https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102043
--- Comment #24 from rguenther at suse dot de <rguenther at suse dot de> --- On Fri, 19 Nov 2021, mikael at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102043 > > --- Comment #23 from Mikael Morin <mikael at gcc dot gnu.org> --- > (In reply to Richard Biener from comment #21) > > (In reply to Bernhard Reutner-Fischer from comment #17) > > > Do we want to address arrays always at position 0 (maybe to help graphite > > > ?) > > > > Helping graphite (and other loop optimizers) would be to not lower > > multi-dimensional accesses to a single dimension (I think that's what > > Sandras patches try to do). > > Or maybe graphite can be taught to handle flattened array access? > > Anyway, does the middle-end support out-of-order array access? > Namely for an array arr(4, 5, 6), arr(:, 1, :) is an array of size (4, 6). > Does the middle-end type system support this? Support as in not miscompile - yes. Dependence analysis is only helped if the shapes of the accesses to the same storage are compatible - a two dimensional and a three dimensional access are not. > In any case, it’s not for gcc 12. Agreed. > > The lower bound doesn't really matter here and > > is well-handled by all code. > > Well, unless the lower bound is negative. ;-) Even a negative lower bound is OK. Problematic is only if you provide an index that accesses the array below the (negative) lower bound ;) But note! In GCC a[i] with a having a lower bound of say -100 will access storage at &a + (i - -100), the lower bound is just to make the effective index >= 0 again, so it can't be used (on its own) to solve the present issue as far as I understand since you _do_ need to access the storage at an effective index < 0.
