[Libre-soc-bugs] [Bug 770] Discussion and Finalisation of Which Cryptographic Primitives to Implement

bugzilla-daemon at libre-soc.org bugzilla-daemon at libre-soc.org
Thu Oct 20 10:38:17 BST 2022


--- Comment #14 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Luke Kenneth Casson Leighton from comment #8)

>     r0 += r1; \
>     r2 ^= r0; \
>     r2 = ROL32(r2, r3);

these are (using reg nums to indicate which REMAP source is needed)

    add r0, r0, r1            RT, RA, RB
    xor r2, r2, r0            RT, RA, RB
    rlwnm r2, r2, r3, 0, 31   rlwnm RA,RS,RB,MB,ME (Rc=0)

however they are all overlapping regs with the exception of RS, meaning
a sequence of svremaps is needed before each operation. slightly annoying
but doable.

example: RT and RA are REMAP0 in the add, but RT must be REMAP2 in the xor.
by turning things around they can line up, reducing the number of svremaps:

    svremap RT=0,RA=1,RB=0
    add r0, r1, r0            RT, RA, RB
    svremap RT=2,RA=2,RB=0    # RB stays = 0
    xor r2, r2, r0            RT, RA, RB
    svremap RS=2,RA=2,RB=3    # RA stays = 2
    rlwnm r2, r2, r3, 0, 31   rlwnm RA,RS,RB,MB,ME (Rc=0)

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