boost::container::flat_map force_copy violates strict-aliasing rule

[prus1337]

Hi everyone!

g++ v13.1.0, v13.2.0, boost v1.82, MSYS2 MINGW64_NT-10.0-19044.

Discussion links:
msys2/MINGW-packages#17977
msys2/MINGW-packages#17977 (comment)

The following example has different behavior depending on compiler optimization flags:

#include <boost/container/flat_map.hpp>

#include <cstdint>
#include <iostream>
#include <string>

int main()
{
    boost::container::flat_map<std::string, size_t> map{};
    const std::string key = "test";
    const size_t value = 13;
	
    auto [it, succeeded] = map.emplace(key, value);
	
    if (succeeded)
    {
        std::cout << "emplace result: " << it->first << " -> " << it->second << "\n";
        std::cout << "emplace iterator result: " << "it == map.end() is " << std::boolalpha << (it == map.end()) << "\n";
		
        it = map.find(key);
        std::cout << "find result: " << it->first << " -> " << it->second << "\n";
        std::cout << "find iterator result: " << "it == map.end() is " << std::boolalpha << (it == map.end()) << "\n"; 
    }
	
    return 0;
}

Expected behavior

Without optimization flags (-O0, -O1) everything fine.
g++ -O0 -o boost_v1.82_flat_map_it_bug boost_v1.82_flat_map_it_bug.cpp

$ ./boost_v1.82_flat_map_it_bug
emplace result: test -> 13
emplace iterator result: it == map.end() is false
find result: test -> 13
find iterator result: it == map.end() is false

Actual behavior

With optimization flags (-O2, -O3) iterator has bad value.
g++ -O2 -o boost_v1.82_flat_map_it_bug boost_v1.82_flat_map_it_bug.cpp

./boost_v1.82_flat_map_it_bug
emplace result: test -> 13
emplace iterator result: it == map.end() is true
find result: test -> 13
find iterator result: it == map.end() is true

The issue does not happen in Linux with gcc 13.2.1 toolchain.

[Mark-MnetCS]

I bumped into a version of this problem that's reproducible on GCC trunk

#include <utility>
#include <boost/container/flat_map.hpp>

namespace bc = boost::container;

int main() {
    bc::flat_map<int, int> map {
        std::pair(0, 1),
        std::pair(1, 2),
        std::pair(2, 4)
    };

    if (map.size() != 3) {
        return 1;
    }

    int i = 0;
    for (const auto &[k, v] : map) {
        ++i;
    }
    if (i != 3) {
        return 2;
    }

    return 0;
}

When this is compiled with -O1 it runs as expected: i == 3 and the exit status is 0.
When compiled with -DBOOST_CONTAINER_DISABLE_FORCEINLINE -O2 the loop gets optimized away, i == 0 and the exit status is 2.

Here's a Compiler Explorer link showing the result: https://godbolt.org/z/7W5eGrKGE
Built with g++ (Compiler-Explorer-Build-gcc-181917d56c0cf4b8c1174a492a8157f4a12d1100-binutils-2.40) 14.0.0 20231219 (experimental)

[Ion Gaztañaga]

Thanks for the report. The strict aliasing violation was needed only for ancient compilers so the following commit removes it for C++11 o newer:

20ad12f