[Libre-soc-bugs] [Bug 238] POWER Compressed Formal Standard writeup

[bugzilla-daemon at libre-soc.org]

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.

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