gcc-4_9 inlines less funcs than gcc-4_8 because of used_as_abstract_origin flag.

[Wei Mi]

We see an inline problem as below caused by r201408
(https://gcc.gnu.org/ml/gcc-patches/2013-08/msg00027.html).

hoo() {
  foo();
  ...
}

foo {
  goo();
  ...
}

foo is func splitted, so its body changes to

foo {
  goo();
  ...
  foo.part();
}

and the used_as_abstract_origin of cgraph node of foo will be set to
true after func splitting.

In ipa-inline, when inlining foo into hoo, the original node of foo
will not be reused as clone node because used_as_abstract_origin of
cgraph node of foo is true and can_remove_node_now_p_1 will return
false, so that a new clone node of foo will be created. This is the
case in gcc-4_9.
In gcc-4_8, the original node of foo will be reused as clone node.

gcc-4_8
foo
  |
goo

gcc-4_9
foo        foo_clone
    \        /
      goo

Because of the difference of whether to create a new clone for foo,
when inlining goo to foo, the overall growth of inlining all callsites
of goo in gcc-4_8 will be less than gcc-4_9 (goo has two callsites in
gcc-4_9 but only one in gcc-4_8). If we have many cases like this,
gcc-4_8 will actually have more inline growth budget than gcc-4_9 and
will inline more aggressively than gcc-4_9.

I don't understand the exact usage of the check about
node->used_as_abstract_origin in can_remove_node_now_p_1, but I feel
puzzled about following two points:

1. https://gcc.gnu.org/ml/gcc-patches/2013-08/msg00027.html said the
patch was to ensure all abstract origin functions do have nodes
attached. However, even if the node of origin function is reused as a
clone node, a new clone node will be created in following code in
symbol_table::remove_unreachable_nodes if only the node that needs
abstract origin is reachable.

          if (TREE_CODE (node->decl) == FUNCTION_DECL
              && DECL_ABSTRACT_ORIGIN (node->decl))
            {
              struct cgraph_node *origin_node
              = cgraph_node::get_create (DECL_ABSTRACT_ORIGIN (node->decl));
              origin_node->used_as_abstract_origin = true;
              enqueue_node (origin_node, &first, &reachable);
            }

2. DECL_ABSTRACT_ORIGIN(decl) seems only useful for debug info of
clone nodes. But now the check of used_as_abstract_origin affect
inline decisions, which should be the same with or without keeping
debug info.

Thanks,
Wei.