Undefined behaviour shifting negative value in rate.c

Clang warn about the following issue:

../../lib/rate.c:746:15: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
    log_scale=OC_Q57(-64);
              ^~~~~~~~~~~
../../lib/mathops.h:134:38: note: expanded from macro 'OC_Q57'
#define OC_Q57(_v) ((ogg_int64_t)(_v)<<57)
                    ~~~~~~~~~~~~~~~~~^

It is unclear to me what the expected and wanted behavior here is. I tested the actual behavior with the following code:

#include <inttypes.h>
#include <stdint.h>
#include <stdio.h>
#define OC_Q57(_v) ((int64_t)(_v)<<57)
int main(int argc, char *argv[])
{
  int64_t log_scale = OC_Q57(-64);
  printf("0x%" PRIx64 "\n", log_scale);
  return 0;
}

The output is 0x8000000000000000, which seem like a useless representation of -64 in Q57 format, but I do not really understand the encoder code and have no idea how it is supposed to behave.

It occured to me that the output from the test program is the same as if the value was not negative, and this program confirm it:

#include <inttypes.h>
#include <stdint.h>
#include <stdio.h>
#define OC_Q57(_v) ((int64_t)(_v)<<57)
int main(int argc, char *argv[])
{
  int64_t log_scale;
  log_scale = OC_Q57(64);
  printf("0x%" PRIx64 "\n", log_scale);
  log_scale = OC_Q57(-64);
  printf("0x%" PRIx64 "\n", log_scale);
  return 0;
}

The output is simple:

0x8000000000000000
0x8000000000000000

Thus a simple fix is to change -64 to 64 to avoid the warning.

mentioned in commit 3afddb5a

mentioned in commit 3a362bfa

mentioned in merge request !23 (merged)

mentioned in commit 72358ce5

mentioned in commit 4549f546

closed with commit 4549f546