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Date: Wed, 3 Mar 2004 23:36:19 -0500 (EST)
From: Daniel Eischen <eischen@vigrid.com>
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To: Peter Grehan <grehan@freebsd.org>
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Subject: Re: User-space context switch and floating-point
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On Thu, 4 Mar 2004, Peter Grehan wrote:

> >>  Any opinions ? I noticed the alpha/sparc64 routines don't save FP.
> > 
> > You don't need to save floating point registers when saving the
> > context from userland.
> 
>   But why is that any different from having to save non-volatile
> integer registers ? For instance, on PPC, gp regs 14-31, and also
> fp regs 14-31, need to be saved by the callee if used since they're
> non-volatile across function calls.
> 
> > Look at the i386 thr_getcontext(); it only needs to save the FP
> > control word.
> 
>   Yes, but I thought the i386 FP calling convention was that all
> FP registers were volatile across calls, so all were preserved
> by the callee.

Yes, but how do you generate code that does:

  double x, y;

  ...
  x = y * 1.5 * sqrt(2.25) - 3.15 + pthread_mutex_lock(&m) + 1.25;
  ...

If on the other hand:

  x = y * 1.5 - sqrt(2.25);
  pthread_mutex_lock(&m);
  x = x + 2.0;

requires FP registers to be saved, then so be it.  I don't know
anything about PPC...

-- 
Dan Eischen