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

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.