[Bug c++/55039] New: std::addressof vs. constexpr

[Bug c++/55039] New: std::addressof vs. constexpr

[bkoz at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55039

             Bug #: 55039
           Summary: std::addressof vs. constexpr
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: bkoz@gcc.gnu.org


If std::addressof can be made constexpr, then the iterator member functions of
std::array should be able to be made constexpr as well (begin/end, etc.)

As it stands, std::addressof calls into std::__addressof, which has a
reinterpret_cast, killing constexpr. 

If some variation of

http://gcc.gnu.org/onlinedocs/gcc/Return-Address.html

could be extended to support this, then the new intrinsic could be used to
implement std::addressof.

[Bug c++/55039] std::addressof vs. constexpr

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55039

--- Comment #1 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2012-10-23 18:15:38 UTC ---
I agree, an intrinsic is needed

[Bug c++/55039] std::addressof vs. constexpr

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55039

--- Comment #2 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-10-23 20:53:41 UTC ---
Daniel, I'm confused: didn't we agree in that other open PR we have got about
reinterpret_cast vs constexpr that actually the Standard should be amended
exactly to allow this meaningful use of reint cast in the impl of address_of?

[Bug c++/55039] std::addressof vs. constexpr

[daniel.kruegler at googlemail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55039

--- Comment #3 from Daniel Krügler <daniel.kruegler at googlemail dot com> 2012-10-23 20:56:36 UTC ---
(In reply to comment #2)
During the Portland meeting the issue

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1384

was discussed and core had consensus *not* to support reinterpret_cast in
constant core expressions. I think this means we need to bite the bullet now.

[Bug c++/55039] std::addressof vs. constexpr

[paolo.carlini at oracle dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55039

--- Comment #4 from Paolo Carlini <paolo.carlini at oracle dot com> 2012-10-23 21:06:09 UTC ---
Ok, thanks, I wasn't there. Then please somebody add a note to that other PR.
In any case, the other PR being about us actually accepting reint casts for the
time being, I think it's Ok for our library to just constexpr for the time
being and add a big comment about this PR, that is about the eventual need of
an intrinsics as implementation detail.

[Bug c++/55039] std::addressof vs. constexpr

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55039

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> ---
Since LWG decided not to allow implementations to add constexpr (sigh), I guess
we don't need this intrinsic.

[Bug c++/55039] std::addressof vs. constexpr

[daniel.kruegler at googlemail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55039

--- Comment #6 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
(In reply to Jason Merrill from comment #5)

Given that there exists the still open issue

http://cplusplus.github.io/LWG/lwg-active.html#2296

should this bugzilla issue remain open?
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Subject: [Bug tree-optimization/63844] [4.8/4.9/5 Regression] open mp parallelization prevents vectorization
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https://gcc.gnu.org/bugzilla/show_bug.cgi?idc844

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2014-11-17
                 CC|                            |jakub at gcc dot gnu.org
           Assignee|unassigned at gcc dot gnu.org      |jakub at gcc dot gnu.org
   Target Milestone|---                         |4.8.4
            Summary|open mp parallelization     |[4.8/4.9/5 Regression] open
                   |prevents vectorization      |mp parallelization prevents
                   |                            |vectorization
     Ever confirmed|0                           |1

--- Comment #3 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
I certainly see the same thing in 4.8, it seems this regressed in r161655.
Anyway, I will have a look tomorrow.

[Bug c++/55039] std::addressof vs. constexpr

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55039

--- Comment #7 from Marc Glisse <glisse at gcc dot gnu.org> ---
std::addressof can be constexpr with -std=gnu++XX, and even in strict mode
std::__addressof can be constexpr for internal use in libstdc++.