[Libre-soc-isa] [Bug 569] svp64 register predicates vs BE arrays of bits

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=569

--- Comment #18 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #17)
> (In reply to Luke Kenneth Casson Leighton from comment #15)
> > if you are volunteering to write the Grant Appplication and develop
> > the simulator and the HDL, after first finishing the delivery of the
> > commitments we are already obliged to complete, then great.
> 
> for what exactly? byteswapping the regfile into memory order?

all of it. the full and comprehensive instruction analysis, modifications
required to the spec, everything.  it's so large that i'd suggest you
write the Grant at the full EUR 50,000.

(In reply to Jacob Lifshay from comment #16)

> no reversals are required because you never touch an integer predicate, you
> go from vector of ints (not a predicate) to the cmpi into a vector of cr
> bits (not an integer predicate so bitreversal doesn't happen) to whatever
> instruction is predicated where the cr vector is used as a predicate (not an
> integer predicate so bitreversal doesn't happen)

to say that the problem may be "avoided" by not using precisely and exactly
one of the beneficial features of SVP64 is... well, i'm not sure what to say

> actually, it's perfectly possible to have that property in BE, simply store
> the bigint in BE:

the example that you gave when picking 32-bit means that
when setting elwidth to 64-bit the words in each 64-bit
element are in the wrong order.

it does not matter which example is picked (any N, M where
N and M are both 16 or greater).

this is the *known* problem of big-endian mode.

> compatibility -- that switching can be completely ignored at the user
> application level). it requires slightly more hardware, but greatly
> simplifies software.

"slightly more hardware" - which needs a full audit and a deeply-comprehensive
review that we absolutely do not have any time or resource to do right now.

please focus on priority tasks.

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