From: William Clodius <wclodius@lanl.gov>
Subject: Re: Pointer Arithmetic (was: Wanted: Ada STL....)
Date: 1999/02/02
Date: 1999-02-02T00:00:00+00:00 [thread overview]
Message-ID: <36B78CDD.1372@lanl.gov> (raw)
In-Reply-To: 797na3$obg$1@nnrp1.dejanews.com
adam@irvine.com wrote:
> <snip>
> I don't know how this relates to STL, but I think I can see what the poster
> is getting at. Pointer arithmetic can be efficient when you're trying to
> step through an array, since you don't have to keep multiplying an index by a
> constant value to access the array elements. I'm not sure how much of a
> difference that makes these days---chips have gotten so fast that
> multiplication might not be that big a deal any more. But anyway, let's
> assume for argument's sake that efficiency is a concern and that we want to
> avoid multiplication if we can.
> <snip>
???
What are your reasons for thinking that indexing an array requires
multiplications that are not also required for the useage of pointers?
For most machines in use the most efficient implementations of arrays is
as a single contiguous block that is accessed by mapping indices
directly to the equivalent pointer operations. (The exceptions are more
easilly mapped to arrays and not to pointers.) It is true that mapping
involving an index other than the most rapidly varying one can be
thought of as involving a multiplication, but the same is true of the
equivalent pointer arithmetic. In both cases the same functionality can
be obtained by adding a (n easilly determined) finite integer to the
current memory index. The analysis for the compiler to do this is so
simple, I would not be surprized if Ada Lovelace discussed it for
Babage's machine in the 1800s. Any compiler that does not perform that
analysis is broken.
It is sometimes thought that because it is relatively easy for the
programmer to move some of the pointer arithmetic outside the loop, that
pointer arithmetic is more efficient, but the algorithms necessary to
perform that motion by the compiler have been available since the first
Fortran compiler, ca. 1957. Any compiler that does not perform that
analysis is a toy and not a professional system. It is more likely that
a programmer will not perform the necessary analysis for the pointers,
than that a compiler will not perform the analysis.
--
William B. Clodius Phone: (505)-665-9370
Los Alamos Nat. Lab., NIS-2 FAX: (505)-667-3815
PO Box 1663, MS-C323 Group office: (505)-667-5776
Los Alamos, NM 87545 Email: wclodius@lanl.gov
next prev parent reply other threads:[~1999-02-02 0:00 UTC|newest]
Thread overview: 102+ messages / expand[flat|nested] mbox.gz Atom feed top
1999-01-31 0:00 Wanted: Ada STL. Reward: Ada's Future Alexy V Khrabrov
1999-01-31 0:00 ` Simon Wright
1999-02-01 0:00 ` Jerry van Dijk
1999-02-01 0:00 ` Marin David Condic
1999-02-01 0:00 ` dennison
1999-02-01 0:00 ` Stanley R. Allen
1999-02-01 0:00 ` Matthew Heaney
1999-02-01 0:00 ` Alexy V Khrabrov
1999-02-01 0:00 ` Matthew Heaney
1999-02-01 0:00 ` Jeff Carter
1999-02-01 0:00 ` Brian Rogoff
1999-02-01 0:00 ` Ehud Lamm
1999-02-02 0:00 ` Brian Rogoff
