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

[Kirill Yu Berezin]

Hello!

I have a code snippet (actually it is a part of larger project):


#include <stdint.h>

#define UDP_PROTO_NUMBER 17

uint32_t calc_16bit_checksum_part(uint8_t* buf, int len, uint32_t ret) {
         uint16_t *ptr = reinterpret_cast<uint16_t*>(buf);
         int i;

         for( i = 0; i < (len / 2); ++i) {
                 ret = ret + ptr[i];
         }
         if ( len%2) {
                 buf[len] = 0;
                 ret += ptr[len/2];
         }

         return ret;
}

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;
     };
     uint8_t tbuf[sizeof(udp_pseudo)];
     udp_pseudo* tmp=reinterpret_cast<udp_pseudo*>(tbuf);

     tmp->src = src;
     tmp->dst = dst;
     tmp->z = 0;
     tmp->proto = UDP_PROTO_NUMBER;
     tmp->len = len;

     return calc_16bit_checksum_part(tbuf, sizeof(tbuf), 0);
}

int main() {
     return calc_ch_udp_pseudo(0x01020304, 0x12131415, 320);
}


When I try to compile it with -O2 and higher I got the following

#c++ -DHAVE_CONFIG_H -I. -I.././include     -g -O3 -Wall -I../include/  
-o snippet snippet.cc
snippet.cc: In function ‘uint32_t calc_ch_udp_pseudo(uint32_t, uint32_t, 
uint16_t)’:
snippet.cc:10:34: warning: ‘tbuf’ is used uninitialized in this function 
[-Wuninitialized]
                  ret = ret + ptr[i];
                              ~~~~~^
snippet.cc:10:34: warning: ‘*((void*)& tbuf +2)’ is used uninitialized 
in this function [-Wuninitialized]
snippet.cc:10:34: warning: ‘*((void*)& tbuf +4)’ is used uninitialized 
in this function [-Wuninitialized]
snippet.cc:10:34: warning: ‘*((void*)& tbuf +6)’ is used uninitialized 
in this function [-Wuninitialized]
snippet.cc: In function ‘int main()’:
snippet.cc:10:34: warning: ‘tbuf’ is used uninitialized in this function 
[-Wuninitialized]
                  ret = ret + ptr[i];
                              ~~~~~^
snippet.cc:10:34: warning: ‘*((void*)& tbuf +2)’ is used uninitialized 
in this function [-Wuninitialized]
snippet.cc:10:34: warning: ‘*((void*)& tbuf +4)’ is used uninitialized 
in this function [-Wuninitialized]
snippet.cc:10:34: warning: ‘*((void*)& tbuf +6)’ is used uninitialized 
in this function [-Wuninitialized]

Compiler :

#gcc --versio
gcc (GCC) 7.1.1 20170528
Copyright (C) 2017 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

My question is. Is this an expected behaviour or I must report a bug ?


Thank you!

Kirill