To back up the other commenter - it's not the same. https://godbolt.org/z/r6e443x1c shows that if you write imperfect C++ clang is perfectly capable of optimizing it.

▲ cogman10 7 hours ago | parent | next [-]

What's strange is I'm finding that gcc really struggles to correctly optimize this.

This was my function

    for (auto v : array) {
        if (v != 0)
            return false;
    }
    return true;

clang emits basically the same thing yours does. But gcc ends up just really struggling to vectorize for large numbers of array.

Here's gcc for 42 elements:

https://godbolt.org/z/sjz7xd8Gs

and here's clang for 42 elements:

https://godbolt.org/z/frvbhrnEK

Very bizarre. Clang pretty readily sees that it can use SIMD instructions and really optimizes this while GCC really struggles to want to use it. I've even seen strange output where GCC will emit SIMD instructions for the first loop and then falls back on regular x86 compares for the rest.

Edit: Actually, it looks like for large enough array sizes, it flips. At 256 elements, gcc ends up emitting simd instructions while clang does pure x86. So strange.

▲

secondcoming 5 hours ago | parent [-]

I;ve had to coerce gcc to emitting SIMD code by using int instead of bool. Also, the early return may be putting it off.

	▲	abbeyj 2 hours ago \| parent [-]
		Doing both of those things does seem to help: https://godbolt.org/z/1vv7cK4bE GCC trunk seems to like using `bool` so we may eventually be able to retire the hack of using `int`.

▲ btdmaster 5 hours ago | parent | prev [-]

I see yeah that makes sense. I wanted to highlight that "magic" will, on average, give the optimizer a harder time. Explicit offset loops like that are generally avoided in many C++ styles in favor of iterators.

▲

delta_p_delta_x 3 hours ago | parent [-]

Even at a higher level of abstraction, the compiler seems to pull through: https://godbolt.org/z/1nvE34YTe

	▲	btdmaster an hour ago \| parent [-]
		It emits a cmp/jmp still when arithmetic would be fine though which is the difference highlighted in the article and examples in this thread. It's nice that it simplifies down to assembly, but the assembly is somewhat questionable (especially that xor eax eax branch target on the other side).