id,summary,reporter,owner,description,type,status,milestone,component,version,severity,resolution,keywords,cc
12183,GCC 6.1 thinks boost::container::string violates strict aliasing,tavianator@…,Ion Gaztañaga,"The following simple test case aborts when built with -O2, but works fine with -fno-strict-aliasing.

{{{
$ cat foo.cpp
#include <boost/container/string.hpp>
#include <cstdlib>
#include <utility>

using boost::container::string;

struct foo
{
  __attribute__((noinline))
  foo(string str)
    : m_str{std::move(str)},
      m_len{m_str.length()}
  { }

  string m_str;
  std::size_t m_len;
};

int main() {
  foo f{""the quick brown fox jumps over the lazy dog""};
  if (f.m_len == 0) {
    std::abort();
  }
  return 0;
}
$ g++ -O2 -Wall foo.cpp -o foo && ./foo
[1]    6375 abort (core dumped)  ./foo
}}}

I reported this as [https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71002 GCC bug 71002] but I'm not sure that the code is actually correct.  It seems to at least access a non-active union member (the is_short bitfield).  There may be other aliasing issues with the union -- in some cases it seems GCC wants you to access the members through the union directly, not through a pointer to a union member; see [https://gcc.gnu.org/bugzilla/show_bug.cgi?id=14319 GCC bug 14319] for example.",Bugs,closed,To Be Determined,container,Boost 1.61.0,Problem,fixed,,