> > We do have the `used_math' optimisation in there which attempts to avoid
> > doing the FP save/restore if the app isn't actually using math.
>
> No, it's more than that. There's a per-processor "used_math" flag to
> determine if we need to _initialize_ the FPU, but on context switches we
> always assume the program we're switching to will _not_ use FP, and we
> just set the "fault on FP" flag and do not normally restore FP state.
This shows room for optimization; if an app is consistently faulting to
use FP after a context switch, in principle the kernel could start to
assume that it will in the next timeslice as well.
> On the other hand, I also wouldn't be surprised if glibc (or similar
I doubt glibc is normally, at least most distros don't ship an SSE
enabled glibc, only an "i686" one.
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