Duff's device
I wouldn't recommend doing this under normal circumstances as you should expect
the compiler to do it but it still makes for alarming interesting reading.
#include <cassert>
#include <numeric>
#include <vector>
#include <cstddef>
int main() {
// Create a container of data to copy
std::vector<int> to(1000);
std::iota(std::rbegin(to), std::rend(to), 0);
// Initialise a container to copy into
std::vector<int> from(to.size());
// Create pointers to the beginning of the contiguous data (because vector)
auto t = to.data(), f = from.data();
// Calculate number of unrolled loops
size_t n = (from.size() + 7) / 8;
// Unroll that loop
switch (from.size() % 8) {
case 0:
do {
*t++ = *f++;
case 7:
*t++ = *f++;
case 6:
*t++ = *f++;
case 5:
*t++ = *f++;
case 4:
*t++ = *f++;
case 3:
*t++ = *f++;
case 2:
*t++ = *f++;
case 1:
*t++ = *f++;
} while (--n > 0);
}
// Check they're identical
assert(to == from && "Containers not identical");
}
https://godbolt.org/z/6Gs1Kefcd
