From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-return-157402-listarch-gcc=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 19293 invoked by alias); 22 Oct 2009 09:12:06 -0000
Received: (qmail 19284 invoked by uid 22791); 22 Oct 2009 09:12:05 -0000
X-SWARE-Spam-Status: No, hits=-1.8 required=5.0 	tests=AWL,BAYES_00,SARE_MSGID_LONG40,SPF_PASS
X-Spam-Check-By: sourceware.org
Received: from mail-vw0-f178.google.com (HELO mail-vw0-f178.google.com) (209.85.212.178)     by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Thu, 22 Oct 2009 09:11:59 +0000
Received: by vws8 with SMTP id 8so5043636vws.0         for <gcc@gcc.gnu.org>; Thu, 22 Oct 2009 02:11:57 -0700 (PDT)
MIME-Version: 1.0
Received: by 10.220.126.205 with SMTP id d13mr3997347vcs.24.1256202717358;  	Thu, 22 Oct 2009 02:11:57 -0700 (PDT)
In-Reply-To: <649555d50910212331ne851950j3ad525632218f789@mail.gmail.com>
References: <649555d50910212331ne851950j3ad525632218f789@mail.gmail.com>
Date: Thu, 22 Oct 2009 17:13:00 -0000
Message-ID: <84fc9c000910220211k7595d1b1q2918b9b32fe72032@mail.gmail.com>
Subject: Re: Dead Store Elimination
From: Richard Guenther <richard.guenther@gmail.com>
To: Pranav Bhandarkar <pranav.bhandarkar@gmail.com>
Cc: gcc@gcc.gnu.org
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable
X-IsSubscribed: yes
Mailing-List: contact gcc-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc/>
List-Post: <mailto:gcc@gcc.gnu.org>
List-Help: <http://gcc.gnu.org/ml/>
Sender: gcc-owner@gcc.gnu.org
X-SW-Source: 2009-10/txt/msg00448.txt.bz2

On Thu, Oct 22, 2009 at 8:31 AM, Pranav Bhandarkar
<pranav.bhandarkar@gmail.com> wrote:
> Hi,
>
> A possible silly question about the dead store elimination pass. From
> the documentation it is clear that the store S1 below is removed by
> this pass (in dse.c)
>
> *(addr) =3D value1; =A0 =A0 =A0 =A0 =A0// S1
> .....
> .....
> *(addr) =3D value2 =A0 =A0 =A0 =A0 =A0// S2 .. No read of "addr" between =
S1 and S2.
> ......
> =A0 =A0 =A0 =A0 =A0 =A0 =3D *(addr) =A0 =A0 =A0 // Load
> .......
> end_of_the_function
>
> However, consider a different example.
>
> *(addr) =3D value1; =A0 =A0 =A0// S1
> ......
> .....
> end_of_the_function.
>
> i.e. there is no store Sn that follows S1 along any path from S1 to
> the end of the function and there is no read of addr following S1
> either. Is the dse pass expected to remove such stores ? (I am
> inclined to think that it should, but I am seeing a case where dse
> doesnt remove such stores) . Further is the behaviour expected to be
> different if the "addr" is based on =A0"fp" ?

Are you talking about the tree dead-store elimination pass or
the RTL one?  Basically *addr =3D value1; cannot be removed
if addr does not point to local memory or if the pointed-to
memory escapes through a call-site that is dominated by this store.

Richard.