Comment 5 for bug 291780

Heres another example, then I do not think the warnings are due to optimization.
I have same warnings with both -O0 and -O3.

#include <stdio.h>

typedef unsigned char u8_t;

void test_cast(unsigned char c1, unsigned char c2, u8_t u1, u8_t u2)
{
  if (c1 == (unsigned char)(~c2)) {
    printf("No warning");
  }
  if (c1 == ~c2) {
    printf("This gives warning");
  }
  if (u1 == (u8_t)(~u2)) {
    printf("This gives warning");
  }
  if ((unsigned char)u1 == (unsigned char)(~u2)) {
    printf("This gives warning");
  }
}

The original code that caused this warnings are the TCP/IP stack lwIP, then I constructed this minimal example.

Original code from lwIP TCP/IP stack:
-------------------------------------

static u8_t ip_reassbitmap[IP_REASS_BUFSIZE / (8 * 8) + 1];
static const u8_t bitmap_bits[8] = { 0xff, 0x7f, 0x3f, 0x1f, 0x0f, 0x07, 0x03, 0x01 };

/.../
      if (ip_reassbitmap[ip_reasslen / (8 * 8)] !=
        (u8_t) ~ bitmap_bits[ip_reasslen / 8 & 7]) {
/.../