Is there a compiler memory barrier for a single variable?

Question

Compiler memory barriers has the effect among other things to force the compiler to make sure all stack variables that are cached in registers are written t

Answer 1

Recollecting one of the threads on lkml, one of the methods for single variable compiler-only barrier is:

#define forget(x) __asm__ __volatile__("":"=m"(x):"m"(x))

Answer 2

Try with { int y = x; *(volatile int*)&x = y; } and inspect the resulting assembly.

Answer 3

I guess you can achieve it by specifying your variable in the list of output values of asm:

__asm__ __volatile__ ("" : "=r" (x) : : )

See Extended Asm for some information.

UPD.

It may be better to use "g" constraint instead of "r" as more permissive.

__asm__ __volatile__ ("" : "=g" (x) : : )

Also, I've found another great howto for inline assembly.

Answer 4

Since you are willing to work with gcc extensions you could use the extensions for atomic instructions for that feature:

__sync_bool_compare_and_swap(&myvar, 0, 0)

would set the value of the variable to 0 if it is already 0 :) and in addition imply a full sequential consistency on that memory location.