[libre-riscv-dev] [Bug 173] dynamic partitioned "shift"

bugzilla-daemon at libre-riscv.org bugzilla-daemon at libre-riscv.org
Fri Feb 14 10:55:44 GMT 2020


--- Comment #22 from Luke Kenneth Casson Leighton <lkcl at lkcl.net> ---
(In reply to Jacob Lifshay from comment #7)
> one important point to remember is that we need to be able to cheaply
> implement the shift instructions, on RISC-V, at least, the shift amount's
> high bits are specifically ignored, so shifting by n bits actually gives the
> result of shifting by n % bit_len where bit_len is the number of bits in the
> field/register being shifted.

this needs to go into all the unit tests, and it needs to be done dynamically.
so, when a *partition length* is 8, the bitmask is 0b111 on the b operand.
when a partition length is 32, the bitmask is 0b1111 on b.

this needs to be in the formal proof as well, michael.

btw we're spending a lot of time on this one, i'm upping the budget.

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