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Re: gcc-4_9 inlines less funcs than gcc-4_8 because of used_as_abstract_origin flag.
- From: Richard Biener <richard dot guenther at gmail dot com>
- To: Wei Mi <wmi at google dot com>,Jan Hubicka <hubicka at ucw dot cz>,GCC Development <gcc at gcc dot gnu dot org>
- Cc: David Li <davidxl at google dot com>,Teresa Johnson <tejohnson at google dot com>,Dehao Chen <dehao at google dot com>
- Date: Wed, 19 Nov 2014 08:21:34 +0100
- Subject: Re: gcc-4_9 inlines less funcs than gcc-4_8 because of used_as_abstract_origin flag.
- Authentication-results: sourceware.org; auth=none
- References: <CA+4CFy7eHnH_cbOv8SON2grM-Wb7nJrSNba-FvDJYi3oaXEQrw at mail dot gmail dot com>
On November 19, 2014 8:13:09 AM CET, Wei Mi <email@example.com> wrote:
>We see an inline problem as below caused by r201408
>foo is func splitted, so its body changes to
>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.
> \ /
>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
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