Warnings about freeing non-heap-allocated object

[Lastique]

On Kubuntu 21.10, gcc 11.2 produces warnings like these when building Boost.Wave 1.79.0 beta1 RC1 (presumably, revision f861b1a305a5dd70286ff84255ad78b1fd46975c):

In file included from /usr/include/x86_64-linux-gnu/c++/11/bits/c++allocator.h:33,
                 from /usr/include/c++/11/bits/allocator.h:46,
                 from /usr/include/c++/11/string:41,
                 from libs/wave/src/token_ids.cpp:18:
In member function ‘__gnu_cxx::new_allocator<char>::deallocate(char*, unsigned long)’,
    inlined from ‘boost::allocator_deallocate<std::allocator<char> >(std::allocator<char>&, boost::allocator_pointer<std::allocator<char> >::type, boost::allocator_size_type<std::allocator<char> >::type)void’ at ./boost/core/allocator_access.hpp:375:17,
    inlined from ‘boost::wave::util::AllocatorStringStorage<char, std::allocator<char> >::Free(void*, unsigned long)’ at ./boost/wave/util/flex_string.hpp:542:36,
    inlined from ‘boost::wave::util::AllocatorStringStorage<char, std::allocator<char> >::~AllocatorStringStorage()’ at ./boost/wave/util/flex_string.hpp:624:17,
    inlined from ‘boost::wave::util::AllocatorStringStorage<char, std::allocator<char> >::reserve(unsigned long)’ at ./boost/wave/util/flex_string.hpp:678:5,
    inlined from ‘boost::wave::util::CowString<boost::wave::util::AllocatorStringStorage<char, std::allocator<char> >, char*>::CowString(char const*, unsigned long, std::allocator<char> const&)’ at ./boost/wave/util/flex_string.hpp:1259:23,
    inlined from ‘boost::wave::util::flex_string<char, std::char_traits<char>, std::allocator<char>, boost::wave::util::CowString<boost::wave::util::AllocatorStringStorage<char, std::allocator<char> >, char*> >::flex_string(char const*, std::allocator<char> const&)’ at ./boost/wave/util/flex_string.hpp:1481:43,
    inlined from ‘boost::wave::get_token_name(boost::wave::token_id)’ at libs/wave/src/token_ids.cpp:243:64:
/usr/include/c++/11/ext/new_allocator.h:145:26: warning: ‘operator delete(void*, unsigned long)’ called on unallocated object ‘boost::wave::util::SimpleStringStorage<char, std::allocator<char> >::emptyString_’ [-Wfree-nonheap-object]
  145 |         ::operator delete(__p
      |         ~~~~~~~~~~~~~~~~~^~~~
  146 | #if __cpp_sized_deallocation
      | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  147 |                           , __t * sizeof(_Tp)
      |                           ~~~~~~~~~~~~~~~~~~~
  148 | #endif
      | ~~~~~~                    
  149 |                          );
      |                          ~
In file included from ./boost/wave/wave_config.hpp:278,
                 from libs/wave/src/token_ids.cpp:22:
/usr/include/c++/11/ext/new_allocator.h: In function ‘boost::wave::get_token_name(boost::wave::token_id)’:
./boost/wave/util/flex_string.hpp:511:1: note: declared here
  511 | SimpleStringStorage<E, A>::emptyString_ =
      | ^~~~~~~~~~~~~~~~~~~~~~~~~

The command line is:

b2 -j 16 --toolset=gcc cxxflags="-flto=auto -ffat-lto-objects -march=nehalem -mtune=skylake -fcf-protection=none -faligned-new -fno-plt -fvisibility-inlines-hidden -O3 -fno-math-errno -fno-rounding-math -fno-signaling-nans -fno-trapping-math -ftree-vectorize -fgcse-sm -fgcse-las -fgcse-after-reload -ftree-loop-im -ftree-loop-distribution -ftree-loop-if-convert -funswitch-loops -frename-registers -fivopts" linkflags="-Wl,-z,relro -Wl,-z,now -Wl,-O1 -Wl,--hash-style=both -Wl,-z,noexecstack -Wl,--as-needed" \
 variant=release cxxstd=17-gnu threading=multi debug-symbols=on runtime-link=shared optimization=speed inlining=full warnings=on link=shared stage

[jefftrull]

Very interesting. I don't think that code has changed. Maybe it's an improved analysis in gcc? I have Kubuntu 20.04 and don't see this.

[Lastique]

I think, the compiler does not understand the invariant that pData_ does not point to SimpleStringStorage<E, A>::emptyString_ when capacity() is zero, hence the warning. If that's the case then I think the warning needs to be silenced with a pragma.

PS: I can see that there are places where you compare pData_ with &SimpleStringStorage<E, A>::emptyString_. This could silence the warning too, but I don't like this, as it will likely break with shared libraries (i.e. there may be multiple instances of SimpleStringStorage<E, A>::emptyString_, and pData_ may point to one of them and not compare equal to a pointer to another). I'm not sure how relevant shared libraries are to Boost.Wave or AllocatorStringStorage in particular. If they are, it is probably a good idea to change the other places to check capacity() instead of pointer comparisons.

[jefftrull]

Can you help me reproduce this? I'm not very familiar with bjam. I always build Wave using b2 from the top level, like this:

./b2 --with-wave

or from within libs/wave like this:

../../b2 build samples test

Neither of those commands seem to generate this warning - and I'm actually on Kubuntu 21.10 myself - though maybe I missed it.

I don't seem to have a bjam in my path, although I know it's kind of the same thing as b2 (?). Thanks for your help.

[Lastique]

I'm on Kubuntu 22.04 now, and the command in the original post still produces the warnings with the current Boost develop. Replace bjam with b2. I'm running the command from Boost root. You can add --with-wave to build only Boost.Wave. Here is the build log: boost_wave_log.txt

$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/11/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 11.2.0-19ubuntu1' --with-bugurl=file:///usr/share/doc/gcc-11/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-11 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-11-gBFGDP/gcc-11-11.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-11-gBFGDP/gcc-11-11.2.0/debian/tmp-gcn/usr --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-serialization=2
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.2.0 (Ubuntu 11.2.0-19ubuntu1) 

[Lastique]

A simpler command that reproduces the warnings (also from Boost root):

b2 -j16 warnings=on --with-wave

(adjust -j to your machine capacity, of course.)

[jefftrull]

Thanks! I can reproduce this now. I'm not sure what exactly to do; this code is actually the work of @andralex and is described in detail here. I've looked at it a bit and other than a pragma, I'm not sure how to safely make a change. The most recent version of the code from the master is here AFAICT and the emptyString_ sentinel is still used. Any thoughts?