https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104408
--- Comment #4 from Tamar Christina <tnfchris at gcc dot gnu.org> --- (In reply to Richard Biener from comment #3) > match.pd just does canonicalization here. SLP discovery could handle this > in the existing swap operands or reassoc support but I guess the desire here > is to pull out a Complex SLP pattern. Yes, though also to optimize the case where you don't have the optab, currently the generated code is much worse at -Ofast. > > So - no perfect idea yet how to reliably match a Complex pattern here but > trying to attack this from the match.pd side sounds wrong. Well the problem is that the scalar code is suboptimal too. even without matching a complex pattern, so the epilogue here does an extra sub on each unrolled step. So I initially figured we'd want to not perform the canonization if it's coming at the expense of sharing. However that looks harder than I though at first as there are multiple points in const-fold.c that will try and force this form. I can probably fix the epilogue post vectorization but that seemed like a worse solution.
