Re: alternative to -ffloat-store on x86?

["Theodore C. Belding" <streak at umich dot edu>]

Hi-
Thanks for the pointer!

My program is in C++; do you have a code snippet that would work in C++,
or would 
__setfpucw ((_FPU_DEFAULT & ~_FPU_EXTENDED) | _FPU_DOUBLE);
work in gcc/g++ as well as g77?
-Ted

--
Ted Belding                               Ted.Belding@umich.edu 
University of Michigan Program for the Study of Complex Systems
http://www-personal.umich.edu/~streak/

On 24 Nov 1998, Dave Love wrote:

> >>>>> "TCB" == Theodore C Belding <Ted.Belding@umich.edu> writes:
> 
>  TCB> I need to force strict IEEE floating-point compliance in a
>  TCB> program that I'm compiling with egcs 1.1 for x86, so that
>  TCB> floating-point results are exactly the same as on other
>  TCB> platforms.
> 
> I can't guarantee exactly that, at least not for Fortran in general.
> 
>  TCB> Is -ffloat-store still the best way to do this, or is there a
>  TCB> better alternative?
> 
> I don't think it's in the 1.1 version, but the development version of
> the g77 manual has this; I think I have recipes that should work on
> other x86 systems.
> 
> Floating point precision
> ------------------------
> 
>    If your program depends on exact IEEE 754 floating point handling it
> may help on some systems--specifically x86 or m68k hardware--to use the
> `-ffloat-store' option or to reset the precision flag on the floating
> point unit *Note Optimize Options::.
> 
>    However, it might be better simply to put the FPU into double
> precision mode and not take the performance hit of `-ffloat-store'.  On
> x86 and m68k GNU systems you can do this with a technique similar to
> that for turning on floating point exceptions *Note Floating-point
> Exception Handling::.  The control word could be set to double
> precision by replacing the `__setfpucw' call with one like this:
>        __setfpucw ((_FPU_DEFAULT & ~_FPU_EXTENDED) | _FPU_DOUBLE);
>    (It is not clear whether this has any effect on the operation of the
> GNU maths library, but we have no evidence of it causing trouble.)
> 
>  TCB> I apologize if this has been discussed before; I wasn't able to find
>  TCB> anything in a web search of the mailing list archives.
> 
> Probably well disguised under a random subject...
>