User-space context switch and floating-point
Daniel Eischen
eischen at vigrid.com
Wed Mar 3 20:36:20 PST 2004
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
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