From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-bugs-return-480314-listarch-gcc-bugs=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 114860 invoked by alias); 13 Mar 2015 20:18:37 -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
Received: (qmail 114597 invoked by uid 48); 13 Mar 2015 20:18:33 -0000
From: "sje at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/59371] [4.8/4.9/5 Regression] Performance regression in GCC 4.8 and later versions.
Date: Fri, 13 Mar 2015 20:18:00 -0000
X-Bugzilla-Reason: CC
X-Bugzilla-Type: changed
X-Bugzilla-Watch-Reason: None
X-Bugzilla-Product: gcc
X-Bugzilla-Component: target
X-Bugzilla-Version: 4.9.0
X-Bugzilla-Keywords: missed-optimization
X-Bugzilla-Severity: normal
X-Bugzilla-Who: sje at gcc dot gnu.org
X-Bugzilla-Status: NEW
X-Bugzilla-Priority: P2
X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org
X-Bugzilla-Target-Milestone: 4.8.5
X-Bugzilla-Flags:
X-Bugzilla-Changed-Fields:
Message-ID: <bug-59371-4-VlIfHYmBBb@http.gcc.gnu.org/bugzilla/>
In-Reply-To: <bug-59371-4@http.gcc.gnu.org/bugzilla/>
References: <bug-59371-4@http.gcc.gnu.org/bugzilla/>
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 7bit
X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/
Auto-Submitted: auto-generated
MIME-Version: 1.0
X-SW-Source: 2015-03/txt/msg01458.txt.bz2

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59371

--- Comment #15 from Steve Ellcey <sje at gcc dot gnu.org> ---
I am not sure yet where and how to improve this automatically but I have found
an interesting hand optimization that could point to a way to fix this.  If I
change the original function:

int foo(int *p, unsigned short c)
{
    signed short i;
    int x = 0;
    for (i = 0; i < c; i++) {
        x = x + *p; p++;
    }
    return x;
}

To:

int foo(int *p, unsigned short c)
{
    signed short i;
    unsigned short new_i;
    int x = 0;

    if (c > 32767)
        for (i = 0; i < c; i++) {
            x = x + *p; p++;
        }
    else
        for (new_i = 0; new_i < c; new_i++) {
            x = x + *p; p++;
        }
    return x;
}


Then GCC 5.0 generates an empty infinite loop for the first for loop and
a compact 4 instruction loop (better even then 4.7) for the second for loop.

I am not sure where or if we can do this optimization in GCC but I am going
to investigate some more.