RFR: 8258010: Debug build failure with clang-10 due to -Wdeprecated-copy [v5]

[Kim Barrett]

On Mon, 11 Jan 2021 02:14:17 GMT, Hao Sun <github.com+16932759+shqking at openjdk.org> wrote:

>> 1. '-Wdeprecated-copy'
>> As specified in C++11 [1], "the generation of the implicitly-defined
>> copy constructor is deprecated if T has a user-defined destructor or
>> user-defined copy assignment operator". The rationale behind is the
>> well-known Rule of Three [2].
>> 
>> Introduced since gcc-9 [3] and clang-10 [4], flag '-Wdeprecated-copy'
>> warns about the C++11 deprecation of implicitly declared copy
>> constructor and assignment operator if one of them is user-provided.
>> Defining an explicit copy constructor would suppress this warning.
>> 
>> The main reason why debug build with gcc-9 or higher succeeds lies in
>> the inconsistent warning behaviors between gcc and clang. See the
>> reduced code example [5]. We suspect it might be return value
>> optimization/copy elision [6] that drives gcc not to declare implicit
>> copy constructor for this case.
>> 
>> Note that flag '-Wdeprecated' in clang-8 and clang-9 would also raise
>> warnings for deprecated defintions of copy constructors. However,
>> '-Wdeprecated' is not enabled by '-Wall' or '-Wextra'. Hence, clang-8
>> and clang-9 are not affected.
>> 
>> ~~2. '-Wimplicit-int-float-conversion'~~
>> ~~Making the conversion explicit would fix it.~~
>> 
>> ~~Flag '-Wimplicit-int-float-conversion' is first introduced in clang-10.~~
>> ~~Therefore clang-8 and clang-9 are not affected. The flag with similar~~
>> ~~functionality in gcc is '-Wfloat-conversion', but it is not enabled by~~
>> ~~'-Wall' or '-Wextra'. That's why this warning does not apprear when~~
>> ~~building with gcc.~~
>> 
>> [1] https://en.cppreference.com/w/cpp/language/copy_constructor
>> [2] https://en.cppreference.com/w/cpp/language/rule_of_three
>> [3] https://www.gnu.org/software/gcc/gcc-9/changes.html
>> [4] https://releases.llvm.org/10.0.0/tools/clang/docs/ReleaseNotes.html
>> [5] https://godbolt.org/z/err4jM
>> [6] https://en.wikipedia.org/wiki/Copy_elision#Return_value_optimization
>> 
>> 
>> Note that we have tested with this patch, debug build succeeded with clang-10 on Linux X86-64/AArch64 machines.
>> Note that '--with-extra-cxxflags=-Wno-implicit-int-float-conversion' should be added when configuration. It's another issue (See JDK-8259288)
>
> Hao Sun has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Define the copy assign operator of class DUIterator_Last as defaulted
>   
>   The copy assignment operator of class DUIterator_Last should also be
>   defined as defaulted, i.e. =default, keeping consistent with the copy
>   constructor.
>   
>   Besides, fix the NIT for the copy ctor definition of class
>   DUIterator_Last.
>   
>   Change-Id: I2f9502f023443163910eea9469b72df5bf1e25e0
>   CustomizedGitHooks: yes

Marked as reviewed by kbarrett (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/1874