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From: "ebotcazou at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/100653] usage of scalar_storage_order
 produces incorrect result
Date: Wed, 26 May 2021 10:49:29 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100653

Eric Botcazou <ebotcazou at gcc dot gnu.org> changed:

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

--- Comment #2 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
> Attached test case produces invalid results with -O2, good with -O1.

See the manual:

     Moreover, the use of type punning or aliasing to toggle the storage
     order is not supported; that is to say, a given scalar object
     cannot be accessed through distinct types that assign a different
     storage order to it.

typedef struct tIp6Addr
{
    unsigned int    s6_addr32[4];
} tIp6Addr;

struct _tBeTimNetAddr
{
    unsigned char isIPv4;
    union
    {
        unsigned int addr;
        tIp6Addr addr6;
    } BB_MSG_ENDIANNESS u;
} BB_MSG_ENDIANNESS;

_tBeTimNetAddr.u.addr6.s6_addr32[0] is accessed in little-endian and
_tBeTimNetAddr.u.addr is accessed in big-endian, but they are the same scal=
ar.=