From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-help-return-54504-listarch-gcc-help=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 3866 invoked by alias); 6 Dec 2013 10:10:38 -0000
Mailing-List: contact gcc-help-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-help.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-help/>
List-Post: <mailto:gcc-help@gcc.gnu.org>
List-Help: <mailto:gcc-help-help@gcc.gnu.org>
Sender: gcc-help-owner@gcc.gnu.org
Received: (qmail 3777 invoked by uid 89); 6 Dec 2013 10:10:37 -0000
Authentication-Results: sourceware.org; auth=none
X-Virus-Found: No
X-Spam-SWARE-Status: No, score=-0.4 required=5.0 tests=AWL,BAYES_00,FREEMAIL_FROM,RCVD_IN_DNSWL_LOW,SPF_PASS autolearn=ham version=3.3.2
X-Spam-User: qpsmtpd, 2 recipients
X-HELO: mail-wg0-f49.google.com
Received: from Unknown (HELO mail-wg0-f49.google.com) (74.125.82.49) by sourceware.org (qpsmtpd/0.93/v0.84-503-g423c35a) with (AES128-SHA encrypted) ESMTPS; Fri, 06 Dec 2013 10:10:36 +0000
Received: by mail-wg0-f49.google.com with SMTP id x12so425518wgg.16        for <multiple recipients>; Fri, 06 Dec 2013 02:10:27 -0800 (PST)
MIME-Version: 1.0
X-Received: by 10.194.206.5 with SMTP id lk5mr2293752wjc.46.1386324627343; Fri, 06 Dec 2013 02:10:27 -0800 (PST)
Received: by 10.195.12.114 with HTTP; Fri, 6 Dec 2013 02:10:27 -0800 (PST)
In-Reply-To: <CADn89gRZPDo1Z4gvime-PTC9aaeO6G5jgbN+0hOSZrnD8M1vtw@mail.gmail.com>
References: <CADn89gRZPDo1Z4gvime-PTC9aaeO6G5jgbN+0hOSZrnD8M1vtw@mail.gmail.com>
Date: Fri, 06 Dec 2013 10:10:00 -0000
Message-ID: <CAFiYyc0kZGi5XgXikvhqH5kHCXgBc=HtrB5OXHrqv2Z+HdNmhg@mail.gmail.com>
Subject: Re: x86 gcc lacks simple optimization
From: Richard Biener <richard.guenther@gmail.com>
To: Konstantin Vladimirov <konstantin.vladimirov@gmail.com>
Cc: GCC Development <gcc@gcc.gnu.org>, GCC-help <gcc-help@gcc.gnu.org>
Content-Type: text/plain; charset=ISO-8859-1
X-SW-Source: 2013-12/txt/msg00041.txt.bz2

On Fri, Dec 6, 2013 at 9:30 AM, Konstantin Vladimirov
<konstantin.vladimirov@gmail.com> wrote:
> Hi,
>
> Consider code:
>
> int foo(char *t, char *v, int w)
> {
> int i;
>
> for (i = 1; i != w; ++i)
> {
> int x = i << 2;
> v[x + 4] = t[x + 4];
> }
>
> return 0;
> }
>
> Compile it to x86 (I used both gcc 4.7.2 and gcc 4.8.1) with options:
>
> gcc -O2 -m32 -S test.c
>
> You will see loop, formed like:
>
> .L5:
> leal 0(,%eax,4), %edx
> addl $1, %eax
> movzbl 4(%edi,%edx), %ecx
> cmpl %ebx, %eax
> movb %cl, 4(%esi,%edx)
> jne .L5
>
> But it can be easily simplified to something like this:
>
> .L5:
> addl $1, %eax
> movzbl (%esi,%eax,4), %edx
> cmpl %ecx, %eax
> movb %dl, (%ebx,%eax,4)
> jne .L5
>
> (i.e. left shift may be moved to address).
>
> First question to gcc-help maillist. May be there are some options,
> that I've missed, and there IS a way to explain gcc my intention to do
> this?
>
> And second question to gcc developers mail list. I am working on
> private backend and want to add this optimization to my backend. What
> do you advise me to do -- custom gimple pass, or rtl pass, or modify
> some existent pass, etc?

This looks like a deficiency in induction variable optimization.  Note
that i << 2 may overflow and this overflow does not invoke undefined
behavior but is in the implementation defined behavior category.

The issue in this case is likely that the SCEV infrastructure does not handle
left-shifts.

Richard.

> ---
> With best regards, Konstantin