Jim Wilson <j...@sifive.com> writes: > On Tue, Jun 23, 2020 at 5:21 AM Richard Sandiford > <richard.sandif...@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.
Thanks for the info. Based on that, I guess GCC should leave the values of extra inactive lanes undefined for now, so that the agnostic case is supported. Maybe in future we could have IFN_LEN_* versions of arithmetic operations too, similar to the IFN_COND_* ones, so that they explicitly ignore the inactive elements. Richard
