15.2.6 Const Function Attribute

The const attribute instructs the compiler to more aggressively optimize calls to the function. The compiler enforces that such functions declared with the attribute do not read objects that affect its return value between successive invocations, such as global objects. However, such functions can safely read objects that do not change their return value, such as non-volatile constants. Note that a function which de-references a pointer argument should not be attributed with const, since the pointer de-reference uses a value which is not a parameter, even though the pointer itself is a parameter. For example:

int __attribute((const)) rv(int a);