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

From: Andreas Schwab
Subject: Re: [Bug-gsl] [PATCH] Disable use of FMA
Date: Mon, 28 Nov 2011 18:44:24 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.0.91 (gnu/linux)

Rhys Ulerich <address@hidden> writes:

> When you say "instable" do you mean that they fail at some particular
> tolerance level?  Could you give an example?  The problem you describe
> sounds like overly-sensitive unit tests rather than a reason to
> entirely avoid FMAs.

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 


Andreas Schwab, address@hidden
GPG Key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."

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