On Fri, Aug 29, 2025 at 6:02 PM Robin Dapp <rdapp....@gmail.com> wrote: > > > I think it might be possible that refactoring how we do VMAT_STRIDED_SLP > > vs VMAT_GATHER/SCATTER, at least and possibly specifically for the > > case of emulated handling would be a good thing. But it'll require > > experiments > > and see how it all fits together. > > I started experimenting some days ago and hit a bit of a roadblock when > checking whether > > uint8_t *pix1; > int i_pix1; > uint8_t *pix2; > int i_pix2; > for( int i = 0; i < 4; i++, pix1 += i_pix1, pix2 += i_pix2 ) > ... > > is suitable for a gather. IMHO the offset type must match the step/stride = > pix1 type = int. As we do not support signed gather offsets we need to use > 64-bit unsigned ones and a 16 * 64 vector isn't directly supported. > > So even though our strided loads do support signed strides, we cannot go via > the > recognize gather > -> recognize strided offset > -> strided load > route because the initial signed-offset gather will be unsupported :/
I did want to relax the initial STMT_VINFO_GATHER_SCATTER_P detection to not require any target support (there's emulated gather after all!). But the above isn't a gather, it's a strided (group) load anyway. > I guess we could define gather_load patterns with signed index but we'd have > no > way of verifying whether a dynamic offset is actually a stride... > > For riscv we could continue to pretend to have the respective gather (via > LMUL, > so using > 1 registers) and later hope that what we recognized is indeed a > strided offset. > > The other option would be to have an alternative to the gather -> strided > connection and additionally check for strided_load support directly? > Currently > I see no way around this given the restrictions imposed by our insns. I think that rather than going via vect_check_gather_scatter in vect_use_strided_gather_scatters_p we should directly query gather optab support with the appropriate offset. Don't you have gather optabs like gatherV4SISI for SImode scalar strides already? Richard. > -- > Regards > Robin >
