There are some truly weird corner cases with various ABIs. My favourite is the Linux x86-32 ABI, which says that unions are passed as structs and (unlike the *BSD / macOS version of the same ABI) does not use the small structure optimisation. Consider this contrived example function:

union X
{
        void *a;
        long b;
};

union X ret()
{
        return (union X){12};
}

On FreeBSD, this compiles to:

        movl    $12, %eax
        retl

But on Linux, it’s:

        movl    4(%esp), %eax
        movl    $12, (%eax)
        retl    $4

On FreeBSD, quite sensibly, a word-sized structure is returned in a word-sized register. On Linux, the caller allocates a word-sized place on the stack and then passes a pointer to it into the callee. The callee then stores the value in the pointer.

I ended up rewriting a bunch of code to cast things via uintptr_t (which, in spite of effectively being a union of an integer and a pointer in the C spec, is not treated as one in the ABI) to avoid overhead on Linux. Fortunately, the 64-bit ABI fixed this particular issue.

This is an insightful article that provides useful information also for people that already have some knowledge of Assembly and C++.