[Bug c++/89056] Optimizer generates bad code for non-void function that fails to return a value

[darryl_okahata at keysight dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89056

--- Comment #6 from Darryl Okahata <darryl_okahata at keysight dot com> ---
(OK, at this point, I'm just whinging, so please feel free to ignore this.)

I just wish the C++ standard instead just allowed an undefined value to be
returned, instead of generating bad optimized code.  With the current state, I
either have to add compiler-specific extensions or unreachable return
statements to insure that correct code is generated (unexpected and violates
POLA).  The issue is that g++ (understandably) can't always detect if there is
always a proper return statement (execution can never hit the end of the
function).  Grossly-oversimplified example (real code is much more
complicated):

    enum E { A, B };

    bool bah(const enum E a)
    {
        if (a == A)
            return false;
        if (a == B)
            return true;
    }

Compiling with (8.2.0):

     g++ -S -O badbad.cc

gives:

    badbad.cc: In function 'bool bah(E)':
    badbad.cc:10:1: warning: control reaches end of non-void function
[-Wreturn-type]
     }
     ^

Understandable, as I don't expect g++ to figure out complicated code
machinations.  However, I don't know all the circumstances under which this
warning means that g++ is generating bad code.  As a result, I have to add
unreachable return statements to insure that g++ does not generate bad
optimized code.  Our code runs on multiple platforms, and so I'd rather avoid
the use of g++ extensions (e.g., __builtin_unreachable() or attributes) and
cluttering #ifdefs.  Adding an unreachable return is undesirable but simple and
portable:

    enum E { A, B };

    bool bah(const enum E a)
    {
        if (a == A)
            return false;
        if (a == B)
            return true;
        return false;     // UNREACHABLE
    }