On 8/2/20 1:31 PM, Bruce Carneal wrote:
import std;
void f0(int[] a, int[] b, int[] dst) @safe {
dst[] = a[] + b[];
}
void f1(int[] a, int[] b, int[] dst) @trusted {
const minLen = min(a.length, b.length, dst.length);
dst[0..minLen] = a[0..minLen] + b[0..minLen];
assert(dst.length == minLen);
}
I was surprised that f0 ran just fine with a.length and b.length geq
dst.length. Is that a bug or a feature?
Assuming it's a feature, are f0 and f1 morally equivalent? I ask
because f1 auto-vectorizes in ldc while f0 does not. Not sure why. As
a guess I'd say that the front end doesn't hoist bounds checks in f0 or
at least doesn't convey the info to the back end in a comprehensible
fashion. Non-guesses welcome.
First, I think this is a bug. A regression in fact. As of 2.077 this
works, and before it did not. There is nothing in the spec that says the
behavior is defined for this case.
Second, it's more than just that. This also runs currently:
void main()
{
auto a = [1, 2, 3];
auto b = [4, 5, 6];
int[] dst = new int[4]; // note the extra element
dst[] = a[] + b[];
writeln(dst[3]);
}
Prior to 2.077, this fails with array length problems.
After that it prints (at the moment): 402653184
If I up the size to 5, it fails with a range violation. I strongly
suspect some off-by-one errors, but this looks unsafe.
-Steve
