Hi Richard, on 2020/9/22 下午10:34, Richard Sandiford wrote: > Richard Sandiford <richard.sandif...@arm.com> writes: >> I'll try to have a patch ready tomorrow morning European time. > > Well, I totally failed to hit that deadline. When testing on Power, > I saw a couple of extra failures, but I now think they're improvements
Thanks for the testing! > rather than regressions. See the point about single-iteration > epilogues below. > > --- > > This patch fixes the fallout that Kewen reported on Power after > the recent change to avoid unnecessary use of partial vectors. > As Kewen said, the problem is that vect_analyze_loop_2 doesn't > know how many epilogue iterations there will be, and so it > cannot make a final decision about whether the number of > iterations forces an epilogue loop to use partial vectors. > > This is similar to the current situation for peeling: we don't know > during initial analysis whether an epilogue loop will itself require > peeling. Instead we decide that during vect_do_peeling, where the > final number of epilogue loop iterations is known. > > The patch takes a similar approach for the decision about whether > to use partial vectors. As the comments in the patch say, the > idea is that vect_analyze_loop_2 should make peeling and partial- > vector decisions based on the assumption that the loop_vinfo will > be used as the main loop, while vect_do_peeling should make them > in the knowledge that the loop_vinfo will be used as an epilogue loop. > > This allows the same analysis to be used for both cases, which we > rely on for implementing VECT_COMPARE_COSTS; see the big comment > in vect_analyze_loop for details. > > I hope the patch makes the (mostly preexisting) structure a bit > more obvious. It isn't what anyone would design from scratch, > but that's the nature of working with a mature vector framework. > > Arranging things this way means that vect_verify_full_masking > and vect_verify_loop_lens now become part of the “can” rather > than “will” test for partial vectors. > > Also, while splitting out the logic that handles epilogues with > constant iterations, I added a check to make sure that we don't > try to use partial vectors to vectorise a single-scalar loop. > This required some changes to the Power tests. Thanks for the great rework! Using partial vector should be more costly than just being with one single scalar iteration, I think this is an improvement. But I'm not sure whether someone would argue that in the context of no-vect-cost-model, it's expected to go with vectorized code, cost-modeling check can punt this when it takes effects. Personally I'm fine on this anyway. BR, Kewen
