Your "id" variable is signed, and right-shifting signed integers yields implementation-defined behaviour according to the C Standard, for example copying instead of shifting the hightest (sign) bit. I'd suggest that you do all bit-shifting and composing/decomposing on unsigned integers, and cast input/output operands. On 19.02.2017 16:44, Chris Pezley wrote: > 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 >
