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

[Richard Smith via llvm-dev]

On Mon, 24 Sep 2018, 20:40 Richard Trieu via cfe-dev, <
cfe-dev at lists.llvm.org> wrote:

> + Erik, who implemented DR1579
>
> Originally, I had the warning similar to GCC's warning, but took it out
> due to not having DR1579 implemented in clang (warning changed in r243594)
>
> Erik in r274291 implemented DR1579, although PR27785 didn't mention
> anything about std::move
>
> It looks like what's happening is that Clang and GCC handles the return
> differently.  Clang needs the std::move call to use the move constructor
> while GCC will use the move constructor with or without the std::move
> call.  This means that the warning is currently correct when running on
> either compiler.
>
> This is a reduced example.  Compiled with Clang, it will print "move
> constructor" then "copy constructor".  GCC will print "move constructor"
> twice.
>

GCC gets the rule "wrong". The rule in question (
http://eel.is/c++draft/class.copy.elision#3.sentence-2) only applies when
the selected B constructor takes A&& as its parameter type.

#include <iostream>
> #include <memory>
>
> struct A {
>   A(int) {}
>   A() {  std::cout << "empty constructor"; }
>   A(const A&) {  std::cout << "copy constructor\n"; }
>   A(A&&) {  std::cout << "move constructor\n"; }
> };
> struct B {
>   B(A a) {}
> };
>
> A getA() { return A(1); }
>
> B run1() {
>   A a = getA();
>   return std::move(a);
> }
>
> B run2() {
>   A a = getA();
>   return a;
> }
>
> int main() {
>   std::cout << "with move:\n";
>   run1();
>   std::cout << "no move:\n";
>   run2();
> }
>
>
>
> On Tue, Sep 18, 2018 at 4:07 PM Richard Trieu <rtrieu at google.com> wrote:
>
>> I'll look into the pessimizing move warning, then after that, the
>> deprecated copy warning.
>>
>> On Sun, Sep 16, 2018 at 10:41 AM Richard Smith <richard at metafoo.co.uk>
>> wrote:
>>
>>> 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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>>>>
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