[Bug c++/111436] New: Wrong code when bit-casting struct through pointer

[Bug c++/111436] New: Wrong code when bit-casting struct through pointer

[josopait at goopax dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111436

            Bug ID: 111436
           Summary: Wrong code when bit-casting struct through pointer
           Product: gcc
           Version: 12.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: josopait at goopax dot com
  Target Milestone: ---

Does gcc produce wrong code here? The program below is part of a bitwise
conversion mechanism, similar to std::bit_cast, but for more general types.

The output should be:
from=17,23
to=17,23

But with -O2 or higher the output is:
from=17,23
to=0,0

I tried with gcc 12, 13, and 14 on x86_64 linux.






#include <iostream>
#include <cstdint>
#include <array>
using namespace std;


template<typename TO, typename FROM>
inline void reinterpret_switch3(TO& to, const FROM& from)
{
  to = *reinterpret_cast<const TO*>(&from);
}

template<typename TO, typename FROM>
inline void reinterpret_switch2(TO& to, const FROM& from)
{
  reinterpret_switch3(to, array<FROM, 1>({ { from } }));
}

template<typename TO, typename FROM>
inline void reinterpret_switch(TO& to, const FROM& from)
{
  array<TO, 1> tmp;
  reinterpret_switch2(tmp, from);
  to = tmp[0];
}


int main(int argc, char** argv)
{
  pair<uint32_t, uint32_t> from = {17, 23};
  pair<int32_t, int32_t> to;
  reinterpret_switch(to, from);

  cout << "from=" << from.first << "," << from.second << endl;
  cout << "to=" << to.first << "," << to.second << endl;
}

[Bug c++/111436] Wrong code when bit-casting struct through pointer

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111436

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
             Status|UNCONFIRMED                 |RESOLVED

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
You are type punning and that will almost definitely cause undefined code due
to alias violations.

You could use memcpy instead of the which will (almost; standard layout
structures [or POD for pre C++11]) always work.

[Bug c++/111436] Wrong code when bit-casting struct through pointer

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111436

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Please re-read the notice that was shown before you created this bug:

"Before reporting a bug, please read the bug writing guidelines, please look at
the list of most frequently reported bugs, and please search for the bug."

"Before reporting that GCC compiles your code incorrectly, compile it with gcc
-Wall -Wextra and see whether this shows anything wrong with your code.
Similarly, if compiling with -fno-strict-aliasing -fwrapv makes a difference,
your code probably is not correct."


Your code produces the expected results with -fno-strict-aliasing and as it
says, that means your code is probably not correct. In this case, it's
definitely not correct.

The reason std::bit_cast doesn't allow more general types is because C++
doesn't support doing it. You can't just reinterpret_cast and expect it to
work.