http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50527
--- Comment #7 from rguenther at suse dot de <rguenther at suse dot de> 2011-09-27 11:08:01 UTC --- On Tue, 27 Sep 2011, vries at gcc dot gnu.org wrote: > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50527 > > --- Comment #6 from vries at gcc dot gnu.org 2011-09-27 10:49:23 UTC --- > > I think we can check if the size is constant in evaluate_stmt and > > compute alignment according to that. > > We can only do that in the last ccp phase that does folding of vla-alllocas. > > If the argument is not constant, it will not be folded in this phase, but it > might be folded during the next ccp phase, when the argument does turn > constant. > > If the argument is constant, it might not be folded in this phase, but it > still > might be folded during the next ccp phase. > > Therefore, in evaluate_stmt, we cannot predict whether the alloca will be > folded, unless we're in the last ccp phase. And the propagation of alignment > of > alloca starts in the first ccp phase. > > > It should only change from > > non-constant to constant, thus properly go down the lattice during > > propagation. > > Currently, the result of an alloca is always constant, to be precise, constant > 0 with only lower bits valid. This is independent of whether the argument is > constant. The parameter I meant. But yes if we don't fold alloca in ccp1 we might fold away alignment tests based on BIGGEST_ALIGNMENT while later ccp might fold it and use less alignment. Maybe don't assume any particular alignment for allocas for vlas then. Richard.
