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

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Hi Alexander, thanks for your reply.

On Tue, Jun 28, 2022 at 9:06 PM Alexander Monakov <amonakov@xxxxxxxxx>
wrote:

> 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:
>

I got your point.

After that, I tried to add __restrict__ to nread as the following shows and
GCC still doesn't optimize it.

#include <cstdint>

// no auto vectorization
void test32(uint32_t *array, uint32_t & __restrict__ nread, uint32_t from,
uint32_t to) {
    for (uint32_t i = from; i < to; i++) {
        array[nread++] = i;
    }
}

However, when I used Clang to compile, I noticed the code was optimized by
Clang. See https://godbolt.org/z/eEz9W7o9z .


> > // 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
>

Clang also optimizes this function. See https://godbolt.org/z/eEz9W7o9z .

-- 
Best regards,
Adonis



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