[Libre-soc-isa] [Bug 1015] rfc for rest of int/fp move/convert ls006

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=1015

--- Comment #14 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #13)
> (In reply to Jacob Lifshay from comment #12)
> > https://libre-soc.org/irclog/%23libre-soc.2023-03-17.log.html#t2023-03-17T08:
> > 57:56
> > me on irc:
> > > oh, btw, i realized that using all the pseudocode functions from the vsx section of the powerisa
> > > spec might not go over so well, so i'm thinking of rewriting the pseudocode for fcvt[f/t]g
> 
> no there is a good reason *to* use the exact same pseudocode: it
> reduces IBM's workload.
> 
> and also ours.

well, with the way fcvttg in particular is written, i think it would be wise to
rewrite it based off book 1 appendix a since that's likely to be much simpler
for readers and for ibm, rather than what is there now which is working around
all the vsx functions by calling them several different times and having two
pages of complex confusing code.
> 
> please find and read my earlier IRC comments about hypothetical
> discussions that may take place with the IBM Hardware Architects.
> 
> don't try to "hide" things like that, it will not be well received,
> it will be viewed as an attempt to deceive, particularly if someone
> after spending considerable time (a) notices they are the same or
> (b) finds these conversations online and sees a conclusion that we
> attempted to deceive.

it was never my intention to deceive, but that the functions used are from a
different section of the book than where we want the code to end up, so we
should base it on the fp code from the scalar floating point section rather
than the vsx section. this also helps avoid issues like us accidentally using
vsx functions that access vsx/vmx registers when we don't want them to, e.g.
si32_CONVERT_FROM_BFP32 can set VSCR.SAT

also, the pseudocode would definitely have comments explaining where the code
came from and what's different, so imho that shouldn't be easily confused with
"hiding" things.

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