[Bug c/79082] New: -Wformat-truncation inconsistent behaviour

[sirl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79082

            Bug ID: 79082
           Summary: -Wformat-truncation inconsistent behaviour
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sirl at gcc dot gnu.org
  Target Milestone: ---

This small testcase warns differently for -O0/g/1/2/3 with this "gcc -c
-Wformat-truncation test.c"

extern int snprintf(char *str, __SIZE_TYPE__ size, const char *format, ...);
void test(char *buffer, int val)
{
  snprintf(buffer, 3, "%2d", val % 100);
  snprintf(buffer, 3, "%2d", (val < 0) ? -(val % 100) : val % 100);
  snprintf(buffer, 3, "%2d", __builtin_abs(val % 100));
  snprintf(buffer, 3, "%2x", val & 0xff);
}

-O0/1 produce 4 warnings, -O2/3 produce 2 warnings and -Og doesn't warn at all.
I believe it should only ever warn for the first case.