On 12/23/2016 3:35 PM, Johan Engelen wrote:
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).
The assumption is that 'a' points to the start of an array [0..SIZE], so there's no overflow.
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.
The vectorized one probably is. But it sure is a lot of code. (The loop speed could possibly be doubled just by unrolling it a few times.)
