Re: Why does different types of array subscript used to iterate affect auto vectorization

[Alexander Monakov <amonakov@ispras.ru>]

On Mon, 27 Jun 2022, Adonis Ling via Gcc-help wrote:

>  Hi all,
> 
> Recently, I met an issue with auto vectorization.
> 
> As following code shows, why uint32_t prevents the compiler (GCC 12.1 + O3)
> from optimizing by auto vectorization. See https://godbolt.org/z/a3GfaKEq6.
> 
> #include <cstdint>
> 
> // no auto vectorization
> void test32(uint32_t *array, uint32_t &nread, uint32_t from, uint32_t to) {
>     for (uint32_t i = from; i < to; i++) {
>         array[nread++] = i;
>     }
> }

Here the main problem is '*array' and 'nread' have the same type, so they might
overlap. Ideally the compiler would recognize that that cannot happen because it
would make 'array[nread++] = i' undefined due to unsequenced modifications, but
GCC is not sufficiently smart (yet). The secondary issue is the same as below:

> // no auto vectorization
> void test_another_32(uint32_t *array, uint32_t &nread, uint32_t from,
> uint32_t to) {
>     uint32_t index = nread;
>     for (uint32_t i = from; i < to; i++) {
>         array[index++] = i;
>     }
>     nread = index;
> }

... here: the issue is that index is unsigned and shorter than pointer type, it
can wrap around from 0xffffffff to 0, making the access non-consecutive. When
you compile for 32-bit x86, this loop is vectorized.

Alexander