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From: "hubicka at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/103223] New: [12 regression] Access attribute
 prevents IPA optimization
Date: Sat, 13 Nov 2021 10:05:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103223

            Bug ID: 103223
           Summary: [12 regression] Access attribute prevents IPA
                    optimization
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hubicka at gcc dot gnu.org
  Target Milestone: ---

Hi,
ipa-fnsummary sets can_change_signature flag which determines whether we can
manipulate parameters of a given function.  It tests:

       /* Type attributes can use parameter indices to describe them.  */
       if (TYPE_ATTRIBUTES (TREE_TYPE (node->decl))
         node->can_change_signature =3D false
Which unfortunately triggers on many C functions now when we introduced the
access attribute.

Updating happens in ipa-param-manipulation and we do have infrastructure ho=
w to
rewrite (suriving) old attributes to new ones, so we could support access
attribute updating (or always map to old indexes when using it).

I don't think possible warnings should inhibit useful optimizations and thi=
s is
a regression wrt compilers before the access attribute.  I am having patch =
to
fix similar issue with fnspec attribute that can be safely removed at signa=
ture
change since we now can preserve info in ipa-modref.

Martin, I wonder if if you would be OK with simply dropping the access when
function signature changes (which I can prepare patch for) or do you want to
dive into updating it?

Once new fuction is created, for every new parameter there is
get_original_index accessor which returns original parameter index (if it
exists).  This could be easily used to update access and drop those entries
that was really optimized out IMO=