[Libre-soc-isa] [Bug 926] add with carry from CR field

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=926

--- Comment #2 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #1)
> imho add-with-carry-in-cr-fields should just disallow Rc=1, simplifying it
> substantially.

i honestly don't know which would be best, here, whether it is mandatory Rc=1
because you have to write it anyway, or, hum.

but it is definitely needed (as an instruction) - parallel carry-adding
would otherwise need the same awfulness as any other ISA lacking carry
flags.

but, also, hum, i am kinda surprised this hasn't been added before, i mean
surely using Rc=1 or cmpi is common to find out if carry should occur, right?

oh one idea i have been mulling over (related here): some bits in SVSTATE
which offset where Rc=1 vector results go.

if this new instruction were to *read* the Rc=1 CR field (to obtain the Carry)
then that offset would be relevant, but most importantly an extra operand
specifying the CRfield src (and dest) for carry-in carry-out would not be
needed.

behind the scenes although it does not look like it, this is a 3-in 2-out
operation which are borderline (expensive on Hazard Management).

source: RA RB CRf
dest  : RT    CRf

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