Richard Biener <rguent...@suse.de> writes: > On Wed, 1 Sep 2021, Richard Sandiford wrote: > >> Richard Biener <richard.guent...@gmail.com> writes: >> > On Tue, Aug 31, 2021 at 11:26 AM Richard Biener via Gcc-patches >> > <gcc-patches@gcc.gnu.org> wrote: >> >> >> >> When doing whole-function SLP we have to make sure the recorded >> >> base alignments we compute as the maximum alignment seen for a >> >> base anywhere in the function is actually valid at the point >> >> we want to make use of it. >> >> Ah, yeah, the danger of optimisations that silently rely on the >> then-current restrictions :-( > > Yeah. > >> >> To make this work we now record the stmt the alignment was derived >> >> from in addition to the DRs innermost behavior and we use a >> >> dominance check to verify the recorded info is valid when doing >> >> BB vectorization. >> >> >> >> Note this leaves a small(?) hole for the case where we have sth >> >> like >> >> >> >> unaligned DR >> >> call (); // does not return >> >> aligned DR >> >> >> >> since we'll derive an aligned access for the earlier DR but the >> >> later DR is never actually reached since the call does not >> >> return. To plug this hole one option (for the easy backporting) >> >> would be to simply not use the base-alignment recording at all. >> >> Alternatively we'd have to store the dataref grouping 'id' somewhere >> >> in the DR itself and use that to handle this particular case. >> > >> > It turns out this isn't too difficult so the following is a patch adjusted >> > to cover that case together with a testcase. >> > >> > Bootstrapped and tested on x86_64-unknown-linux-gnu. >> > >> > OK? >> >> TBH I know nothing about this group id scheme, so I'm not really >> in a position to comment. But it LGTM from the (few) bits I do understand. >> >> I guess we're leaving the same easter egg for loop optimisation if >> we ever support early exits, but I'm not sure what to do about that. > > We're currently not recording alignment from masked DRs > (aka DR_IS_CONDITIONAL_IN_STMT), I suppose we'd need to mark > all stmts after early exits in this way then since in the end > they will have to be masked on the early exit. > > So we _might_ be fine there ...
Yeah, for a pure SVE-like implementation that's probably true. But we also have the option of vectorising an early exit by branching if the condition is true for *any* element, then handling the remaining elements with an epilogue. It would be quite a different approach from what we're doing now, and wouldn't necessarily require up-front if-conversion. But the point is that it's theoretically possible, just as whole-function SLP was theoretically possible (but seemingly some way off) when this code was written :-) Thanks, Richard
