On Thu, 24 Dec 2020 17:27:39 GMT, Kim Barrett <kbarr...@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. > > src/hotspot/share/opto/node.hpp line 1458: > >> 1456: // initialize to garbage >> 1457: >> 1458: DUIterator_Last (const DUIterator_Last& that) : >> DUIterator_Fast(that) {} > > Since DUIterator_Last is just delegating both the copy constructor and > assignment operator to the base class, their copy constructor and > assignment operator would be better as the default (either implicit or > explicit using `=default`) rather than explicit code. Agree. c2 actually never uses the assignment operator of DUIterator_Last. It needs the copy constructor in this line. https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/node.hpp#L1499 you can delete the following code or leave it =default. - void operator=(const DUIterator_Last& that) - { DUIterator_Fast::operator=(that); } ------------- PR: https://git.openjdk.java.net/jdk/pull/1874
