[libre-riscv-dev] SV Prefix questions

Luke Kenneth Casson Leighton lkcl at lkcl.net
Wed Jun 26 09:34:49 BST 2019

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On Wed, Jun 26, 2019 at 9:16 AM Jacob Lifshay <programmerjake at gmail.com> wrote:
> I tried to write a reduction calculation that uses the balancing the
> last two iterations part of setvl and I discovered that it works just
> fine without that. Therefore, I think we should switch the algorithm
> to the following:
> let mut regs = [0u64; 128];
> let mut vl = 0;

 ... added:

 can explore it there.

 fitting with that algorithm:

 * bits 24..20 clearly fit MAXVL.
 * bits 19..15 (rs2) just as clearly have no purpose whatsoever for sv.setvl :)
 * rvv's vsetvli appears to be absolutely no different from vsetvl

 i managed to fill in 4 of the possible 8 permutations (from the
pseudocode), it works well, basically, when rs2==0, it's a "sv.setvl
or sv.setvli", and if rs2 != 0, that can be used as a (scalar) opcode.


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