I think I see what's going on. Your oneof field gets stored as a union that
looks like this in the generated code:
union MsgDataUnion {
MsgDataUnion() {}
::google::protobuf::internal::ArenaStringPtr first_;
::google::protobuf::uint32 second_;
} msg_data_;
In DemoMessage::mutable_first() we call
msg_data_.first_.UnsafeSetDefault(), but this is technically incorrect
because we are calling a method on one of the types in the union without
first initializing it. For example it could be that the union is actually
in the uint32 form and yet we are attempting to call an ArenaStringPtr
method on the data. I think this works out OK in practice but is probably
technically undefined behavior and so we should fix it.
On Mon, Nov 14, 2016 at 10:05 AM, <changchunzhao.t...@gmail.com> wrote:
> Adam, yes, forgot providing my setup, here it is:
>
> $ g++ --version
> g++ (GCC) 4.4.7 20120313 (Red Hat 4.4.7-3)
> Copyright (C) 2010 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions. There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE
> .
>
> $ uname -a
> Linux demo-host 2.6.32-358.el6.x86_64 #1 SMP Tue Jan 29 11:47:41 EST 2013
> x86_64 x86_64 x86_64 GNU/Linux
>
> *And generated code around the problematic line*:
> 262 inline ::std::string* DemoMessage::mutable_first() {
> 263 if (!has_first()) {
> 264 clear_msg_data();
> 265 set_has_first();
> 266 msg_data_.first_.UnsafeSetDefault(&::google::protobuf::internal::
> GetEmptyStringAlreadyInited());
> 267 }
>
> I have also attached the generated files from my system for your reference.
>
> Thanks again!
>
> Regards,
> Changchun
>
> On Monday, November 14, 2016 at 12:50:56 PM UTC-5, Adam Cozzette wrote:
>>
>> Thanks, Changchun. I have just a few more questions to try to debug the
>> problem:
>> - What version of g++ are you using? (g++ --version should tell you)
>> - What architecture are you building for?
>> - Could you send the generated code around line 266 in demo.pb.h just so
>> that it's clear what lines the compiler is warning about?
>>
>> On Mon, Nov 14, 2016 at 7:55 AM, <changchun...@gmail.com> wrote:
>>
>>> Adam, Thanks for your reply.
>>>
>>> Here I attached a simple .proto file to demonstrate this warning message:
>>>
>>> $ protobuf-3.1/bin/protoc --cpp_out=. demo.proto
>>>
>>> $ /usr/bin/g++ -Iprotobuf-3.1/include -c -Wall -O2 -Wno-long-long demo.
>>> pb.cc -o demo.pb.o
>>> In member function 'virtual bool demo::DemoMessage::MergePartia
>>> lFromCodedStream(google::protobuf::io::CodedInputStream*)':
>>> cc1plus: warning: dereferencing pointer 'prephitmp.1630' does break
>>> strict-aliasing rules
>>> demo.pb.h:266: note: initialized from here
>>> cc1plus: note: initialized from here
>>>
>>> Please let me know if anything else you need to debug further.
>>>
>>> Thanks,
>>> Changchun
>>>
>>> On Friday, November 11, 2016 at 4:45:03 PM UTC-5, Adam Cozzette wrote:
>>>>
>>>> What version of g++ are you using? Also, do you have a way of
>>>> determining what actual line of code the compiler is warning about there?
>>>>
>>>> I tried building with -Wstrict-aliasing=2 at commit df8390790a and saw
>>>> a few warnings like this:
>>>>
>>>> ./google/protobuf/generated_message_util.h:94:66: warning:
>>>> dereferencing type-punned pointer might break strict-aliasing rules
>>>> [-Wstrict-aliasing]
>>>> const T& get() const { return reinterpret_cast<const T&>(union_); }
>>>>
>>>> But that doesn't look like it's quite the same warning as what you are
>>>> seeing.
>>>>
>>>> On Fri, Nov 11, 2016 at 12:39 PM, <changchun...@gmail.com> wrote:
>>>>
>>>>> I have below option in my .proto file:
>>>>>
>>>>> *option cc_enable_arenas = true;*
>>>>>
>>>>> But g++ for Linux gives below warning:
>>>>>
>>>>> *cc1plus: warning: dereferencing pointer 'prephitmp.4925' does break
>>>>> strict-aliasing rules*
>>>>>
>>>>> Will this cause issue and how to solve it?
>>>>>
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