RFR: 8258010: Debug build failure with clang-10 due to -Wdeprecated-copy [v4]

[Vladimir Kozlov]

On Wed, 6 Jan 2021 06:14:11 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:
> 
>   Split the PR, addressing -Wdeprecated-copy only
>   
>   As suggested by kimbarrett, we should focus on warnings produced by
>   '-Wdeprecated-copy' in this PR. Because JDK-8259288 is a very different
>   problem and might be reviewed by folks from different teams.
>   
>   Will create a new PR to address JDK-8259288.
>   
>   Change-Id: I1b9f434ab6fcdf2763a46870eaed91641984fd76
>   CustomizedGitHooks: yes

Still good.

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Marked as reviewed by kvn (Reviewer).

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