[Bug c++/33819] New: Miscompiled shift of C++ bitfield

[ian at airs dot com]

When this test is compiled and run using -O2 with gcc 4.2 and with mainline, it
fails (tested on i686-pc-linux-gnu).  It succeeds with 4.1.

class s
{
public:
  s(long long aa) : a(aa), i1(0) { }
  long long id() const { return (this->a << 16) >> 16; }
  bool operator< (s sv) { return this->a < sv.id(); }
private:
  long long a : 48;
  int i1 : 16;
};
s g(1);
int
main(int, char**)
{
  if (g < (1LL << 38) - 1)
    return 0;
  else
    return 1;
}

I haven't pinned down the exact problem.  However, it appears to be due to the
fact that C++ creates the bitfield as a 48-bit signed integer type.  The left
shift is done in that 48-bit type, not in the long long type which should be
used.  The left shift and right shift in the 48 bit yield -1, which is
incorrect; it should yield ((1LL << 38) - 1).


-- 
           Summary: Miscompiled shift of C++ bitfield
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ian at airs dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33819