On 12/04/11 09:18, David Brown wrote:
> For example, if you have code like this:
>
> static int x;
> void test(void) {
> x = 1;
> asm volatile ("" : : : "memory");
> x = 2;
> }
>
> The variable "x" is not volatile - can the compiler remove the
> assignment "x = 1"? Perhaps with aggressive optimisation, the compiler
> will figure out how and when x is used, and discover that it doesn't
> need to store it in memory at all, but can keep it in a register
> (perhaps all uses have ended up inlined inside the same function). Then
> "x" is no longer in memory - will it still be affected by the memory
> clobber?
I don't think either assignment can be removed, at least not when
the program is compiled with threads enabled.
Even though x is local to this translation unit it is not local to
this thread. The compiler must respect memory barriers in this
case: if it did not, POSIX threads support would not work. There
was a fantastically long thread about this
"Optimization of conditional access to globals: thread-unsafe?" in
Oct 2007.
Andrew.
