.ada>


Jon says

<<Second, while 13.13.2(9) sez that for composites (including arrays)
the associated Read and Write attributes call the Read/Write
attributes for each _component_, I don't see why this can't be one of
those cases where "as if semantics" apply.  For example, for one
dimensional arrays, just read the thing as if you called Read/Write
for each element.>>

Sure, this is a possible optimization, but not such a simple one. For one
thing, we deliberately introduce another level of abstraction in GNAT,
namely the system.stream_attributes package in s-stratt.ads. This is provided
so that it is possible to substitute alternative semantics for stream
handling for primitive elements, and for example GLADE takes advantage
of this to substitute XDR (target independent) semantics.

Now it would be easy enough to at least provide an extra entry point for
string processing in system.stream_attributes for the special case of
string (and maybe wide_string), so that the front end could still do
the optimization, but this would not generalize to all arrays, and in
any case, the generalization to all arrays is not always possible,
it can be derailed by alignment considerations, or by packing, or
by the specification of component size.

Note that the additional layer here does not introduce extra call overhead
since it can be inlined, but it does make it virtually impossible using the
current interface, to specialize the case of stream attributes on strings.

As I say, the interface can easily be change. Handling stream_io more
efficiently on strings has been down on our list of possible optimziations
for a long time, but it's not particularly high priority -- so far it
has not been a significant performance issue for any of our customers!

Robert Dewar
Ada Core Technologies

P.S. I strongly encourage people to do profiling on their applications.
Nothing like finding out what *actually* takes the time, as opposed to
guessing at what might be critical optimizations!

Much the most convincing kind of input here is profiles from a real
application, not one construted to make the point, that show that
x% of time is spent in string stream attributes, where x is high enougj
to worry about.