Libbpf eBPF macro barrier_var
The barrier_var macro is used to prevent the compiler from reordering memory operations for a specific variable.
Definition
#define barrier_var(var) asm volatile("" : "+r"(var))
Usage
This macro is a variable-specific compiler (optimization) barrier. It's a no-op which makes compiler believe that there is some black box modification of a given variable and thus prevents compiler from making extra assumption about its value and potential simplifications and optimizations on this variable.
E.g., compiler might often delay or even omit 32-bit to 64-bit casting of a variable, making some code patterns unverifiable. Putting barrier_var() in place will ensure that cast is performed before the barrier_var() invocation, because compiler has to pessimistically assume that embedded
This is a variable-specific variant of more global barrier().
Example
Docs could be improved
This part of the docs is incomplete, contributions are very welcome