Re: GCC floating point usage

[Dale Johannesen <dalej at apple dot com>]

On Monday, October 14, 2002, at 09:59  AM, Zack Weinberg wrote:
> On Mon, Oct 14, 2002 at 12:37:14PM -0400, David Edelsohn wrote:
> > > > > > > Stefan Bylund writes:
> > Stefan> Is it possible to somehow instruct GCC to only generate
> > Stefan> floating point instructions for C/C++ float/double
> > Stefan> operations and not for internal non-floating point
> > Stefan> optimizations etc?
> >
> > 	If you do not want to use the FPU, then invoke the compiler with
> > the -msoft-float PowerPC option.  If the FPRs are present and it is
> > efficient to use them, the compiler will use them.
> Come now, what he wants is not unreasonable.  "If floating point
> operations appeared in the source code, generate hardware floating
> point instructions; otherwise, generate only integer-unit
> instructions."  I (that is, CodeSourcery) have a customer that needs
> precisely this; in fact I posted a patch to implement it for PPC last
> fall.  Now seems like a reasonable time to bring the issue back up.
Apple's group has users who want this also.  From one viewpoint the real
problem is that the compiler's idea of "efficient to use them" is not
always accurate; but doing better may require information not available
to the compiler, as in Stefan Bylund's case.  Overall I think a patch
of this sort is a good idea.

(We also have users who carefully write double stores to zero out an 
array,
and are annoyed when the compiler changes them into two int stores.
[Actually a mix may be better than either.]  I wouldn't mind having 
Zach's
switch, or another switch, disable this behavior also.)