From: robert_dewar@my-dejanews.com
Subject: Re: Pointer Arithmetic (was: Wanted: Ada STL....)
Date: 1999/02/03
Date: 1999-02-03T00:00:00+00:00 [thread overview]
Message-ID: <799f5t$7je$1@nnrp1.dejanews.com> (raw)
In-Reply-To: 798lej$69d$1@plug.news.pipex.net
In article <798lej$69d$1@plug.news.pipex.net>,
"Nick Roberts" <Nick.Roberts@dial.pipex.com> wrote:
> All unnecessary, Adam. Nowadays, (most) optimising
> compilers do what you suggest automatically, without the
> need for a pragma.
Adam is claiming that the pointer arithmetic in C speeds
up generated code by eliminating stride computations in
some programs. While it is true that of course there are
programs in which stride computations can be eliminated,
the wide spread use of pointers in C in general *damages*
the quality of the code by introducing uncontrolled
aliasing that the compiler cannot deal with.
If in C we see an isolated statement
*p++ = 5;
as opposed to
x[i++] = 5;
it is true we saved the stride computation in accessing
the array, BUT, in the absence of global information quite
unlikely to be available in the case Adam is looking at
(i.e. the case where the index computation cannot be
strength reduced), the assignment to *p++ potentially
destroys all sorts of aliasing linkes, requiring things
to be dumped from registers etc. This latter effect is
FAR more damaging than the stride computation, which you
might want to note is free on many machines in many cases
(e.g. on the ia32 for strides of 1,2,4,8).
The design of C in general is effective in allowing the
user to do some optimization when using a completely stupid
compiler that does no optimization at all. For example, the
register declaration is in this category.
But with a modern optimizing compiler, the excessively low
level of C code (in particular its over-reliance on
explicit pointers) is counter productive (it is interesting
to note that most modern optimizing C compilers simply
ignore the register declaration, they can do a better job
than the programmer of figuring out what to keep in
registers), but these same optimizing compilers cannot undo
the (mis)use of pointer arithmetic for, e.g. array
referencing.
Robert Dewar
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next prev parent reply other threads:[~1999-02-03 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 ` Jeff Carter
1999-02-08 0:00 ` Michael F Brenner
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 ` Richard D Riehle
1999-02-03 0:00 ` robert_dewar
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 ` Tucker Taft
1999-02-05 0:00 ` Brian Rogoff
1999-02-05 0:00 ` Richard D Riehle
1999-02-05 0:00 ` Brian Rogoff
1999-02-06 0:00 ` Matthew Heaney
1999-02-05 0:00 ` John English
1999-02-05 0:00 ` Nick Roberts
1999-02-04 0:00 ` Ehud Lamm
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 ` dennison
1999-02-09 0:00 ` Corey Minyard
1999-02-09 0:00 ` robert_dewar
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$1@nnrp1.dejanews.com>
1999-02-06 0:00 ` Larry Kilgallen
1999-02-05 0:00 ` robert_dewar
1999-02-05 0:00 ` robert_dewar
1999-02-05 0:00 ` Larry Kilgallen
1999-02-05 0:00 ` robert_dewar
[not found] ` <79f24e$t14 <36BB4162.52FC6D9F@averstar.com>
1999-02-05 0:00 ` dennison
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
1999-02-04 0:00 ` Al Christians
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
1999-02-05 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
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 ` Nick Roberts
1999-02-03 0:00 ` William Clodius
1999-02-03 0:00 ` robert_dewar
1999-02-03 0:00 ` Nick Roberts
1999-02-03 0:00 ` robert_dewar [this message]
1999-02-03 0:00 ` Robert A Duff
1999-02-01 0:00 ` Wanted: Ada STL. Reward: Ada's Future Stanley R. Allen
1999-02-01 0:00 ` Jerry van Dijk
1999-02-01 0:00 ` Marin David Condic
1999-02-01 0:00 ` dennison
1999-02-05 0:00 ` Corey Minyard
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