From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-348699-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 4116 invoked by alias); 1 Mar 2011 06:44:55 -0000
Received: (qmail 4106 invoked by uid 22791); 1 Mar 2011 06:44:55 -0000
X-SWARE-Spam-Status: No, hits=-2.8 required=5.0	tests=ALL_TRUSTED,AWL,BAYES_00
X-Spam-Check-By: sourceware.org
Received: from localhost (HELO gcc.gnu.org) (127.0.0.1)    by sourceware.org (qpsmtpd/0.43rc1) with ESMTP; Tue, 01 Mar 2011 06:44:50 +0000
From: "carrot at google dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/47920] strange code generated for expression (a+7)/8
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Keywords:
X-Bugzilla-Severity: normal
X-Bugzilla-Who: carrot at google dot com
X-Bugzilla-Status: UNCONFIRMED
X-Bugzilla-Priority: P3
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: ---
X-Bugzilla-Changed-Fields:
Message-ID: <bug-47920-4-f7ShwS83Ma@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-47920-4@http.gcc.gnu.org/bugzilla/>
References: <bug-47920-4@http.gcc.gnu.org/bugzilla/>
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
Content-Type: text/plain; charset="UTF-8"
MIME-Version: 1.0
Date: Tue, 01 Mar 2011 06:44:00 -0000
Mailing-List: contact gcc-bugs-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Id: <gcc-bugs.gcc.gnu.org>
List-Archive: <http://gcc.gnu.org/ml/gcc-bugs/>
List-Post: <mailto:gcc-bugs@gcc.gnu.org>
List-Help: <mailto:gcc-bugs-help@gcc.gnu.org>
Sender: gcc-bugs-owner@gcc.gnu.org
X-SW-Source: 2011-03/txt/msg00012.txt.bz2

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47920

--- Comment #3 from Carrot <carrot at google dot com> 2011-03-01 06:44:47 UTC ---
(In reply to comment #1)
> Presumably because arithmetic right-shift by 3 isn't the same as a division by
> 8 when (a+7) is negative.  Changing the types to unsigned gives the code you
> want.

You are right. Right shift and division are different for negative numbers.

Thumb1 uses a different code sequence since it doesn't support conditional
execution

t08:
    add    r0, r0, #7
    asr    r3, r0, #31
    lsr    r3, r3, #29
    add    r0, r3, r0
    asr    r0, r0, #3
    bx    lr

If we use the similar code sequence in thumb2 we can reduce one instruction
because the logical shift and the following add can be merged into one
instruction

t08:
    add    r0, r0, #7
    asr    r3, r0, #31
    add    r0, r0, r3, LSR #29
    asr    r0, r0, #3
    bx    lr

I wonder if this will be faster.