Re: Microarchitectural support for counting

On Sat, 5 Oct 2024 15:11:29 +0000, EricP wrote:

MitchAlsup1 wrote:

On Fri, 4 Oct 2024 18:11:23 +0000, EricP wrote:
>

--------------------------

>
Not necessary, you purge all of the younger instructions from the
thread at retirement, but none of the instructions associated with
the new <interrupt> thread at the front.

>
That's difficult with a circular buffer for the instruction queue/rob
as you can't edit the order. For a branch mispredict you might be able
to mark a circular range of entries as voided, and leave the entries
to be recovered serially at retire.

Sooner or later, the pipeline designer needs to recognize the of
occuring
code sequence pictured as::
     INST
     INST
     BC-------\
     INST     |
     INST     |
     INST     |
/----BR       |
|    INST<----/
|    INST
|    INST
\--->INST
     INST
So that the branch predictor predicts as usual, but DECODER recognizes
the join point of this prediction, so if the prediction is wrong, one
only nullifies the mispredicted instructions and then inserts the
alternate instructions while holding the join point instructions until
the alternate instruction complete.

But voiding doesn't look like it works for exceptions or conflicting
interrupt priority adjustments. In those cases purging the interrupt
handler and rejecting the hand-off looks like the only option.

Nullify instructions from the mispredicted paths. On hand off to ISR,
adjust recovery IP to past the last instruction that executed properly;
nullifying between exception and ISR.