[Libre-soc-bugs] [Bug 1135] add FPSCR and Rounding classes to ieee754fpu

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=1135

--- Comment #13 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #7)
> (In reply to Luke Kenneth Casson Leighton from comment #4)
> > * hold off the write to the regfile until it is known
> >   - if the exception has occurred (in which case "CANCEL WRITE" is raised)
> >   - if there is NO CHANCE of the exception occurring
> >     (in which case "DROP SHADOW" is raised)
> 
> I assumed that, i don't know how you got it into your head that I meant we
> should rip that out.

no: you listed *as this task* that a FULL BLOWN OUT OF ORDER CORE
would be written, in order that Shadowing could even be used!

that means that *under this task* you have to do get the Shadow Matrices
in, write the unit tests, do EVERYTHING that *I* know is tens of
thousands of Euros worth of work!


> according to my plan, fp ops will shadow until they know the sticky bits
> haven't been changed and no fp traps are needed, at which point they drop
> the shadow.

to do that requires an OoO core as a prerequisite. you're *literally*
designing an ENTIRE Out-of-Order Core for the scant budget of
EUR 2500 or less...

AS PART OF THIS TASK!!

you see what i mean that you are unable to perform budget scoping and
time management?


>  if either of those are needed, then standard trap procedures are
> followed, except that setting sticky bits is more like a branch
> misprediction in that it just restarts the following insns instead of a trap
> where it changes MSR and goes to e.g. PC=0x700.

yyep. now you know why TestIssuer only does one instruction at a time.

see OP_TRAP and in particular notice how "sc" is treated as
"a type of trap".

there is absolutely no way in hell that a tiny budget of this size
could possbly cover the FULL writing of an OoO core necesaary to
do what you are proposing.

lesson hopefully taken on board: do things INCREMENTALLY, in small
chunks, leveraging what *already exists*.

we'll get more grants, don't worry. but we need to demonstrate an
ability to complete what's in front of us.

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