From: David Kastrup <dak@gnu.org>
To: Mark H Weaver <mhw@netris.org>
Cc: guile-devel@gnu.org
Subject: Re: Growable arrays?
Date: Tue, 12 Jun 2012 22:47:56 +0200 [thread overview]
Message-ID: <871ulkntoj.fsf@fencepost.gnu.org> (raw)
In-Reply-To: <87haug1d89.fsf@netris.org> (Mark H. Weaver's message of "Tue, 12 Jun 2012 16:34:14 -0400")
Mark H Weaver <mhw@netris.org> writes:
> Hi David,
>
> David Kastrup <dak@gnu.org> writes:
>> Mark H Weaver <mhw@netris.org> writes:
>>> Simpler data structures can usually be implemented with less memory,
>>> shorter code sequences with fewer conditional branches and less space in
>>> the instruction cache, which in turn means they can be implemented more
>>> efficiently. Therefore, to allow efficient compilation, primitive data
>>> structures should be very simple, with more complex structures built on
>>> simpler ones instead of the other way around.
>>>
>>> For example, consider resizable vectors vs fixed-size vectors. A
>>> fixed-size vector can be represented as a single memory block that
>>> contains both the length and the elements together. A resizable vector
>>> requires an additional level of pointer indirection, which inevitably
>>> means slower accesses and greater code size. Furthermore, fixed-size
>>> vectors allow the possibility of compiling in an unsafe mode where
>>> out-of-bounds checks can be skipped.
>>
>> I have a really hard time swallowing an efficiency argument for Scheme
>> that is weak enough in comparison with the associated drawbacks not to
>> find consideration in the C++ standard template library.
>
> C++, like Scheme, already supports fixed-size vectors in the core
> language, so it would be redundant to include them in a library.
A vector with run-time determined size? Which variant of C++ offers
that?
>> What kind of performance gains are we talking about in a typical
>> vector-heavy application?
>
> If C++ supported _only_ resizable vectors, such that there was no way
> to avoid the additional level of pointer indirection, and all derived
> data structures had to be built upon these doubly-indirected vectors,
> then I'd expect that the efficiency impact would be quite significant
> in both time and space.
Reality check: C++ does not offer structs/classes containing vectors of
run-time determinable size. You need to allocate a pointer for them.
You are apparently confusing fixed size with compile-time size.
--
David Kastrup
next prev parent reply other threads:[~2012-06-12 20:47 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-06-09 12:32 Growable arrays? David Kastrup
2012-06-09 14:43 ` Krister Svanlund
2012-06-09 17:35 ` David Kastrup
2012-06-11 4:23 ` Daniel Hartwig
2012-06-11 4:37 ` David Kastrup
2012-06-11 5:00 ` Daniel Hartwig
2012-06-11 7:25 ` David Kastrup
2012-06-11 9:01 ` Daniel Hartwig
2012-06-11 9:13 ` Daniel Hartwig
2012-06-11 10:38 ` David Kastrup
2012-06-11 11:57 ` Daniel Hartwig
2012-06-11 12:13 ` Noah Lavine
2012-06-11 12:28 ` David Kastrup
2012-06-11 23:50 ` Mark H Weaver
2012-06-12 9:34 ` David Kastrup
2012-06-12 20:34 ` Mark H Weaver
2012-06-12 20:47 ` David Kastrup [this message]
2012-06-12 21:03 ` Mark H Weaver
2012-06-12 21:18 ` David Kastrup
2012-06-11 8:14 ` Thien-Thi Nguyen
2012-06-11 9:08 ` Andy Wingo
2012-06-11 9:55 ` David Kastrup
2012-06-11 11:25 ` Andy Wingo
2012-06-11 12:00 ` David Kastrup
2012-06-11 12:12 ` David Kastrup
2012-06-11 12:20 ` David Kastrup
2012-06-11 13:04 ` Daniel Hartwig
2012-06-11 14:19 ` David Kastrup
2012-06-11 15:24 ` Stefan Israelsson Tampe
2012-06-11 15:27 ` Andy Wingo
2012-06-11 16:03 ` David Kastrup
2012-06-11 12:20 ` Daniel Hartwig
2012-06-11 12:36 ` David Kastrup
2012-06-11 12:02 ` Ludovic Courtès
2012-06-12 13:36 ` Hans Aberg
2012-06-14 14:33 ` Mark H Weaver
2012-06-14 14:47 ` David Kastrup
2012-06-14 15:23 ` Daniel Hartwig
2012-06-14 15:34 ` David Kastrup
2012-06-14 16:56 ` Daniel Hartwig
2012-06-14 17:15 ` David Kastrup
2012-06-14 17:23 ` Daniel Hartwig
2012-06-14 17:49 ` David Kastrup
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