[Libre-soc-isa] [Bug 1023] crternlut/crbinlut analysis needed of CR regfile usage

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=1023

Luke Kenneth Casson Leighton <lkcl at lkcl.net> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |FIXED
       The table of|# lkcl =
                   |lkcl = 1200
  payments (in EUR)|# jacob =                   |jacob = 300
     for this task;|                            |
        TOML format|                            |
             Status|IN_PROGRESS                 |RESOLVED

--- Comment #10 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
the answer here remained a firm "NO" following a detailed and
unscheduled explanation of Dependency Matrces.

the premise that grouping similar instructions together is ok because they
already have GPR and CR paths from regfiles was demonstrated to be false.


grouping of instructions behind shared Reservation Stations into the
same pipelines requires the DM Cells to contain the *union* of the Register
Profiles for those pipelines. thus whilst it is fine to group ALU and
Logical together because they both share XER CR0 RT RS RA and RB,
it is *not* fine to group madd with that same group because RC incteases
the size of *all* the DM Cells by 15% just for serving that one extra
register.

thus grouping CR Ops with these instructions similarly increases the
total number of registers handled by every DM Cell in front of those
RSes.

this is why the CDC6600 and the 68000 have 3 separate register files with
very little crossover instructions as it keeps the DMs lean and sparse,
right where they are critically important to keep gate count down.

jacob was unaware of all of this and it had to be explained under duress.

-- 
You are receiving this mail because:
You are on the CC list for the bug.