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

[Richard Biener]

On Wed, Nov 19, 2014 at 8:21 AM, Richard Biener
<richard.guenther@gmail.com> wrote:
> On November 19, 2014 8:13:09 AM CET, Wei Mi <wmi@google.com> wrote:
>>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.
>
> I think we need to keep the functions but do not need to account for them in the unit size if we otherwise could remove them

Btw - please make sure to open a bug so this issue doesn't get lost.

Thanks,
Richard.

> Richard.
>
>>Thanks,
>>Wei.
>
>