Not sure about the platform question, I tried building it for a Cortex-M3 (32-bit "small" ARM) but that still doesn't issue the memcpy() call. Instead it just loads pairs of words from memory then moves those into the FPU for comparison. The loads are marked with "@unaligned" but not sure what that actually means and didn't spend the time to research it now.
I assume you're talking about this:
long x = <whatever>
double y = *(double*)&x;
If you got rid of this rule a huge number of optimizations would be lost, since the compiler would have to assume that any memory, of any type, could change if anything at all is modified through a pointer. You'd be surprised how slow everything would be. You'd also be surprised how much code violates this and gets away with it. Time bombs waiting to happen. ;-)
memcpy(), and more generally accessing bytes through char*, are how you avoid this kind of undefined behavior. It comes up so often that C++20 added std::bit_cast for this same use: https://en.cppreference.com/w/cpp/numeric/bit_cast
Here's how I would have written it, without memcpy():
static int cmp_double(const void *va, const void *vb)
{
const double a = *(const double *) va, b = *(const double *) vb;
return a < b ? -1 : a > b;
}
Edit: missed const:s in function body oops.