[Libre-soc-bugs] [Bug 238] POWER Compressed Formal Standard writeup
bugzilla-daemon at libre-soc.org
bugzilla-daemon at libre-soc.org
Wed Nov 18 13:50:30 GMT 2020
https://bugs.libre-soc.org/show_bug.cgi?id=238
--- Comment #35 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to cand from comment #34)
> I'm still missing why permutations would be ruled out. Even a pure
> permutation array covering all possibilities would be 4^4 = 256 values = 8
> bits. It can represent XXXX, XYZW, all.
a permutation is defined mathematically as mutually exclusive moving, no
copying. 4-factorial i.e 4x3x2x1. that is the definition of a permutation and
you only need 4 bits to specify a 4-permutation (12 options).
* X goes in 1 of 4 places
* that leaves 3 to choose from for Y
* that leaves 2 for Z
* W goes in the remaining slot.
YZWX. WYXZ.
except... we actually need repetitions.
XXZY. ZWWW.
that is not called a "permutation". it is 4 separate and distinct indexed
*selections* and for that we need 8 bits.
it would be really nice to only need 4 bits!
as it is we can probably get away with using MV.SWIZZLE by having the swizzle
be part of prefix and place that on a standard C.mv op.
--
You are receiving this mail because:
You are on the CC list for the bug.
More information about the libre-soc-bugs
mailing list