[Libre-soc-bugs] [Bug 911] svshape2 instruction (with offsets)

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=911

--- Comment #5 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #4)
> (In reply to Jacob Lifshay from comment #1)
> > notes: offset will need to be signed.
> 
> this makes me jittery as it is an extra bit.  at least 3 bits are needed
> to be able to offset to a byte within a 64 bit register.  now it is 4 bits,
> they are very precious in SVSTATE. i think there is room.

assuming you meant SVSHAPE0-3, not SVSTATE.

imho it'd make more sense to just expand SVSHAPE0-3 to 64-bits...we'd want more
than i4 for offset, since many instructions can only access every other or
every 4th register iirc, so offset should be able to cover the rest of the
bytes.

also, it just occurred to me we don't need signed offset after all, e.g. if you
want offset -2 for bytes, just use offset -2 % 8 = 6 instead and use the
previous register in the sv.* instruction.

rather than:
svshape2 offset=-1, remap=ra
sv.add/elwid=8 *32, *64, *32

do:
svshape offset=6, remap=ra
sv.add/elwid=8 *32, *63, *32

or:
svshape offset=14, remap=ra
sv.add/elwid=8 *32, *62, *32 # if you can't address every register...

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