[cfe-dev] [llvm-dev] New warnings when building trunk with GCC 9

[Richard Smith via cfe-dev]

Yes, we should produce this warning in C++11 mode too. (I could be
misrecalling, but I think the rationale for the current behaviour is based
on historical GCC behaviour.)

On Sun, 16 Sep 2018, 10:04 David Blaikie via cfe-dev, <
cfe-dev at lists.llvm.org> wrote:

> Fair point made on that thread - that this is a DR, so technically the
> std::move is pessimizing even in C++11 mode. Richard: Any thoughts on
> generalizing the warning to cover these cases even in C++11 mode?
>
> On Sat, Sep 15, 2018 at 2:37 AM Dávid Bolvanský <david.bolvansky at gmail.com>
> wrote:
>
>> There is a new discussion related to -Wredundant-move warning on GCC
>> bugzilla.
>>
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87300
>>
>> pi 14. 9. 2018 o 9:53 David Blaikie <dblaikie at gmail.com> napísal(a):
>>
>>>
>>>
>>> On Fri, Sep 14, 2018 at 12:48 AM Stephan Bergmann <sbergman at redhat.com>
>>> wrote:
>>>
>>>> On 13/09/2018 18:22, David Blaikie via llvm-dev wrote:
>>>> > On Thu, Sep 13, 2018 at 12:13 AM Dávid Bolvanský via llvm-dev
>>>> > <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>>> >
>>>>  /home/davidbolvansky/trunk/llvm/unittests/ExecutionEngine/Orc/CompileOnDemandLayerTest.cpp:79:40:
>>>>
>>>> >       required from here
>>>> >
>>>>  /home/davidbolvansky/trunk/llvm/include/llvm/ExecutionEngine/Orc/CompileOnDemandLayer.h:314:29:
>>>> >     warning: redundant move in return statement [-Wredundant-move]
>>>> >     314 |         return std::move(Err);
>>>>
>>>> Note that the move (into the implicit JITSymbol(Error) ctor) is only
>>>> redundant if the compiler implements a fix for
>>>> <http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1579>
>>>> "Return by converting move constructor".  (But not sure whether the
>>>> LLVM
>>>> compiler baselines imply that, anyway.
>>>
>>>
>>> Looks like that was accepted for C++14, not 11. So I don't believe it's
>>> valid to remove the std::move in C++11 code. (& I believe Clang's warning
>>> correctly diagnoses this only in the right language versions) - so we
>>> probably want to disable the buggy GCC warning here, then.
>>>
>>>
>>>> In LibreOffice it forced me to
>>>> introduce ugly #ifs, to not have to disable that warning outright,
>>>> <
>>>> https://cgit.freedesktop.org/libreoffice/core/commit/?id=dc06c8f4989fc28d0c31ebd333e53dfe0e0f5f66>
>>>>
>>>> "-Werror=redundant-move (GCC 9), take two".)
>>>>
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