[Libre-soc-isa] [Bug 968] document shift-and-add instruction

[bugzilla-daemon at libre-soc.org]

https://bugs.libre-soc.org/show_bug.cgi?id=968

--- Comment #2 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
part of the justification for this instruction does come from twofish
(as well as LD-ST-address-calculate-with-a-shift)

#define ENCRYPT_RND( A,B,C,D, T0, T1, xkey, r ) \
    T0 = g0(A,xkey); T1 = g1(B,xkey);\
    C ^= T0+T1+xkey->K[8+2*(r)]; C = ROR32(C,1);\
    D = ROL32(D,1); D ^= T0+2*T1+xkey->K[8+2*(r)+1]   

and there is additional shifting occuring in creation of a q-table:

        ae = i>>4; be = i&0xf;   
        ao = ae ^ be; bo = ae ^ ROR4BY1(be) ^ ((ae<<3)&8);   
        ae = t[0][ao]; be = t[1][bo];   
        ao = ae ^ be; bo = ae ^ ROR4BY1(be) ^ ((ae<<3)&8);   
        ae = t[2][ao]; be = t[3][bo];

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