[linuxsh-dev] Lazy FPU switching

[NIIBE Y. <gn...@ch...>]

Jesper Skov wrote:
 > If I'm not mistaken, the SH implementation of lazy FPU switching isn't
 > really lazy. It always saves the FPU state on a context switch:

The meaning of "lazy" is different.

If the process uses FPU, the state is saved at the context switch.
Saved state is not re-loaded, until FPU is used again (this part is
"lazy").

Well, in other word, current implementation does lazy re-loading of
FPU state, while it doesn't do lazy saving.

 > I think the current implementation is probably due to being based on
 > the i386 where (apparently) there's no gain in lazy switching.

There's gain: Reducing context switch time.

 > Probably would make sense to clear out the FPU when not used,
 > allowing the FPU circuitry to be powered down [if that's possible
 > in the SH4 CPUs].

AFAIK, there's no such power down mode for FPU?  Is there?

BTW, I have another plan.

With new GCC, we have the flag -m4-nofpu, I will introduce this flag
for SH-4 kernel to avoid using FPU for division.
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