[Bug c/87060] New: Condition check is optimized out for volatile unsigned char / short

[yso at melexis dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87060

            Bug ID: 87060
           Summary: Condition check is optimized out for volatile unsigned
                    char / short
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yso at melexis dot com
  Target Milestone: ---

Compilers from http://gcc.godbolt.org/ are used to reproduce the issue.

Compiler: x86-64 gcc (trunk); gcc (GCC-Explorer-Build) 9.0.0 20180821
(experimental) 
Options: -Os

Test code:  
```
volatile unsigned char u8;
void test_u8 (void)
{
    if (u8 + 1) { u8 = 1; }
}
```

Disassembly:  
```
test_u8:
        mov     al, BYTE PTR u8[rip]
        mov     BYTE PTR u8[rip], 1
        ret
```

Condition check is optimized out, while unsigned overflow is perfectly
legitimate operation.

- also can be reproduced in gcc 8/7/6/5 and 4 (after 4.1.2)
- also with unsigned short type