From owner-freebsd-hackers  Wed Jun 12 11:15: 9 2002
Delivered-To: freebsd-hackers@freebsd.org
Received: from warspite.cnchost.com (warspite.concentric.net [207.155.248.9])
	by hub.freebsd.org (Postfix) with ESMTP
	id E6B6F37B405; Wed, 12 Jun 2002 11:15:02 -0700 (PDT)
Received: from bitblocks.com (adsl-209-204-185-216.sonic.net [209.204.185.216])
	by warspite.cnchost.com
	id OAA26361; Wed, 12 Jun 2002 14:15:02 -0400 (EDT)
	[ConcentricHost SMTP Relay 1.14]
Message-ID: <200206121815.OAA26361@warspite.cnchost.com>
To: hackers@freebsd.org
Cc: jdp@freebsd.org
Subject: linker bug?
Date: Wed, 12 Jun 2002 11:14:59 -0700
From: Bakul Shah <bakul@bitblocks.com>
Sender: owner-freebsd-hackers@FreeBSD.ORG
Precedence: bulk
List-ID: <freebsd-hackers.FreeBSD.ORG>
List-Archive: <http://docs.freebsd.org/mail/> (Web Archive)
List-Help: <mailto:majordomo@FreeBSD.ORG?subject=help> (List Instructions)
List-Subscribe: <mailto:majordomo@FreeBSD.ORG?subject=subscribe%20freebsd-hackers>
List-Unsubscribe: <mailto:majordomo@FreeBSD.ORG?subject=unsubscribe%20freebsd-hackers>
X-Loop: FreeBSD.ORG

In order to measure call overhead on an Athlon XP system I
compiled and ran the following program and saw some curious
results!

    $ cat foo.c
    #include <stdlib.h>

    void func() { }

    void    (*funp)() = 0;

    int main(int argc, char **argv) {
	    int i, j;
	    if (argv[1][0] != '?')	/* defeat compile-time optimization */
		    funp = &func;
	    i = atoi(argv[1]);
	    for (j = i; j > 0; --j)
		    (*funp) ();
    }
    $ cc -O -fomit-frame-pointer foo.c
    $ time a.out 1000000000
    a.out 1000000000  4.11s user 0.01s system 97% cpu 4.215 total

Then I did a static link and saw the time increase by 10 seconds!

    $ cc -O -fomit-frame-pointer -static foo.c
    $ time a.out 1000000000
    a.out 1000000000  14.28s user 0.01s system 96% cpu 14.759 total

nm reveals the problem.

    $ cc -O -fomit-frame-pointer foo.c && nm a.out |grep func
    08048490 T func
    $ cc -O -fomit-frame-pointer -static foo.c && nm a.out |grep func
    080481c4 T func

Here is what
    void func() {}
gets compiled to:

	    .p2align 2,0x90
    .globl func
		    .type            func,@function
    func:
	    ret

This is on a 4.6-RC system with gcc-2.95.3.  The fact that
func is aligned on a 16 byte boundary in the -dynamic case is
likely conincidental.  gcc-3.1 seems to put it on an 8 byte
with -dynamic and 4 byte boundary with -static.

So the question is: does the linker ignore alignment
altogether or did I miss some magic flag?

-- bakul

To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-hackers" in the body of the message