[Libre-soc-bugs] [Bug 755] add grev instruction (OP_GREV)

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=755

--- Comment #29 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
QTY 64of LUT2s:

   Number of cells:                640
     $_ANDNOT_                     192
     $_AND_                         64
     $_MUX_                         64
     $_ORNOT_                      128
     $_OR_                         192

six layers of those is only 3840 cells, which is not bad.  it's certainly
one hell of a lot less than a LUT3:

   Number of cells:               2048
     $_ANDNOT_                     576
     $_AND_                        192
     $_MUX_                         64
     $_NAND_                       192
     $_NOT_                         64
     $_ORNOT_                      128
     $_OR_                         832

six layers --> 12,288 cells which given that's excluding the grev-muxes is
far too much - brings it to over 20,000 cells.

a LUT2 would be workable and cover 256 possible instructions rather than
forcing us to do grevi and grorc as separate instructions. might actually
save gates.

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