https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86617
Bug ID: 86617
Summary: Volatile qualifier is ignored sometimes for unsigned
char
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: ---
I'm using http://gcc.godbolt.org/ to reproduce this issue.
Compiler: x86-64 gcc (trunk); gcc (GCC-Explorer-Build) 9.0.0 20180719
Options: -Os
Test code:
```
volatile unsigned char u8;
void test (void)
{
u8 = u8 + u8;
u8 = u8 - u8;
}
```
Disassembly:
```
test:
mov al, BYTE PTR u8[rip]
add eax, eax
mov BYTE PTR u8[rip], al
mov al, BYTE PTR u8[rip]
mov BYTE PTR u8[rip], 0
ret
```
In the addition expression `u8 + u8`, volatile variable u8 is copied to the
register only once and then register is doubled. This seems incorrect, as
volatile variable shall be read out from the memory also for the second term of
the sum.
In the subtraction expression `u8 - u8`, result (0) is calculated at compile
time without explicit subtraction.
Also can be reproduced in:
- x86-64 gcc 8.1; gcc (GCC-Explorer-Build) 8.1.0
- x86-64 gcc 7.3; gcc (GCC-Explorer-Build) 7.3.0
- x86-64 gcc 6.3; gcc (GCC-Explorer-Build) 6.3.0
- x86-64 gcc 5.4; gcc (GCC-Explorer-Build) 5.4.0
When g++ is used to compile the same code, everything is fine!
Compiler: x86-64 gcc (trunk); g++ (GCC-Explorer-Build) 9.0.0 20180719
Options: -Os
Disassembly:
```
_Z3foov:
mov al, BYTE PTR u8[rip]
mov dl, BYTE PTR u8[rip]
add eax, edx
mov BYTE PTR u8[rip], al
mov al, BYTE PTR u8[rip]
mov dl, BYTE PTR u8[rip]
sub eax, edx
mov BYTE PTR u8[rip], al
ret
u8:
.zero 1
```
