From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <amonakov@ispras.ru>
Received: from mail.ispras.ru (mail.ispras.ru [83.149.199.84])
 by sourceware.org (Postfix) with ESMTPS id B0538382893E
 for <gcc-help@gcc.gnu.org>; Tue, 28 Jun 2022 13:06:19 +0000 (GMT)
DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org B0538382893E
Received: from [10.10.3.121] (unknown [10.10.3.121])
 by mail.ispras.ru (Postfix) with ESMTPS id 682F740755C5;
 Tue, 28 Jun 2022 13:06:13 +0000 (UTC)
Date: Tue, 28 Jun 2022 16:06:13 +0300 (MSK)
From: Alexander Monakov <amonakov@ispras.ru>
To: Adonis Ling <adonis0147@gmail.com>
cc: gcc-help@gcc.gnu.org
Subject: Re: Why does different types of array subscript used to iterate
 affect auto vectorization
In-Reply-To: <CAG5qfXii3dyLDsdULLyZSoigMZjMjk7LmOmjwgZSB44cAQzf-g@mail.gmail.com>
Message-ID: <3eb44329-3b12-896c-14c4-3473d43aed3d@ispras.ru>
References: <CAG5qfXii3dyLDsdULLyZSoigMZjMjk7LmOmjwgZSB44cAQzf-g@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
X-Spam-Status: No, score=-2.8 required=5.0 tests=BAYES_00, KAM_DMARC_STATUS,
 SPF_HELO_NONE, SPF_PASS, TXREP,
 T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6
X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on
 server2.sourceware.org
X-BeenThere: gcc-help@gcc.gnu.org
X-Mailman-Version: 2.1.29
Precedence: list
List-Id: Gcc-help mailing list <gcc-help.gcc.gnu.org>
List-Unsubscribe: <https://gcc.gnu.org/mailman/options/gcc-help>,
 <mailto:gcc-help-request@gcc.gnu.org?subject=unsubscribe>
List-Archive: <https://gcc.gnu.org/pipermail/gcc-help/>
List-Post: <mailto:gcc-help@gcc.gnu.org>
List-Help: <mailto:gcc-help-request@gcc.gnu.org?subject=help>
List-Subscribe: <https://gcc.gnu.org/mailman/listinfo/gcc-help>,
 <mailto:gcc-help-request@gcc.gnu.org?subject=subscribe>
X-List-Received-Date: Tue, 28 Jun 2022 13:06:21 -0000

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