[Bug middle-end/81445] New: Dynamic stack allocation not optimized into static allocation

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81445

            Bug ID: 81445
           Summary: Dynamic stack allocation not optimized into static
                    allocation
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: wilco at gcc dot gnu.org
  Target Milestone: ---

Many compilers optimize small dynamic allocations into static allocation. This
is more efficient as it allows multiple small dynamic allocations to be merged
into a single static allocation. In some cases it may remove all dynamic
allocations, avoiding the need for a frame pointer (which frees up an extra
register on various targets in addition to reducing prolog/epilog overheads).

Obviously allocations cannot be in a loop (including tail-recursion), and the
total size should be limited to avoid increasing stack size by too much.

void f(void*);
void alloca (int x)
{
  if (x < 100)
    f (__builtin_alloca (x));
  f (__builtin_alloca (16));
}