Re: [PATCH 1/7 v5] ifn/optabs: Support vector load/store with length

[Jim Wilson <jimw@sifive.com>]

On Tue, Jun 23, 2020 at 5:21 AM Richard Sandiford
<richard.sandiford@arm.com> wrote:
> MVE and Power both set inactive lanes to zero.  But I'm not sure about RVV.
> AIUI, for RVV the approach instead would be to reduce the effective vector
> length for the final iteration of the vector loop, and I'm not sure
> whether in that situation it makes sense to say that the other elements
> still exist and are guaranteed to be zero.
>
> I'm the last person who should be speculating on that though.  Let's see
> whether Jim has any comments.

The RVV spec supports two policies for tail elements, i.e. elements
beyond the current vector length.  They can be undisturbed or
agnostic.  In the undisturbed case, the trail elements retain their
old values.  In the agnostic case, the implementation can choose to
either retain their old values, or set them to all ones, and this
choice can be different from lane to lane.  The latter case is useful
because registers may be wider than the execution unit, and current
vector length may not be a multiple of the width of the execution
unit.  So for instance if the vector registers can hold 8 elements,
and the execution unit works on 4 elements at a time, and the current
vector length is 2, then it might make sense to leave the last four
elements unmodified to avoid an iteration across the registers, but
the third and fourth elements might be set to all ones because you
have to write to them anyways.  The choice is left up to the
implementation because we have multiple parties designing vector
units, and some are target for low cost embedded market, and some are
target for high performance, and they couldn't agree on a single best
way to implement this.  The software is expected to choose agnostic
only if it doesn't care about what happens to tail elements, and
undisturbed if you want to preserve them.  The value of all ones was
chosen to discourage software developers from trying to use the values
in tail elements.  The choice of undisturbed or agnostic can be
changed every time you set the current vector length and type.

In most cases, I think RVV programs will use agnostic for tail
elements, since we can change the vector length at will, and it will
be rare that we will care about elements beyond the current vector
length.

Tail elements can't cause exceptions so there is no need to worry
about whether those elements hold valid values.

Jim