Warnings on GCC 10 + LTO

[phprus]

Commit: 26cedfa

GCC: gcc version 10.2.1 20200825 [revision c0746a1beb1ba073c7981eb09f55b3d993b32e5c] (SUSE Linux)

CMake command:

CC=gcc-10 CXX=g++-10 cmake -DCMAKE_VERBOSE_MAKEFILE=ON -DCMAKE_BUILD_TYPE=Release -DCMAKE_CXX_STANDARD=17 -DBOOST_JSON_STANDALONE=ON -DBUILD_TESTING=ON -DBOOST_JSON_BUILD_TESTS=ON -DBOOST_JSON_BUILD_FUZZERS=OFF -DBOOST_JSON_BUILD_EXAMPLES=OFF -DCMAKE_CXX_FLAGS="-flto -fno-fat-lto-objects" ../..

Warnings:

[ 12%] Linking CXX executable boost_json-limits
cd /home/.../boostorg/json/json-26cedfabfc1ff13cd6fbc3d8ae03c33724d2343d/build/gcc10/test && /usr/bin/cmake -E cmake_link_script CMakeFiles/boost_json-limits.dir/link.txt --verbose=1
/usr/bin/g++-10  -flto -fno-fat-lto-objects -O3 -DNDEBUG   CMakeFiles/boost_json-limits.dir/limits.cpp.o CMakeFiles/boost_json-limits.dir/main.cpp.o CMakeFiles/boost_json-limits.dir/__/src/src.cpp.o  -o boost_json-limits 
In function ‘relocate’,
    inlined from ‘__ct ’ at /home/.../boostorg/json/json-26cedfabfc1ff13cd6fbc3d8ae03c33724d2343d/include/boost/json/impl/array.ipp:92:17,
    inlined from ‘insert’ at /home/.../boostorg/json/json-26cedfabfc1ff13cd6fbc3d8ae03c33724d2343d/include/boost/json/impl/array.ipp:458:26,
    inlined from ‘testArray.constprop’ at /home/.../boostorg/json/json-26cedfabfc1ff13cd6fbc3d8ae03c33724d2343d/test/limits.cpp:169:13:
/home/.../boostorg/json/json-26cedfabfc1ff13cd6fbc3d8ae03c33724d2343d/include/boost/json/impl/array.hpp:122:17: warning: ‘__builtin_memmove’ writing between 24 and 103079215080 bytes into a region of size 0 overflows the destination [-Wstringop-overflow=]
  122 |     std::memmove(
      |                 ^
In member function ‘term’,
    inlined from ‘resize.constprop’ at /home/.../boostorg/json/json-26cedfabfc1ff13cd6fbc3d8ae03c33724d2343d/include/boost/json/impl/string.ipp:357:19:
/home/.../boostorg/json/json-26cedfabfc1ff13cd6fbc3d8ae03c33724d2343d/include/boost/json/detail/string_impl.hpp:313:23: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
  313 |             s_.buf[n] = 0;
      |                       ^
/home/.../boostorg/json/json-26cedfabfc1ff13cd6fbc3d8ae03c33724d2343d/include/boost/json/impl/string.ipp: In member function ‘resize.constprop’:
/home/.../boostorg/json/json-26cedfabfc1ff13cd6fbc3d8ae03c33724d2343d/include/boost/json/detail/string_impl.hpp:62:14: note: at offset [100, 1001] to object ‘buf’ with size 15 declared here
   62 |         char buf[sbo_chars_ + 1];
      |              ^
In member function ‘term’,
    inlined from ‘append’ at /home/.../boostorg/json/json-26cedfabfc1ff13cd6fbc3d8ae03c33724d2343d/include/boost/json/detail/impl/string_impl.ipp:172:13,
    inlined from ‘append’ at /home/.../boostorg/json/json-26cedfabfc1ff13cd6fbc3d8ae03c33724d2343d/include/boost/json/impl/string.ipp:239:35,
    inlined from ‘testString.constprop’ at /home/.../boostorg/json/json-26cedfabfc1ff13cd6fbc3d8ae03c33724d2343d/test/limits.cpp:192:17:
/home/.../boostorg/json/json-26cedfabfc1ff13cd6fbc3d8ae03c33724d2343d/include/boost/json/detail/string_impl.hpp:313:23: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
  313 |             s_.buf[n] = 0;
      |                       ^
/home/.../boostorg/json/json-26cedfabfc1ff13cd6fbc3d8ae03c33724d2343d/test/limits.cpp: In function ‘testString.constprop’:
/home/.../boostorg/json/json-26cedfabfc1ff13cd6fbc3d8ae03c33724d2343d/include/boost/json/detail/string_impl.hpp:62:14: note: at offset [886, 4294968294] to object ‘buf’ with size 15 declared here
   62 |         char buf[sbo_chars_ + 1];
      |              ^

[grisumbras]

I've investigated the errors and as far as I can see, the static analyzer GCC uses when lto is enabled is kind of faulty. If the library is built in debug mode gcc do see stuff better and the warning doesn't appear. I recommend you to disable the warning (at least, when building Json).

[vinniefalco]

Is this an shipping release of GCC or is it a beta?

[phprus]

@vinniefalco
This is GCC from openSUSE 15.2.

@grisumbras
Maybe disable this warning for the whole json library?

Like https://github.com/chriskohlhoff/asio/blob/master/asio/include/asio/detail/push_options.hpp and https://github.com/chriskohlhoff/asio/blob/master/asio/include/asio/detail/pop_options.hpp in ASIO?

[vinniefalco]

Do you think that's best?

[grisumbras]

I'm not sure it's actually doable. The warning is emitted by the linker and it seems LTO "objects" don't store pragma information, so disabling this warning with a pragma doesn't do anything. The only way I've managed to disable the warning is to pass "-Wno-stringop-overflow" to the linker. I guess, we could add linking flags to tests via build scripts.

[phprus]

@grisumbras, you're right, pragma doesn't work. Linker flag required.

[grisumbras]

I'm inclined to do nothing here.