On Wed, 23 Dec 2020 01:45:58 GMT, Hao Sun <github.com+16932759+shqk...@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/AArch64 machines. I think the two issues described here are distinct and should be dealt with in separate bugs and PRs. Their only relation is that both arise with using clang-10. But they are very different problems, in very different parts of the code, and probably ought to be reviewed by folks from different teams. ------------- PR: https://git.openjdk.java.net/jdk/pull/1874
