[Libre-soc-bugs] [Bug 784] Implement cl* instructions for carry-less operations

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=784

--- Comment #13 from Jacob Lifshay <programmerjake at gmail.com> ---
(In reply to Luke Kenneth Casson Leighton from comment #5)
>   73         for i in range(self.width):
>   74             with m.Switch(Cat(self.a[i], self.b[i])):
>   75                 with m.Case('11'):
>   76                     # both have no leading zeros so far, so set carry
>   77                     m.d.comb += [
>   78                         addend1[i].eq(1),
>   79                         addend2[i].eq(1),
>   80                     ]
>   81                 with m.Case('01', '10'):
>   82                     # different number of leading zeros, so clear carry
>   83                     m.d.comb += [
>   84                         addend1[i].eq(0),
>   85                         addend2[i].eq(0),
>   86                     ]
>   87                 with m.Case('00'):
>   88                     # propagate results from lower bits
>   89                     m.d.comb += [
>   90                         addend1[i].eq(1),
>   91                         addend2[i].eq(0),
>   92                     ]
> 
> that's just:
> 
>    comb += addend1.eq(a ^ b)
>    comb += addend2.eq(a | b)

I explicitly have it more complex so I can explain all the cases and to match
1:1 with the reference algorithm.

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