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Perry E. Metzger wrote:
<blockquote cite="mid:87ljyj20gx.fsf@snark.cb.piermont.com" type="cite">
<pre wrap="">"Robert G. Brown" <a class="moz-txt-link-rfc2396E" href="mailto:rgb@phy.duke.edu"><rgb@phy.duke.edu></a> writes:
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<pre wrap="">On Tue, 26 Aug 2008, Michael H. Frese wrote:
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<pre wrap="">C is not much better. I once worked a young computational
programmer for almost a week to get him to prove to himself that a C
source program couldn't walk through a 2-d array the hard way as
fast as a Fortran source program unless the stepping was coded by
hand. He didn't believe that a 2-d array in C is syntactically a 1-d
array of pointers to 1-d arrays, and the row pointers must be
fetched from memory! And separate compilation of functions
</pre>
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<pre wrap=""><!---->
As I said already, he's wrong....
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<pre wrap="">Perhaps, but don't most C programmers allocate such an array as a single
vector and then repack the indices?
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<pre wrap=""><!---->
I've never seen anyone allocate "as a single vector and repack the
indices", though I'm sure that a counterexample exists in someone's
code out there somewhere. In any case, one has no need to do such a
thing.
(This is not to say that when one calls malloc, if you're calling
malloc to allocate an array, that you don't pass it a single size_t
indicating what you're looking for, but that's a different issue.)
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I am not sure if this is what you mean but, anyone that has been
programming in C long enough (hrm...to use malloc at least once ;) )
_should_ know that malloc reserves X bytes of memory and doesn't care
nor needs to know what the memory is used for. <br>
<br>
As for the contiguous nature of the assignment, doing otherwise would
be horrendously inefficient given that most processors take for granted
this memory mapping to optimize cache usage and pre-fetches (noting
that actual memory allocation is done by the OS as pages). I am
currently unable to dig out the reference but there are very few
processors (none that are Beowulf COTS material iirc) that implement
any sort of semantics to detect the actual fetching pattern (ie:
understand that the data is being fetched by strides of x bytes). Also
I am actually doing some studies on comparing the use of data
structures (or not) in some simple C code to see if the use of
structures has a significant impact on the cache's fetching
capabilities....efforts which might become useless (stay readable to
the human and let the compiler do it's work) since (from GCC-4.3.1's
manpage):<br>
<br>
-fipa-struct-reorg<br>
Perform structure reorganization optimization, that change
C-like structures layout in order to better utilize spatial locality.
This transformation is<br>
affective for programs containing arrays of structures.
Available in two compilation modes: profile-based (enabled with
-fprofile-generate) or static<br>
(which uses built-in heuristics). Require -fipa-type-escape
to provide the safety of this transformation. It works only in whole
program mode, so it<br>
requires -fwhole-program and -combine to be enabled.
Structures considered cold by this transformation are not affected (see
--param<br>
struct-reorg-cold-struct-ratio=value).<br>
<blockquote cite="mid:87ljyj20gx.fsf@snark.cb.piermont.com" type="cite">
<pre wrap="">Perry
</pre>
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Eric Thibodeau<br>
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