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

From: Rhys Ulerich
Subject: Re: [Bug-gsl] [PATCH] Disable use of FMA
Date: Mon, 28 Nov 2011 09:37:33 -0600

Hi Andreas,

On Sat, Nov 26, 2011 at 6:12 PM, Andreas Schwab <address@hidden> 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.  Add a configure check to disable fp
> contractions.

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.

FWIW, I am aware of FMA gives the wrong answer for comparisons like
(a*x + b*y) == 0 when x == y and b == -a.  The fix is never check
strict equality for floats and instead check if |a*x + b*y| < tol.
The problem in those cases is not the use of FMAs but rather the code
being incorrect in the presence of floating point arithmetic.

- Rhys

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