[libre-riscv-dev] [Bug 189] Create partitioned right shift using the existing partitioned left shift

bugzilla-daemon at libre-riscv.org bugzilla-daemon at libre-riscv.org
Thu Feb 27 22:10:09 GMT 2020


--- Comment #7 from Michael Nolan <mtnolan2640 at gmail.com> ---
(In reply to Jacob Lifshay from comment #6)

> If we do it as microcode, we should try to have it still be a constant-time
> operation, since it's commonly assumed to be constant-time by crypto code.

Rol ra, rb, rc getting translated to

shl tmp1, rb, rc
shr tmp2, rb, (32-rc)
or  ra, tmp1, tmp2

should be constant time with respect to the data being manipulated. The actual
timing probably depends on what else is in the pipe, but that's true of other
instructions as well.

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