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Re: [Bug-gsl] [PATCH] Disable use of FMA

From: Tuomo Keskitalo
Subject: Re: [Bug-gsl] [PATCH] Disable use of FMA
Date: Thu, 01 Dec 2011 18:13:48 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110921 Thunderbird/3.1.15


What's your architecture/platform, compiler & version?

msbdf should never increase or decrease order by more than one, unless it is reset, so there's something really wrong there. Please post all lines until previous "msbdf_apply:". Is this coming from test_extreme_problems? I too bet on a compiler issue.

On 12/01/2011 05:13 AM, Rhys Ulerich wrote:

The algorithms in cblas (at least) become instable when the compiler is
allowed to contract floating-point expressions.  This manifests in the
failure of the ode-initval2 test.
This is the failure I get:

gsl: msbdf.c:1336: ERROR: msbdf_apply too large order change
-- order change -2

This comes from test_extreme_problems, with the "ringmod" problem.
Enabling DEBUG in msbdf.c shows this difference between
-ffp-contract=off and -ffp-contact=fast:

--- dstep: 9.81743e-17 3.08687e-48 9.07047e-10 -9.07047e-10 -9.07047e-10 
9.07047e-10 -3.62750e-13 -2.20616e-27 -6.93678e-59 -1.81409e-16 -1.81409e-16 
1.81409e-16 1.81409e-16 1.57079e-14 4.93899e-46
+-- dstep: 9.81743e-17 -8.68549e-36 9.07047e-10 -9.07047e-10 -9.07047e-10 
9.07047e-10 -3.62750e-13 -2.20616e-27 5.60182e-46 -1.81409e-16 -1.81409e-16 
1.81409e-16 1.81409e-16 1.57079e-14 -1.96797e-42

I don't know that region of GSL well enough to understand the
discrepancy between 3.08687e-48 and -8.68549e-36; -6.93678e-59  and
5.60182e-46; or 4.93899e-46 and -1.96797e-42.  All of them seem like
floating point noise near zero to me...


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