http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50527
--- Comment #5 from rguenther at suse dot de <rguenther at suse dot de> 2011-09-27 09:28:42 UTC --- On Tue, 27 Sep 2011, vries at gcc dot gnu.org wrote: > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50527 > > --- Comment #3 from vries at gcc dot gnu.org 2011-09-27 09:21:12 UTC --- > > Or alternatively (given we re-compute alignment together with folding > > alloca), > > assign the same alignment as folding would. > > At the point that we determine the alloca alignment during propagation in > visit_stmt, we cannot predict whether that alloca will be folded (during the > same or later ccp phase). > > So the only way to achieve other alignment is to be conservative a bit longer > for vla-allocas with respect to alignment: > - keep align at 1 byte during ccp. > - if we fold during ccp, assign align calculated at folding > - after we are sure there is no more folding (at expand, or f.i. at the end of > the second ccp phase if we limit folding to the first 2 ccp phases, to take > advantage of the larger alignment in the middle-end), we assign > BIGGEST_ALIGNMENT. I think we can check if the size is constant in evaluate_stmt and compute alignment according to that. It should only change from non-constant to constant, thus properly go down the lattice during propagation. We don't want to force excessive alignment on the replacement decls as that might require re-aligning the stack which is expensive. > > The question is of course what standards say about the alignment of > > alloca (4) > > I think alloca is non-standard. But in the context of > fold_builtin_alloca_for_var, alloca is the implementation vehicle of vlas, so > the question is what the standard says about alignment of vlas. Indeed.
