Peculiar behavior with bit-shifting

[Chris Pezley <chris@xxxxxxxxxx>]

I've encountered a rather peculiar behavior in gcc (see below) and I'm 
unsure where to inquire about this. Would this be the correct place?

When combining two int32_t with the bitwise-or operator into an int64_t, 
I've noticed that the less-significant 32 bits will contain something 
extra which will pollute the more-significant bits.

gcc version/OS: (Ubuntu 5.4.0-6ubuntu1~16.04.4) 5.4.0 20160609

Steps to reproduce:
1. Compile example in attachment:
    gcc -Wall incorrect_bitshifting.c
    No warning / error output.
2. Run with ./a.out
    output:
        Higher, lower: aaaa, aaaaaaaa
        result: ffffffffaaaaaaaa
        original: aaaaaaaaaaaa

Expected behavior:
Result matches original.

Actual behavior:
The upper 32 bits are polluted.

Work-around:
bitwise-and ing the lower 32 bits will provide the correct result:
(int64_t)high_part << 32 | low_part & 0xFFFFFFFF

#include <stdio.h>
#include <stdint.h>


int main( void )
{
    int32_t high_part;
    int32_t low_part;
    int64_t together;

    const int64_t id = 0xAAAAAAAAAAAA;

    high_part = id >> 32;
    low_part = id & 0xFFFFFFFF;
    together = (int64_t)high_part << 32 | low_part;

    printf("Higher, lower: %x, %x\n", high_part, low_part);
    // Higher, lower: aaaa, aaaaaaaa
    printf("result: %lx\n", together);
    // result: ffffffffaaaaaaaa
    printf("original: %lx\n", id);
    // original: aaaaaaaaaaaa
}