Re: ../../gdbsupport/new-op.cc:137:1: error: ‘void operator delete [](void*, std::size_t)’ is a usual (non-placement) deallocation function in C++14 (or with -fsized-deallocation) [-Werror=c++14-compat]

[Andrew Burgess <aburgess@redhat.com>]

Simon Marchi via Gdb-patches <gdb-patches@sourceware.org> writes:

> On 2022-03-02 12:22, Andrew Burgess wrote:
>> Simon Marchi via Gdb-patches <gdb-patches@sourceware.org> writes:
>> 
>>> On 2022-03-02 11:30, Andrew Burgess wrote:
>>>> * 周春明(日月) via Gdb-patches <gdb-patches@sourceware.org> [2022-03-01 15:48:47 +0800]:
>>>>
>>>>>
>>>>> Hi GDB maintainers,
>>>>>
>>>>> I tried to build new GDB12, but encounter below error, anyone could tell me how to fix it? Thanks!
>>>>>
>>>>>  CXX new-op.o
>>>>> ../../gdbsupport/new-op.cc:32:13: error: ‘void operator delete(void*, std::size_t)’ is a usual (non-placement) deallocation function in C++14 (or with -fsized-deallocation) [-Werror=c++14-compat]
>>>>>  extern void operator delete (void *p, std::size_t) noexcept;
>>>>>  ^
>>>>> ../../gdbsupport/new-op.cc:33:13: error: ‘void operator delete [](void*, std::size_t)’ is a usual (non-placement) deallocation function in C++14 (or with -fsized-deallocation) [-Werror=c++14-compat]
>>>>>  extern void operator delete[] (void *p, std::size_t) noexcept;
>>>>>  ^
>>>>> ../../gdbsupport/new-op.cc:119:1: error: ‘void operator delete(void*, std::size_t)’ is a usual (non-placement) deallocation function in C++14 (or with -fsized-deallocation) [-Werror=c++14-compat]
>>>>>  operator delete (void *p, std::size_t) noexcept
>>>>>  ^
>>>>> ../../gdbsupport/new-op.cc:137:1: error: ‘void operator delete [](void*, std::size_t)’ is a usual (non-placement) deallocation function in C++14 (or with -fsized-deallocation) [-Werror=c++14-compat]
>>>>>  operator delete[] (void *p, std::size_t) noexcept
>>>>
>>>> I was able to reproduce this on Ubuntu 16.04.1 with their gcc 5.4.0.
>>>> I was unable to easily rebuild GCC 5.4.0 on my current development
>>>> machine to check if this is reproducible with upstream gcc, or is just
>>>> something impacting Ubuntu.  However, you can configure like:
>>>>
>>>>   ../src/configure ...configure-flags-here... CXXFLAGS="-Wno-error=c++14-compat"
>>>>
>>>> to disable this warning/error which I believe should be fine.
>>>>
>>>> For why this error is occurring, I'm honestly not 100% sure what the
>>>> error is telling us.  I _think_ what it's saying is that the delete
>>>> operator that we're declaring/defining conflicts with a "usual
>>>> deallocation function", which is added in c++14 and means something
>>>> specific.  I guess the idea is that maybe we're just randomly defining
>>>> this version of delete for some reason, and then, if/when we move on
>>>> to c++14 this function will get called unexpectedly by the language
>>>> runtime in some situations.
>>>>
>>>> As time moved on I think this warning was relaxed, possibly with this
>>>> commit:
>>>>
>>>>   https://gcc.gnu.org/legacy-ml/gcc-patches/2015-05/msg01883.html
>>>>
>>>> All this makes me wonder if the usual deallocation functions are ever
>>>> actually used, and indeed, I applied the patch below, and GDB still
>>>> seems to build fine, so this might be an alternative approach.  Maybe
>>>> we should commit this to master?
>>>>
>>>> Thanks,
>>>> Andrew
>>>>
>>>> ---
>>>>
>>>> diff --git a/gdbsupport/new-op.cc b/gdbsupport/new-op.cc
>>>> index 1d066ba..4faa557 100644
>>>> --- a/gdbsupport/new-op.cc
>>>> +++ b/gdbsupport/new-op.cc
>>>> @@ -27,11 +27,6 @@
>>>>  #include "host-defs.h"
>>>>  #include <new>
>>>>  
>>>> -/* These are declared in <new> starting C++14.  Add these here to enable
