[Bug c++/103954] GCC did not generate correct code for template function with O2

[woodhead99 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103954

--- Comment #2 from Ming Zhi <woodhead99 at gmail dot com> ---
thanks for your great answer! I did not know the effect of strict aliasing till
today. It looks I need to update my knowledge base about c++ right now.

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________________________________
From: pinskia at gcc dot gnu.org <gcc-bugzilla@gcc.gnu.org>
Sent: Monday, January 10, 2022 12:22:24 AM
To: woodhead99@gmail.com <woodhead99@gmail.com>
Subject: [Bug c++/103954] GCC did not generate correct code for template
function with O2

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103954

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

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

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---


enum EnumSeriProto : guint32
{
    seriNone = 0,
    seriRidl = 1,
    seriPython = 2,
    seriJava = 3,
    seriInvalid = 4
};

....


        EnumSeriProto dwSeriProto = seriNone;
        ret = GetSeriProto( pResp, dwSeriProto );

...
    return oCfg.GetIntProp(
        propSeriProto, ( guint32& )dwSeriProto );
...
    gint32 GetIntProp( gint32 iProp, guint32& val ) const
    {
        return GetPrimProp( iProp, val );
    }
...
    template< typename PrimType, ... >
    gint32 GetPrimProp(
        gint32 iProp, PrimType& val ) const
    {
        gint32 ret = 0;
        if( m_pConstCfg == nullptr )
            return -
# 1212 "/usr/local/include/rpcf/configdb.h" 3 4
                   14
# 1212 "/usr/local/include/rpcf/configdb.h"
                         ;

        auto pdb2 = static_cast< const CConfigDb2* const >( m_pConstCfg );
        do{
            const Variant* p =
                pdb2->GetPropertyPtr( iProp );
            if( p == nullptr )
            {
                ret = -
# 1220 "/usr/local/include/rpcf/configdb.h" 3 4
                      2
# 1220 "/usr/local/include/rpcf/configdb.h"
                            ;
                break;
            }
            val = *p;

        }while( 0 );

        return ret;
    }


enum types don't alias the underlaying type and therefore this code is
undefined at runtime.
Use Either a memcpy (or an union) inside GetSeriProto or use
-fno-strict-aliasing.

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