[Bug middle-end/50527] inconsistent vla align

["vries at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

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.