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cannot compile c++ generated code #8484
Description
Version: 3.15.8
Language: C++
Windows 10, MSVC 16.9.3 with clang-cl compiler (LLVM 11.1)
Message definition:
syntax = "proto3";
message StringMsg
{
string value = 1;
}
Compilation of generated pb.cc fails for this message with "error : variable does not have a constant initializer".
Compiles with repeated modifier.
Variable is:
PROTOBUF_ATTRIBUTE_NO_DESTROY PROTOBUF_CONSTINIT StringMsgDefaultTypeInternal StringMsg_default_instance;
Activity
TeBoring commentedon Apr 22, 2021
Do you see if there is a constexpr constructor for StringMsgDefaultTypeInternal in .pb.cc?
TeBoring commentedon Apr 22, 2021
Could you also paste the full error log?
Eddie-cz commentedon Apr 22, 2021
There is constexpr constructor, code compiles fine with msvc compiler, only clang-cl (both 11.1 or 12.0) thinks this message variable doesn't satisfy constinit restrictions introduced by PROTOBUF_CONSTINIT (guess it has problems with string type).
Should I put repeated modifier for that string param, or manualy remove PROTOBUF_CONSTINIT from pb.cc, it compiles fine.
Log:
etherCATMessages.pb.cc(114,79): error : variable does not have a constant initializer
etherCATMessages.pb.cc(114,31): message : required by 'require_constant_initialization' attribute here
protobuf\3.15.8\include\google/protobuf/port_def.inc(571,30): message : expanded from macro 'PROTOBUF_CONSTINIT'
TeBoring commentedon Apr 23, 2021
https://godbolt.org/z/75rfhqY4E
I cannot reproduce it in the godbolt. Could you help advice a reproduce?
TeBoring commentedon Apr 23, 2021
https://godbolt.org/z/j9nv47n5P
Still no error
Eddie-cz commentedon Apr 26, 2021
Issue is isolated to
PROTOBUF_CONSTINIT google::protobuf::internal::ArenaStringPtr sptr(&google::protobuf::internal::fixed_address_empty_string);
Same error as described above. To reproduce, you must define your ExplicitlyConstructed<std::string> empty_string variable; in different compilation unit, in this case in different dll, same as it's for protobuf variables. When I test your code in one compilation unit, it's ok, problem is with protobuf dll usage.
ngg commentedon May 26, 2021
We ran into the same problem.
If I try to compile libprotobuf into a dll, and compile the protoc executable that uses this dll with clang-cl, then I get an error like this from
plugin.pb.cc:I tried using protobuf 3.15.2 and 3.17.1
sbenzaquen commentedon May 26, 2021
It seems to be an interaction between
__declspec(dllimport)and constant initialization.I can't test with clang-cl, so my tests below are with MSVC.
If I write:
then
pis dynamically initialized (at least in the latest MSVC)If the symbol is not
dllimport, thenpis constant initialized as expected.I think what's happening is that clang is following this behavior but since it also supports the
[[clang::require_constant_initialization]]attribute it is failing there because it is indeed not constant initialized.We might want to disable the attribute in clang-for-windows.
The attribute is not required for correctness. It is there mostly to make sure we really are constant initialized in the platforms that support it for code size efficiency.
ngg commentedon May 26, 2021
I also checked that if I compile with clang in C++20 mode, then
constinitis used, and the same error still appears.I did not check what happens with MSVC, but if C++20 is enabled (using
/std:c++latest) andconstinitis used then I think it wouldn't compile either. (clang seems to be right, this is not supposed to work as constantly initialized on Windows)sbenzaquen commentedon May 26, 2021
I don't know about that.
This works fine:
So it can do constant initialization for it, but it just chooses not to when
constexpris not there.tbh, I don't know enough about the semantics of
dllexport/dllimport.kelteseth commentedon Jun 29, 2021
I can confirm this issue with clang 12 on Windows.
This fixes the problem for me
mmurrian commentedon Sep 10, 2021
That was my solution. I changed line 571 of port_def.inc to:
#if !defined(_MSC_VER) && __has_cpp_attribute(clang::require_constant_initialization)Same problem on Visual Studio 2019, clang-cl, Windows.
Eddie-cz commentedon Sep 16, 2021
Will make pull request as suggested by mmurrian, kinda suprised multiple versions were released in last 6 month and nobody was concerned with fact that library is producing not compilable code under msvc clang-cl environment.
acozzette commentedon Sep 22, 2021
This should be fixed now with #8993.
martenrichter commentedon Jul 9, 2022
I am currently trying to build libquiche for windows with a cmake build and clang as compiler.
The patch seems to be removed from the repository.
And I am getting:
....webtransport\third_party\quiche\quiche\quic\core\proto\cached_network_parameters.pb.cc(40,127): error : variable does not have a constant initializer [...webtransport\build\gquiche.vcxproj] ....third_party\quiche\quiche\quic\core\proto\cached_network_parameters.pb.cc(40,31): message : required by 'require_constant_initialization' attribute here [...webtransport\build\gquiche.vcxproj] C:\vcpkg\installed\x64-windows\include\google/protobuf/port_def.inc(651,30): message : expanded from macro 'PROTOBUF_CONSTINIT' [...webtransport\build\gquiche.vcxproj]If I patch port_def.inc analog to the previous patch:
#elif !defined(_MSC_VER) && __has_cpp_attribute(clang::require_constant_initialization) && \ ((defined(__APPLE__) && __clang_major__ >= 13) || \ (!defined(__APPLE__) && __clang_major__ >= 12))it is fixed again.
Can you consider adding this check again?
Eddie-cz commentedon Jul 9, 2022
Yey, this is getting ridiculous more and more. I made another pull request, it is not approved yet. #10232
martenrichter commentedon Jul 9, 2022
@Eddie-cz: Thanks for making the new PR, it much better then mine, and yes using protocol buffers on windows is pretty complicated.