g++ - boost spirit V2 qi bug associated with optimization level

Question

I develop my code in my spare time. Preferably in debug mode. Recently, when I tried to build release version, then I got the error (runtime, output: 1 2 then failure). I located the piece of code (below), which contains the error, and I found, that the error only occurs, when optimization level is -Os, -Ofast, -O2, -O3 but not -O, -O0, -O1, -Og. In release mode I am constrained in debug abilities. What is the cause of the error? What is the method to find such errors?

#!/usr/bin/env bash -vex WARN="-W -Wall -Wextra" INCLUDE="-isystem /c/libs/boost-trunk" OPT="-O2" g++ -x c++ - -std=gnu++1y $INCLUDE

$WARN $OPT -o a <<__EOF && ./a && echo -e "e[1;32msucceedede[0m" || echo -e "e[1;31mfailede[0m"

#include <iterator>
#include <string>
#include <iostream>

#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/support_multi_pass.hpp>

using namespace boost::spirit;

template< typename Iterator >
struct skipper
        : qi::grammar< Iterator >
{

    skipper();

private :

    typename skipper::base_type::start_type skipper_;

};

template< typename Iterator >
skipper< Iterator >::skipper()
    : skipper::base_type(skipper_, "skipper") 
{
    std::cerr << 1 << std::endl;
    auto const ana =
            *~qi::char_('*') > +qi::char_('*')
            ;
    std::cerr << 2 << std::endl;
    skipper_ =
            qi::space
            | ("/*" > ana > *(~qi::char_("/*") > ana) > '/')
            | ("//" > *(qi::char_ - qi::eol) > (qi::eol | qi::eoi))
            ; // R"(s+|(/*[^*]**+([^/*][^*]**+)*/)|(//[^
]*))"
    std::cerr << 3 << std::endl;
}

using input_type = std::string;
using input_iterator_type = std::istreambuf_iterator< typename input_type::value_type >;
using base_iterator_type = multi_pass< input_iterator_type >;

template struct skipper< base_iterator_type >;

using skipper_type = skipper< base_iterator_type >;

int main()
{
    skipper_type const skipper_;
    std::cerr << 4 << std::endl;
    return EXIT_SUCCESS;
}
__EOF

gcc -v 2>&1 | tail -n1:

gcc version 4.8.1 (rev5, Built by MinGW-W64 project) 

Answer 1

It's a bug in your code, nothing wrong with the compiler or the optimization levels.

The cinch is with expression templates (like the ones used by Boost Proto, and hence by Boost Spirit). They are only valid to the end of their enclosing full expression ^[1]

The canonical workaound is:

 BOOST_SPIRIT_AUTO(ana, *~qi::char_('*') > +qi::char_('*'));

You can find it here: http://svn.boost.org/svn/boost/trunk/libs/spirit/example/qi/typeof.cpp and it got first introduced in the comments at this blog post: http://boost-spirit.com/home/articles/qi-example/zero-to-60-mph-in-2-seconds/

The explicit fix I tested (which worked nicely on my box, no warnings remaining in valgrind):

auto const ana = boost::proto::deep_copy(
        *~qi::char_('*') > +qi::char_('*'))
        ;

Spirit X3 promises to remove this wart. Slightly related, I think Protox11 also removes this issue by being aware of references at all times.

---

^[1] Grep the standard for lifetime extension of temporaries. The expression templates keep references to the literals used (the rest has value semantics anyways), but the temporaries aren't bound to (const) references. So they go out of scope. Undefined Behaviour results