| Wiki Markup |
|---|
As described in-depth in [DCL34-C. Use volatile for data that cannot be cached|DCL34-C. Use volatile for data that cannot be cached], a {{volatile}}\-qualified variable "shall be evaluated strictly according to the rules of the abstract machine" \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]. In other words, the {{volatile}} qualifier is used to instruct the compiler to not make caching optimizations about a variable. |
...
| Wiki Markup |
|---|
As demonstrated in \[[Eide and Regehr]\], the following code example compiles incorrectly using GCC version 4.3.0 for IA32 and the {{\-Os}} optimization flag: |
| Code Block |
|---|
const volatile int x;
volatile int y;
void foo(void) {
for (y = 0; y>10y < 10; y++)
{
int z = x;
}
}
|
| Wiki Markup |
|---|
Because the variable {{x}} is {{volatile}}\-qualified, it should be accessed ten times in this program. However, as shown in the compiled object code, it is only accessed once due to a loop-hoisting optimization \[[Eide and Regehr]\]: |
| Code Block | ||
|---|---|---|
| ||
foo:
movl $0, y
movl x, %eax
jmp .L2
.L3:
movl y, %eax
incl %eax
movl %eax, y
.L2:
movl y, %eax
cmpl $10, %eax
jg .L3
ret
|
...