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From: "pexu@gcc-bugzilla.mail.kapsi.fi" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/61882] attribute weak ignored for function templates
Date: Tue, 07 Mar 2023 15:43:43 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D61882

Pekka S <pexu@gcc-bugzilla.mail.kapsi.fi> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pexu@gcc-bugzilla.mail.kaps
                   |                            |i.fi

--- Comment #2 from Pekka S <pexu@gcc-bugzilla.mail.kapsi.fi> ---
This is problem is still present on GCC 13 to a certain degree.

For an example, the following construct is not possible, as `weak_template'=
 is
essentially always considered being present unless using -O0.  Note that `w=
eak'
behaves as expected even it is not extern "C".

If weak address is sourced from GOT (PIC/PIE) and the symbol is undefined, =
the
program will likely crash, as the address is zero (typically optimized as an
unconditional indirect branch).=20=20

Purely static builds are affected as well, but as there is no indirect bran=
ch,
the non-existing branch is likely replaced by a no-operation during linking=
 and
possibly goes unnoticed for simple constructs like these.

[[gnu::weak]] extern void weak();
template <typename T>
[[gnu::weak]] extern void weak_template();

void call ()
{
  auto f0 =3D weak;
  auto f1 =3D weak_template<int>;
  if (f0) f0();
  if (f1) f1(); /* NB: else statement would be always ignored here.  */
  // indirect_call(f1); /* see below.  */
}

It is possible to overcome this issue by not calling the acquired address
directly but passing it first to another function.  This helper function mu=
st
not be inlined / optimized (which might require additional tricks documente=
d in
the manual and so forth).

[[gnu::noipa]] static void indirect_call (auto f)
{
  if (f) f();
}=