C global anonymous struct / union

Question

I have a uint64 variable which often only requires high or low 32 bit access. I am using a 32-bit ARM Cortex M0, and to help with speed and I am trying to overlap the uint64 var

Answer 1

You could get fancy and use assembly to achieve what you want, or use C++.

In assembly

                EXPORT AB
                EXPORT A
                EXPORT B
AB
A               SPACE 4
B               SPACE 4

In C:

extern uint64_t AB;
extern uint32_t A;
extern uint32_t B;

Then do what you want.

In C++, something like this:

union MyUnionType
{
    uint64_t ab;
    struct 
    {
        uint32_t a;
        uint32_t b;
    };
} ;

MyUnionType MyUnion;

uint64_t &ab = MyUnion.ab;
uint32_t &a  = MyUnion.a;
uint32_t &b  = MyUnion.b;

But, to be honest, this is all wasted effort. Accessing a or MyUnion.a is going to have the same code generated by the compiler. It knows the size and offsets of everything, so it's not going to calculate anything at runtime, but just load from the right address it knows ahead of time.