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Re: [PATCH] PR libstdc++/88782 avoid ODR problems in std::make_shared


On 21/01/19 11:32 +0000, Jonathan Wakely wrote:
On 21/01/19 12:16 +0100, Richard Biener wrote:
On Mon, Jan 21, 2019 at 12:08 PM Jonathan Wakely <jwakely@redhat.com> wrote:

On 21/01/19 09:13 +0100, Richard Biener wrote:
On Fri, Jan 18, 2019 at 10:29 PM Jonathan Wakely <jwakely@redhat.com> wrote:

The old version of _Sp_counted_ptr_inplace::_M_get_deleter (up to GCC
8.2.0) expects to be passed a real std::typeinfo object, so mixing that
with the new definition of the __shared_ptr constructor (which always
passes the fake tag) leads to accessing the fake object as a real
std::typeinfo. Instead of trying to make it safe to mix the old and new
definitions, just stop using that function. By passing a reference to
__shared_ptr::_M_ptr to the __shared_count constructor it can be set
directly, without needing to obtain the pointer via the _M_get_deleter
back-channel. This avoids a virtual dispatch (which fixes PR 87514).

This means that code built against new libstdc++ headers doesn't use
_M_get_deleter at all, and so make_shared works the same whether RTTI is
enabled or not.

Also change _M_get_deleter so that it checks for a real type_info object
even when RTTI is disabled, by calling a library function. Unless
libstdc++ itself is built without RTTI that library function will be
able to test if it's the right type_info. This means the new definition
of _M_get_deleter can handle both the fake type_info tag and a real
type_info object, even if built without RTTI.

If linking to objects built against older versions of libstdc++ then if
all objects use -frtti or all use -fno-rtti, then the caller of
_M_get_deleter and the definition of _M_get_deleter will be consistent
and it will work. If mixing -frtti with -fno-rtti it can still fail if
the linker picks an old definition of _M_get_deleter and an old
__shared_ptr constructor that are incompatible. In that some or all
objects might need to be recompiled.

Can you try summarizing whatever caveats result from this in
the 8.3/9 changes.html parts?  I have a hard time extracting that
myself from the above ;)  (small example what kind of simplest code might
run into this helps)

There are *fewer* caveats compared to previous releases.

There's an example in PR 87520 showing problems mixing -frtti and
-fno-rtti code, although it's a contrived example that uses explicit
instantiation of shared_ptr internals to demonstrate the problem. The
real issue reported privately to me is harder to reproduce, and will
be even harder with this fix committed. I'll try to document this
potential problem mixing -frtti and -fno-rtti (there are still others,
e.g. PR 43105 lists one).

The problems described in PR 88782 only happen if you use 8.2.1
snapshots post-20181122 and mix code compiled by that snapshot with
code compiled by earlier releases. If you only use 8.3.0 (or 9.1.0)
and no snapshots post-20181122 then, the problem can't happen. I don't
think we need to document transient issues that only existed for two
months with snapshots.

OK, I had the impression that with the fix in there's still more cases
that have issues than before whatever rev. triggered the original issue
on the GCC 8 branch.  If that's not so then I'm fine.

Nope, this makes some problem cases work, and introduces no new
problem cases compared to 8.2.0.

Oh, I guess I should mention that mixing std::make_shared compiled
with 8.2.0 and compiled with later versions will produce a bit of code
bloat compared to using the same version for all objects. That's
because they instantiate different constructors now, so the linker
will keep both definitions, instead of discarding one. (That's how the
bug is fixed, by ensuring the new code generates different symbols
which aren't affected by the behaviour of the old symbols). But that
happens with a lot of libstdc++ fixes, and we don't usually document
it.


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