[Bug tree-optimization/58817] optimize alloca with constant size

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58817

--- Comment #3 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #1)
> Transforming VLAs that way isn't a good idea, at least if the size isn't
> really small, at least when the VLA isn't in a scope that dies at the
> function's end (or if there is a chance the function might be inlined
> because of that optimization).  Because, unlike normal alloca, VLAs are
> deallocated already when leading the scope they were declared in, and tons
> of programs rely on that deallocation, otherwise you might have too big
> stack requirements.

Currently we are turning VLAs into regular arrays, but not alloca. From your
explanations, I understand we should basically do the converse? (do it for
alloca but not VLA)