"Uninitialized array" warnings by c++ with -O2

[K]

> That snippet invokes undefined behavior at runtime (violates C++ aliasing rules),
> so just fix it, rather than bother with bugreports.  E.g. look for -fstrict-aliasing
> in GCC documentation, or read the C++ standard.  With -fno-strict-aliasing,
> which is a workaround for broken code you won't get any warnings about
> uninitialized uses.
>
> 	Jakub
>

Yes this code has problems with aliasing. But anyway warning messages 
are extremely misleading.
And I found that that a version which I beleve mustn't have aliasing 
problems still generates same warnings.

The code:

  static uint32_t calc_16bit_checksum_part(uint8_t* buf, int len, 
uint32_t ret) {
         struct ui16{
                 uint16_t d;
         };
         ui16 *ptr = (ui16*)buf;
         for( int i = 0; i < (len / 2); ++i) {
                 ret += ptr[i].d;
         }
         if ( len%2) {
                 ret += buf[len-1];
         }

         return ret;
}

static uint32_t calc_ch_udp_pseudo(uint32_t src, uint32_t dst, uint16_t 
len) {
         struct udp_pseudo {
                 uint32_t src;
                 uint32_t dst;
                 uint8_t  z;
                 uint8_t  proto;
                 uint16_t len;
         } tmp;

         tmp.src = src;
         tmp.dst = dst;
         tmp.z = 0;
         tmp.proto = UDP_PROTO_NUMBER;
         tmp.len = len;

         auto ret = calc_16bit_checksum_part((uint8_t*)&tmp, 
sizeof(tmp), 0);
         return ret;
}

and messages

static uint16_t calc_16bit_checksum(uint8_t* buf, int len) {
                  ^~~~~~~~~~~~~~~~~~~
snippet.cc: In function ‘int main()’:
snippet.cc:66:31: warning: ‘tmp.calc_16bit_checksum_part(uint8_t*, int, 
uint32_t)::ui16::d’ is used uninitialized in this function [-Wuninitialized]
                  ret += ptr[i].d;
                         ~~~~~~~^
snippet.cc:66:31: warning: ‘*((void*)(& 
tmp)+2).calc_16bit_checksum_part(uint8_t*, int, uint32_t)::ui16::d’ is 
used uninitialized in this function [-Wuninitialized]
snippet.cc:66:31: warning: ‘*((void*)(& 
tmp)+4).calc_16bit_checksum_part(uint8_t*, int, uint32_t)::ui16::d’ is 
used uninitialized in this function [-Wuninitialized]
snippet.cc:66:31: warning: ‘*((void*)(& 
tmp)+6).calc_16bit_checksum_part(uint8_t*, int, uint32_t)::ui16::d’ is 
used uninitialized in this function [-Wuninitialized]
snippet.cc:66:31: warning: ‘*((void*)(& 
tmp)+8).calc_16bit_checksum_part(uint8_t*, int, uint32_t)::ui16::d’ is 
used uninitialized in this function [-Wuninitialized]
snippet.cc:66:31: warning: ‘*((void*)(& 
tmp)+10).calc_16bit_checksum_part(uint8_t*, int, uint32_t)::ui16::d’ is 
used uninitialized in this function [-Wuninitialized]
[


ui16 is an aggregate type and uint8_t is unsigned char, so there will be 
no undefined behaviour.

Kirill.