[Libre-soc-dev] svp64 review

[Luke Kenneth Casson Leighton]

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On Mon, Jul 25, 2022 at 6:22 AM Jacob Bachmeyer via Libre-soc-dev
<libre-soc-dev at lists.libre-soc.org> wrote:

> > in other words, i took the concept of "Sub-PC" very seriously and
> > treated it literally as part of the [absolutely] critical Context, aka
> > a peer of PC and MSR.
> >
>
> If nested traps are possible, the trap handler still must preserve
> SVSRR1 somewhere.

i missed this earlier - yes, due to the strict rules surrounding
keeping SVSTATE up-to-date and precise-interruptible, it's
even possible to perform function calls within those Vertical-First
Loops.

this is what the (new) SVLR is for: to save SVSTATE into SVLR
at *exactly* the same time as PC is stored into LR.

the caveat on that one though is that if the function call is using
REMAP at the time, the REMAP SPRs have to be explicitly
saved/restored.  there's 4 of them so it's a bit much to have
SVLRSHAPE0-3 as well.

given that read/write of SVLR has to be added to the branch pipeline,
we can get away with adding one more SPR to the branch
pipeline but not five.

l.