1999-02-02 0:00 ` robert_dewar
1999-02-04 0:00 ` Ehud Lamm
1999-02-03 0:00 ` John English
1999-02-03 0:00 ` Matthew Heaney
1999-02-03 0:00 ` Brian Rogoff
1999-02-04 0:00 ` Stephen Leake
1999-02-04 0:00 ` Hyman Rosen
1999-02-05 0:00 ` Stephen Leake
1999-02-05 0:00 ` Hyman Rosen
1999-02-04 0:00 ` Matthew Heaney
1999-02-04 0:00 ` Brian Rogoff
1999-02-05 0:00 ` Stephen Leake
1999-02-05 0:00 ` Brian Rogoff
1999-02-05 0:00 ` John English
1999-02-05 0:00 ` Brian Rogoff
1999-02-06 0:00 ` Matthew Heaney
1999-02-05 0:00 ` Tucker Taft
1999-02-05 0:00 ` Brian Rogoff
1999-02-05 0:00 ` Richard D Riehle
1999-02-05 0:00 ` Nick Roberts
1999-02-05 0:00 ` John English
1999-02-04 0:00 ` Ehud Lamm
1999-02-04 0:00 ` Brian Rogoff
1999-02-05 0:00 ` Matthew Heaney
1999-02-05 0:00 ` Brian Rogoff
1999-02-08 0:00 ` John English
1999-02-04 0:00 ` Pat Rogers
1999-02-04 0:00 ` Larry Kilgallen
1999-02-04 0:00 ` Pat Rogers
1999-02-04 0:00 ` Larry Kilgallen
1999-02-05 0:00 ` robert_dewar
1999-02-05 0:00 ` Larry Kilgallen
1999-02-05 0:00 ` Tom Moran
1999-02-05 0:00 ` dewar
1999-02-05 0:00 ` Tom Moran
1999-02-05 0:00 ` dewar
1999-02-05 0:00 ` dennison
1999-02-06 0:00 ` dewar
1999-02-08 0:00 ` dennison
1999-02-08 0:00 ` robert_dewar
1999-02-08 0:00 ` dennison
1999-02-09 0:00 ` robert_dewar
1999-02-09 0:00 ` dennison
1999-02-09 0:00 ` Nick Roberts
1999-02-07 0:00 ` Simon Wright
1999-02-08 0:00 ` Corey Minyard
1999-02-08 0:00 ` Open Source Licensing (was: Wanted: Ada STL. Reward: Ada's Future) dennison
1999-02-08 0:00 ` Corey Minyard
1999-02-09 0:00 ` robert_dewar
1999-02-09 0:00 ` Corey Minyard
1999-02-09 0:00 ` dennison
1999-02-09 0:00 ` Corey Minyard
1999-02-09 0:00 ` Wanted: Ada STL. Reward: Ada's Future robert_dewar
1999-02-09 0:00 ` dennison
1999-02-10 0:00 ` robert_dewar
1999-02-07 0:00 ` Simon Wright
[not found] ` <36ba730b.35540068@ <79fmg1$fn0$1@nnrp1.dejanews.com>
1999-02-06 0:00 ` Larry Kilgallen
1999-02-05 0:00 ` robert_dewar
1999-02-05 0:00 ` robert_dewar
1999-02-05 0:00 ` Larry Kilgallen
1999-02-05 0:00 ` robert_dewar
1999-02-05 0:00 ` dennison
1999-02-05 0:00 ` robert_dewar
1999-02-05 0:00 ` dennison
1999-02-06 0:00 ` Nick Roberts
1999-02-05 0:00 ` Tucker Taft
[not found] ` <79f24e$t14 <36BB4162.52FC6D9F@averstar.com>
1999-02-05 0:00 ` robert_dewar
1999-02-05 0:00 ` dennison
1999-02-04 0:00 ` Al Christians
1999-02-05 0:00 ` John English
1999-02-09 0:00 ` micro_ada
1999-02-05 0:00 ` Nick Roberts
1999-02-08 0:00 ` John English
1999-02-02 0:00 ` Pointer Arithmetic (was: Wanted: Ada STL....) adam
1999-02-02 0:00 ` William Clodius [this message]
1999-02-03 0:00 ` adam
1999-02-03 0:00 ` robert_dewar
1999-02-03 0:00 ` Jean-Pierre Rosen
1999-02-03 0:00 ` William Clodius
1999-02-03 0:00 ` Nick Roberts
1999-02-03 0:00 ` robert_dewar
1999-02-03 0:00 ` Nick Roberts
1999-02-03 0:00 ` robert_dewar
1999-02-03 0:00 ` Robert A Duff
1999-02-02 0:00 ` Wanted: Ada STL. Reward: Ada's Future Richard D Riehle
1999-02-03 0:00 ` robert_dewar
1999-02-01 0:00 ` Jeff Carter
1999-02-08 0:00 ` Michael F Brenner
1999-02-05 0:00 ` Corey Minyard
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