Re: How to diagnose a false positive warning

Am 13.05.26 um 05:43 schrieb Michael Welsh Duggan via Gcc-help:
> g++ 16 ends up triggering a new false positive warning in some of my
> code that didn't trigger in g++ 15.  An example of this warning:
> 
> In file included from /home/md5i/src/ipfix/master/tests/build.hpp:6,
>                   from /home/md5i/src/ipfix/master/tests/read.cpp:7:
> In function ‘void ipfix::convert::to_chunk_unsafe(auto:65&&, T) [with T = unsigned int; auto:65 = std::span<unsigned char>]’,
>      inlined from ‘build::message& build::message::set_domain(build::u32)’ at /home/md5i/src/ipfix/master/tests/build.hpp:202:29,
>      inlined from ‘virtual void ReadTest_EmptyMessages_Test::TestBody()’ at /home/md5i/src/ipfix/master/tests/read.cpp:262:19:
> /home/md5i/src/ipfix/master/src/../include/detail/conversions.hpp:244:14: error: writing 4 bytes into a region of size 0 [-Werror=stringop-overflow=]
>    244 |   std::memcpy(data(chunk), start, size(chunk));
>        |   ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> In member function ‘virtual void ReadTest_EmptyMessages_Test::TestBody()’:
> cc1plus: note: destination object is likely at address zero
> 
> I'm fairly certain that this is a false positive, but the code is too
> complex to reproduce here.  I started to work on a minimal reproducer,
> but, again, the code is quite layered and complex.  I know I can ignore
> the warning using diagnostic pragmas, but I prefer not to do that unless
> completely necessary.  Can anyone recommend a way to debug why this
> warning is triggered?

As far is I know, -Wstringop-overflow is basically broken,
see for example https://gcc.gnu.org/PR88443

You can try to disable the warning with option pragmas, like

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wstringop-overflow"
#pragma GCC diagnostic pop

etc.  There are dozens / hundreds of test cases that expose false
positives, dunno if adding yet another one would help much.

Johann