Re: register sets

On Tue, 15 Jul 2025 4:56:53 +0000, John Savard wrote:

On Thu, 17 Apr 2025 13:35:36 +0000, Scott Lurndal wrote:
>

Many modern CPUs including intel/amd have mechanisms that the OS can use
to determine if floating point registers have been used since the user
process was dispatched, including a trap to the OS on the first floating
point use.  This allows them to avoid saving and restoring the FP
registers during context switches.

>
That certainly is helpful. But if a user program happens not to have
done any floating-point computations during a particular time-slice,
but did perform some earlier which it will later continue, it could be
that while the FP registers don't need to be saved, they will still
need to be restored on returning to that task from the old copy.

John, you misunderstand, the HW flag bits keep track of whether
the FP or SIMD registers have been used "THIS TIME SLICE".

So a mechanism like that still has to be used carefully, and it
doesn't just make the whole problem disappear.

It almost does once you understand.

John Savard