From: Adonis Ling <adonis0147@gmail.com>
To: Alexander Monakov <amonakov@ispras.ru>
Cc: gcc-help@gcc.gnu.org
Subject: Re: Why does different types of array subscript used to iterate affect auto vectorization
Date: Tue, 28 Jun 2022 23:23:52 +0800 [thread overview]
Message-ID: <CAG5qfXi-jE1wtgsKBECR88R8c6FhnWBP3Dj07eTv5d6g4fZHYQ@mail.gmail.com> (raw)
In-Reply-To: <3eb44329-3b12-896c-14c4-3473d43aed3d@ispras.ru>
Hi Alexander, thanks for your reply.
On Tue, Jun 28, 2022 at 9:06 PM Alexander Monakov <amonakov@ispras.ru>
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
next prev parent reply other threads:[~2022-06-28 15:24 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-27 2:47 Adonis Ling
2022-06-28 13:06 ` Alexander Monakov
2022-06-28 15:23 ` Adonis Ling [this message]
2022-06-28 15:38 ` Alexander Monakov
2022-06-28 15:47 ` Adonis Ling
2022-06-28 15:56 ` Alexander Monakov
2022-06-28 16:00 ` Adonis Ling
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