[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 #36 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #35)
> (In reply to Jacob Lifshay from comment #34)
> > (In reply to Luke Kenneth Casson Leighton from comment #33)
> > > fantastic. do you have an indication of the gate count (and LUT4 resources,
> > > synth_ecp5)?
> > 
> > I haven't checked, but I expect the additional resources required to be on
> > the order of 500 2-in muxes (384 (6 layers * 64 bits) for the grev itself,
> > the rest for misc routing in/out)
> 
> pffh, that's not bad at all. should be fine to look at a LUT2-based version
> which will give 16 instructions and dozens of immediates in a 32-bit op

imho a lut2 version is enough additional latency that, if we're not worried
about that, we should just merge with the shift/rotate logic, since imho it
should be similar latency cost and will save a bunch of area.

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