[libre-riscv-dev] [Bug 305] Create Pipelined ALU similar to alu_hier.py

[bugzilla-daemon@libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=305

--- Comment #3 from Michael Nolan <mtnolan2640@gmail.com> ---
Luke's Reply:

On Fri, May 8, 2020 at 3:03 PM Michael N <mtnolan2640@gmail.com> wrote:

>> On May 8, 2020, at 9:28 AM, Michael Nolan <mtnolan2640@gmail.com> wrote:
>>
>> Now that I have branches working in the simulator, I was thinking of starting work on the integer ALU. Taking a look at alu_hier.py, I think I'll keep the interface but replace the implementation with a pipelined one.
>>
>> --Michael
>>
> I noticed alu_hier was nicely broken up into different function units (adder, branch unit, multiplier, etc.).

yes, with an "op" - which in our case will basically be "decoder.InternalOp"

> Is there a nice way you know of to do that with the nmutil pipeline stages?

yyyess.... except it's too complicated to explain right just this
minute.  raise the question in the bugreport.

> Like a way to merge stage 1 of the adder pipe with stage one of the branch pipe to get the stage 1 of the alu?

ok if you are talking about *outside* of ReservationStations, you are
mixing "purposes".  the Branch pipeline *does not* interact - at all -
with the ALU pipeline.

can you look again at Mitch's ScoreBoard Mechanics1.pdf

damnit this needs to be on the bugreport, i haven't got time, 10m until meeting

see page 8, diagram at the top.

Those 4 batches of Function Units are *completely separate*.  they do
*NOT* interact.

basically, the first ALU, do it entirely as combinatorial just as in
alu_hier.py and assume only one clock cycle.  leave out MUL and DIV
because those i believe need to be minimum 2 stages for MUL and a
*lot* for DIV.

actually you know what, screw it: make separate ones for MUL and DIV,
pretty much exactly like in that diagram on p8.  we're doing a
radically different design as a proof-of-concept.

* add sub neg compare logical shift all go in their own
Reservation-Station-style FunctionUnit
* MUL in its own separate RS/FU
* DIV likewise

l.

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