>>>> -   compilation using C++11. */
>>>> -extern void operator delete (void *p, std::size_t) noexcept;
>>>> -extern void operator delete[] (void *p, std::size_t) noexcept;
>>>
>>> The story of this is that ASan gave some alloc-dealloc mismatch warnings
>>> if we didn't define these specific delete operators, so we defined them
>>> in this commit:
>>>
>>>   https://gitlab.com/gnutools/binutils-gdb/-/commit/5fff6115feae7aaa23c0ae8d144e1c8418ee2ee1
>> 
>> I saw that commit, but it wasn't clear (to me) that _every_ supplied
>> delete was actually needed, or if the patch author just went wide.
>> 
>> The patch even says:
>> 
>>   There could be 16 operators delete but there are only 6, GDB uses 2 of
>>   them.  It depends on libraries and compiler which of the operators
>>   will get used.
>> 
>> But, that does seem to indicate that if I built with a different
>> compiler/c++-runtime, then I might end up using different delete
>> functions. 
>> 
>>>
>>> But for the variants of delete that are only introduced in C++14, it
>>> meant adding functions without an equivalent declaration when building
>>> in C++11, which produced some -Wmissing-declarations warnings.  So these
>>> declarations were added in this commit:
>>>
>>>   https://gitlab.com/gnutools/binutils-gdb/-/commit/b038b53f1ff4bf00ecdead1db23eddc4fd654305
>> 
>> OK, so, I think you're saying that if we compile with C++14 then we
>> might need the delete functions I proposed deleting.
>> 
>> Just for my sanity, GDB currently targets C++11, right?  But, I guess
>> you're suggesting you'd like to keep these functions in place so we
>> _can_ compile with C++14 if we want?
>
> That was my idea, yes.  We require GDB to be buildable in C++11, but we
> also regularly build it in C++14 and C++17 as well, since recent
> compilers default to that version (and we don't force
> -std=c++11/-std=gnu++11).
>
>>> The idea being that in C++11, these delete operators won't get used, but
>>> there will be a declaration to avoid the warning, and in C++14 the
>>> declarations will duplicate those found in headers, which is harmless.
>>>
>>> And now here we are, we have a C++11 compiler that complains about
>>> declaring these delete operators.
>>>
>>> So we can't simply remove the declarations and / or definitions, we
>>> would just re-introduce the problems fixed by these commits.
>> 
>> I think this last statement depends on the "wanting to compile with
>> C++14" idea above, right? Otherwise...
>
> Yes.
>
>>> I think that a clean way to fix this would be to conditionally define
>>> these delete operators based on the C++ version.  So we would remove the
>>> declarations, as you do in your commit, but place the definitions under
>>> an "#if __cplusplus >= xyz".
>> 
>> ... if we did this, and only ever compile with C++11, then this would be
>> equivalent to what I proposed, right?
>
> Yes (but we currently don't restrict to C++11).
>
>> I ask the above not (just) to be pedantic, but (mostly) to ensure I've
>> properly understood what's going on.
>> 
>> Below is a patch which maybe does what you suggest.  No commit message,
>> but if this is what you were thinking then I can write this up.
>
> I think it is what I suggested, by per Pedro's message, that is probably
> not good.  From what I understand, it is possible for a library GDB
> links with to be built with a more recent C++, and call some "recent"
> delete operator.  So even if we build GDB in C++11, we have to cover the
> more recent delete operators introduced in C++14.

ACK.  Thanks for explaining though.  At least I feel I understand what's
going on now.

Thanks,
Andrew