[Bug c++/50776] New: unused object optimized out, despite having constructor

["nyh at math dot technion.ac.il" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50776

             Bug #: 50776
           Summary: unused object optimized out, despite having
                    constructor
    Classification: Unclassified
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: nyh@math.technion.ac.il


Consider the following simple program:

        #include <cstdio>
        class Ident {
        public:
                Ident(const char *ident){
                        // This constructor prints a message!
                        printf("yo\n");
                }
        };

        static Ident id("$Id: hello $");

        main(){
                printf("hello\n");
        }

When you compile it with g++ without additional parameters, its output rightly
looks like:

        yo
        hello

I.e., the object "id" gets instantiated, and its constructor prints the message
"yo".
However, if you compile it with optimization enabled - g++ -O2 - the output is
DIFFERENT: the "id" object is optimized out (because it is static, but nobody
uses it in this source file), and therefore its constructor no longer runs.

I think it is a bug for optimization to change the behavior (not just the
performance) of a program. I think that if an object has a non-trivial
constructor with any side effects beside setting class fields, then we cannot
optimize its construction out because this would change the program behavior.