[Libre-soc-dev] MVL unclear

[Luke Kenneth Casson Leighton]

On Fri, May 21, 2021 at 2:05 PM Lauri Kasanen <cand at gmx.com> wrote:


> Which you could ensure by writing a couple of insns to check what
> you're setting VL to. Without needing MVL.
>

exactly.  in extremely tight loops that is considered inefficient design,
and a waste of instructions when the amount of hardware needed to
be added inside setvl to perform the exact same job of the additional
2 to 8 instructions (one of which is a branch) is trivial.

in some cases that setup cost could end up exceeding the number
of instructions inside the inner loop.

people with a hell of a lot of expertise have thought about this for
decades.

l.