[Rd] Floating point control (was: [R] Variance for Vector of Constants is STILL Not Zero)

Prof Brian Ripley ripley at stats.ox.ac.uk
Sat Feb 18 16:53:41 CET 2006

I believe this to be a Windows-only problem.  Setting the FPU control is 
OS x CPU specific (e.g. Solaris and FreeBSD do it differently on Sparc and 
ix86), and may well not be allowed for user processes.

But we should be more aggressive about combating it on Windows.

On Sat, 18 Feb 2006, Duncan Murdoch wrote:

> Over on R-help, the old problem of floating point precision has come up
> again (see my example below, where calling RSiteSearch can change the
> results of the var() function).
> The problem here is that on Windows many DLLs set the precision of the
> fpu to 53 bit mantissas, whereas R normally uses 64 bit mantissas.
> (Some Microsoft docs refer to these as 64 bit and 80 bit precision
> respectively, because they count the sign and exponent bits too).

(I have a solution to that in place in R-devel.  As some other chips, e.g. 
Sparc, are more strictly IEC60559 compliant, we should and now do achieve 
full accuracy on chips with an effective 53-bit mantissa ('effective' 
since the top bit is not actually stored).)

> When R calls out to the system, if one of these DLLs gets control, it
> may change the precision and not change it back.  This can happen for
> example in calls to display a file dialog or anything else where a DLL
> can set a hook; it's very hard to predict.
> I consider this to be very poor programming; DLLs shouldn't
> unnecessarily change the operating environment of their caller.
> However, it's something we've got to live with.
> Currently R itself sets the FPU precision to 64 bit mantissas when it
> starts and preserves it across dyn.load calls.  I think we need to be
> more aggressive about protecting the precision.  Specifically, in any
> case where we know we are directly calling an external function we
> should protect the precision across the call.
> A problem is that the C runtime library also makes calls to system
> functions, so some of those calls are probably risky too.  It's not
> reasonable to protect all C library calls, but I think we should fairly
> aggressively test for changes, fix them, and optionally report them.
> Another problem is that R itself is used as a DLL.  Should it set the
> precision to 64 bit mantissas, or try to maintain whatever precision the
> caller gave it?  I'd lean towards documenting a requirement for 64 bit
> precision on entry and documenting that we may change the precision to
> 64 bits.
> Yet another problem is that Microsoft's .NET only supports 53 bit
> precision, according to some documentation I've read.  Do we need to
> interoperate with .NET?
> I don't know if this is a Windows-only problem, or if it occurs on any
> other systems, but I think the only way to know is to add the tests on
> all systems.
> I'd like to suggest the following:
>  - We add R level functions to get and set the floating point control bits.
>  - We save the value that is set, and work aggressively to make sure it
> doesn't get changed by other mechanisms, with debugging options to
> report all unexpected changes.
> I don't know how portable any of this will be.  Is the _controlfp()
> function standard C, or is it only available on some of our platforms?
> Duncan Murdoch
> On 2/17/2006 11:05 PM, Duncan Murdoch wrote:
>> My guess is that you've got a video driver or some other software that's
>> messing with your floating point processor, reducing the precision from
>> 64 bit to 53 or less.  I can reproduce the error after running
>> RSiteSearch, which messes with my fpu in that way:
>> > var(rep(0.2, 100))
>> [1] 0
>> > RSiteSearch('fpu')
>> A search query has been submitted to http://search.r-project.org
>> The results page should open in your browser shortly
>> > var(rep(0.2, 100))
>> [1] 1.525181e-31
>> (I'm not blaming RSiteSearch for doing something bad, it's the system
>> DLLs that it calls that are at fault.)
>> I think this is something we should address, but it's not easy.
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Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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