[Libre-soc-dev] we need two XLENs

[Luke Kenneth Casson Leighton]

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On Fri, Sep 9, 2022 at 3:57 PM Richard Wilbur <richard.wilbur at gmail.com> wrote:

> The infamous double round?

not a problem: as i said, the function which implements the
FP operation is hidden and may perform the necessary
rounding without poisoning the specification pseudocode.

please see the pseudocode
https://libre-soc.org/openpower/isa/fparith/

A-Form

   fadds FRT,FRA,FRB (Rc=0)
    fadds. FRT,FRA,FRB (Rc=1)

Pseudo-code:

    FRT <- FPADD32(FRA, FRB)

this is first redefined to FPADDHALF and it is the function FPADDHALF
that becomes directly responsible for performing the correct and required
rounding *BEHIND THE SCENES*.

we absolutely cannot have

  IF XLEN==64 then do one thing
  ELSE IF XLEN == 32 then do something else
  ELSE IF OTHER BULLSHIT

the ISA WG will freak out.  *hiding* the overridden meanings
behind functions on the other hand will keep simplicity for
Scalar implementors especially those not in the least bit
interested in XLEN.

l.