Risc V greatly underperforms

Niels Möller nisse at lysator.liu.se
Wed Oct 6 15:18:32 UTC 2021

Hans Petter Selasky <hps at selasky.org> writes:

> No, this is not what I tried to express. I meant that if the Risc-V is
> modified to consumes a fixed number of parallell instructions, N, per
> clock, instead of just one, that the performance would be comparable
> to that of x86.

Not with the current instruction set. 

As Torbjörn has explained is some detail, you can produce difference and
borrow with two *independent* instructions, so with sufficient
parallelism (send the work to two independent execution units, running
in parallel) you can get both in a single cycle.

But to produce sum and carry out, you need two *dependent* instructions,
so even if you have plenty of execution units to send the work to, it
will still be two cycles. x86 (like most modern processors) will do it
in one cycle. To get it into one cycle on RISC V, you would have to
somehow fuse instructions, which is a lot more complexity than

And on top of that, there's also a severe penalty on carry *input*.


Niels Möller. PGP-encrypted email is preferred. Keyid 368C6677.
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