GNU bug report logs - #12350
Composites identified as primes in factor.c (when HAVE_GMP)

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From: Pádraig Brady <P <at> draigBrady.com>
To: Torbjorn Granlund <tg <at> gmplib.org>
Cc: 12350 <at> debbugs.gnu.org, nisse <at> lysator.liu.se, Jim Meyering <jim <at> meyering.net>
Subject: bug#12350: Composites identified as primes in factor.c (when HAVE_GMP)
Date: Mon, 08 Oct 2012 15:43:41 +0100

On 10/08/2012 02:21 PM, Torbjorn Granlund wrote:
> Jim Meyering <jim <at> meyering.net> writes:
>
>    However, this little command does a lot of I/O, too:
>    191M input, 77M output.
>
> Sure.  I've never seen significant variance for such stuff, measurable
> as CPU time.
>
> I tested with a Sandybridge i3-2120T now.  The range takes 32 s.
>
> In both cases, the systems run GNU/Linux.  The kernel version is 3.2.
>
> The factoring speed varies very much with GCC version.  I particular the
> trial division code has a very tight loop, and such loops have more
> compiler reliance.  GCC 4.6 an later generate code that executes 4 insns
> per (unsuccessful) division.

gcc (GCC) 4.6.0 20110603 (Red Hat 4.6.0-10)
With -march=native -O2

> It is also important to use a 32bit binary.  We should perhaps have

s/to use/to not use/ I presume. Yep I'm on 64 bit.

> provided better 32-bit code paths to be used for numbers < 2^32 on
> 32-bit hardware.  Now, Pádraig's example needs about 3x more time for a
> 32-bit binary on the same hardware.

thanks,
Pádraig.

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