On Friday, 23 December 2016 at 22:11:31 UTC, Walter Bright wrote:
enum SIZE = 100000000;
void foo(int* a, int* b) {
int* atop = a + 1000; // should be `a + SIZE`, right?
ptrdiff_t offset = b - a;
for (; a < atop; ++a)
*a &= *(a + offset);
}
This code is not equivalent with the plain foreach loop.
Execution is different when a > (size_t.max-SIZE). Thus the
"ptrdiff" loop cannot be vectorized (or can only be vectorized
when guarded with a check for potential overflow first).
LDC:
https://godbolt.org/g/YcCJdZ
(note the funny jmp .LBB1_6 to a ret instruction... what's that?!)
GDC does something more complex:
https://godbolt.org/g/3XeI9p
Just for info. Don't know which is faster, but I'm guessing the
vectorized foreach loop.
