stack height reduction in tree-SSA

["Rakesh Kumar, Noida" <rakesh dot kumar at noida dot hcltech dot com>]

Hi,

  Sharing stack space for objects with disjoint
lifetimes is a well known problem with GCC (especially
for the kernel people). Consider the code

  if (i)
    {
      char a[100];
      use (a);
    }
  else
    {
      char b[100];
      use (b);
    }

Apparantly, 'a' and 'b' can share same stack slot. I feel
that this problem could have been fixed in tree-SSA, but
exactly opposite is done. During 'lower' pass, record_vars
saves the variable declarations in a link list, which are
later expanded to RTL during RTL generation pass in expand_vars.
Below is the dump of 'lower' pass for the above test case.

foo (i)
{
  char b[100];
  char a[100];

  if (i != 0) goto <D1052>; else goto <D1053>;
  <D1052>:;
  use (&a);
  goto <D1054>;
  <D1053>:;
  use (&b);
  <D1054>:;
  return;
}

Is it that good to forget the scope of variable and allocate each
one a new stack slot? I'm unable to get any reasons why the stack
slots can't be shared. Please make me aware of the issues involved.

Thanks and Regards,
Rakesh Kumar