On Wed, 22 Feb 2006, Duncan Murdoch wrote: > On 2/22/2006 3:52 AM, Prof Brian Ripley wrote: >> I've managed to track this down. The setting of the FPU control word on a >> ix86 machine changes the precision of (gcc) long double calculations in the >> FPU, as well as those of double. So if it gets changed to PC_53, long >> doubles lose accuracy even though 10 bytes are carried around. >> >> For some discussion of extended-precision issues, see Priest's annex to >> Goldberg's paper at http://www.validlab.com/goldberg/paper.pdf. >> >> Many other OSes other than Windows on ix86 do consider the FPU control word >> when doing context-switching. There is (often heated) debate as to what >> the correct default should be, see e.g. the thread begining > > I think Windows also preserves the FPU control word across context switches. > The problem is that it doesn't do a context switch when you make a call into > the system. In particular, the shell services (file dialogs, URL > recognition, etc.) involve a large number of DLLs, all running within the > same context. MSVC++ normally chooses 53 bit precision as the default, and > will set that when a DLL starts up, or whenever a function in it asks to > reset the FPU.
Yes, I think you are correct. In that case the difference is that it is considered legitimate for a shared resource (a DLL) to change the control word in the context that initializes it. Since as I understand it Win64 does not have a working controlfp nor the ability to change precision (from http://msdn2.microsoft.com/en-us/library/e9b52ceh.aspx) in its native mode, maybe one day this will go away. Brian Ripley -- Brian D. Ripley, [EMAIL PROTECTED] 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 ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
