[Bug c++/12747] New: GCC generates poor code for temporary structs

[jbuck at gcc dot gnu dot org]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12747

           Summary: GCC generates poor code for temporary structs
           Product: gcc
           Version: 3.3.2
            Status: UNCONFIRMED
          Keywords: pessimizes-code
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: jbuck at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org

This problem has been discussed for a decade, but I couldn't find a clear example
in the Bugzilla database, so here is one.

In this example we have a simple C++ bit vector class.  operator[] is overloaded
in the usual way, to return a proxy object, so that C-like array notation can be
used.  However, the code sucks: typically we wind up with many dead stores to
the stack.  The example includes two routines: copy, which uses the operator[],
and copy_2, which directly calls getBit and setBit.  The compiler should be able
to generate equally good code for copy and copy_2.