needless deep recursion in gt-c-decl.h

[Per Bothner <per at bothner dot com>]

The gt_ggc_mx_lang_tree_node routine in the generated
gt-c-decl.h file causes a stack overflow when I tried
it under Darwin.  Perhaps I need to increase the stack
limit, but the deep recursion is a performance problem
that needlessly increases the working set.

Specifically, we should iterate, not recurse, on the
field that is most likely to be a long list, the TREE_CHAIN.
I don't understand how gengtype works, but the generated
code should be something like this:

void
gt_ggc_mx_lang_tree_node (x_p)
      void *x_p;
{
  union lang_tree_node * const x = (union lang_tree_node *)x_p;
  for (;;) {
    if (! ggc_test_and_set_mark (x))
      return;
  {
    ...
    gt_ggc_m_tree_node ((*x).generic.common.type);
    x = (*x).generic.common.chain;
    if (x != NULL_TREE)
      continue;
    return;
  }
}

All the comparisons against tag1 and tag2 may also be a performance
problem.  I don't know if the compiler is smart enough to convert
  if (tag2 == (TS_REAL_CST) ...;
  if (tag2 == (TS_VECTOR)) ...;
to:
  if (tag2 == (TS_REAL_CST) ...;
  else if (tag2 == (TS_VECTOR)) ...;
Assuming it is, it would still be an advantage if we could
order the type tests by frequency, unless the compiler is
smart enough to compile all the tests into a switch.  Is it?
--
	--Per Bothner
per@bothner.com   http://www.bothner.com/per/