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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/104539] Failed to inline a very simple template function
 when it's explicit instantiated.
Date: Tue, 15 Feb 2022 08:13:28 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104539

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
   Last reconfirmed|                            |2022-02-15
          Component|tree-optimization           |c++
     Ever confirmed|0                           |1
                 CC|                            |hubicka at gcc dot gnu.org,
                   |                            |jason at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
      Known to fail|                            |12.0

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
t.ii:9:20: missed:   not inlinable: int g()/1 -> int f() [with int <anonymo=
us>
=3D 0]/0, function body can be overwritten at link time

so the issue seems to be that explicitely instantiating changes

_Z1fILi0EEiv/1 (int f() [with int <anonymous> =3D 0]) @0x7ffff6697220
  Type: function definition analyzed
  Visibility: semantic_interposition public weak comdat
comdat_group:_Z1fILi0EEiv one_only
  Aux: @0x42d7d00
  References:
  Referring:
  Function flags: body
  Called by: _Z1gv/0
  Calls:

to

_Z1fILi0EEiv/0 (int f() [with int <anonymous> =3D 0]) @0x7ffff6697000
  Type: function definition analyzed
  Visibility: forced_by_abi semantic_interposition public weak
comdat_group:_Z1fILi0EEiv one_only
  Aux: @0x7ffff6697220
  References:
  Referring:
  Function flags: body
  Called by: _Z1gv/1 (can throw external)
  Calls:

the explicit instantiation lacks COMDAT (but has comdat_group) and it
has forced_by_abi.

I'm not sure the C++ standard calls out any semantic difference for
explicit vs. implicit instantiations but maybe the Itanium ABI does.

Jason?  Honza?=