[Bug c++/38855] New: evaluation of reinterpret_cast with -O2

[Bug c++/38855] New: evaluation of reinterpret_cast with -O2

[ElGringo at gmx dot at]

---code---
#include <iostream>
#include <iomanip>

using namespace std;

int main() {
        float f(1.0);
//      cout << "Value: ";   // works with any cout before eval!!
        unsigned int  i = *reinterpret_cast<unsigned int*>(&f);
        cout << hex << i << endl;

        cout << sizeof(float) << endl;
        cout << sizeof(unsigned int) << endl;
}
--- end code ---

run w/o -O2:
bernhard@xbernhard$ c++ floatmini.cpp -o floatmini
bernhard@xbernhard$ ./floatmini
3f800000
4
4
=> works

with -O2
bernhard@xbernhard$ c++ floatmini.cpp -o floatmini -O2
bernhard@xbernhard$ ./floatmini
8048779
4
4
=> fails

* If codeline 8 is active (see comment), works too with -O2.
* unsigned int&  i = reinterpret_cast<unsigned int&>(f);
  fails the same way.

Is that line not standard conform?

------------------------------------
bernhard@xbernhard$ c++ -v
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v
--enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr
--enable-shared --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix --enable-nls
--with-gxx-include-dir=/usr/include/c++/4.2 --program-suffix=-4.2
--enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-mpfr
--disable-libmudflap --enable-targets=all --enable-checking=release
--build=i486-linux-gnu --host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.2.3 (Debian 4.2.3-2)


-- 
           Summary: evaluation of reinterpret_cast with -O2
           Product: gcc
           Version: 4.2.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ElGringo at gmx dot at
 GCC build triplet: i486-linux-gnu
  GCC host triplet: i486-linux-gnu
GCC target triplet: i486-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38855

[Bug c++/38855] evaluation of reinterpret_cast with -O2

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2009-01-15 14:37 -------
It is not.  You are violating C++ aliasing rules.  Use memcpy or a union.


-- 

rguenth at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38855