Re: Darwin multilibs

[Michael Meissner <gcc-mail at the-meissners dot org>]

On Mon, May 26, 2003 at 09:39:05AM -0400, Stan Shebs wrote:
> Geoff Keating wrote:
> 
> >Dara Hazeghi <dhazeghi@yahoo.com> writes:
> >
> >
> >>Hello,
> >>
> >>is there any reason we're still building soft-float libs on Darwin
> >>still? Stan Shebs mentioned in a message back in November
> >>(http://gcc.gnu.org/ml/gcc/2002-04/msg01549.html) that multilibs
> >>weren't strictly necessary on Darwin. Would a patch disabling them
> >>then be welcome? Thanks,
> >>
> >
> >I believe that soft-float is used for the kernel and drivers.
> >
> >
> The reason I was doubtful about the right thing to do is that kernel
> and kexts aren't supposed to be using floating point at all, and
> -msoft-float is just used to prevent float reg refs from getting in
> by accident.  What I think kernel folks would most like is a libgcc
> where adddf3 etc is not defined at all, so that mistaken references
> show up as a link error, but that's a pretty unusual variant of
> multilib.

Note, at least when I was PowerPC maintainer several years ago, the register
allocator would often times use floating point registers for memory to memory
copies (particularly for block move operations), since it avoids extra spills.
Some Linux crashes were attributed to these optimizations in the kernel in code
run during interrupt handling (when the GPRs were saved, but not the FPRs), and
Linux started building with -msoft-float to prevent this.  I would imagine
Darwin has similar issues.

So, it is not to just prevent errant programmers from using floating point, but
also to suppress an optimization that is not valid in the kernel context.

-- 
Michael Meissner
email: gnu@the-meissners.org
http://www.the-meissners.org