From owner-freebsd-chat  Tue Jul 24 12:14:49 2001
Delivered-To: freebsd-chat@freebsd.org
Received: from meow.osd.bsdi.com (meow.osd.bsdi.com [204.216.28.88])
	by hub.freebsd.org (Postfix) with ESMTP
	id 0AB7437B405; Tue, 24 Jul 2001 12:14:46 -0700 (PDT)
	(envelope-from jhb@FreeBSD.org)
Received: from laptop.baldwin.cx (john@jhb-laptop.osd.bsdi.com [204.216.28.241])
	by meow.osd.bsdi.com (8.11.4/8.11.2) with ESMTP id f6OJDNv22533;
	Tue, 24 Jul 2001 12:13:23 -0700 (PDT)
	(envelope-from jhb@FreeBSD.org)
Message-ID: <XFMail.010724121328.jhb@FreeBSD.org>
X-Mailer: XFMail 1.4.0 on FreeBSD
X-Priority: 3 (Normal)
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 8bit
MIME-Version: 1.0
In-Reply-To: <20010724200251.A72789@dogma.freebsd-uk.eu.org>
Date: Tue, 24 Jul 2001 12:13:28 -0700 (PDT)
From: John Baldwin <jhb@FreeBSD.org>
To: j mckitrick <jcm@FreeBSD-uk.eu.org>
Subject: Re: stack use preference
Cc: Greg Lehey <grog@FreeBSD.org>, freebsd-chat@FreeBSD.org
Sender: owner-freebsd-chat@FreeBSD.ORG
Precedence: bulk
List-ID: <freebsd-chat.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-chat>
List-Unsubscribe: <mailto:majordomo@FreeBSD.ORG?subject=unsubscribe%20freebsd-chat>
X-Loop: FreeBSD.org


On 24-Jul-01 j mckitrick wrote:
>| Yes.  On x86, doing
>| 
>|         enter $8, $0    ; 8 bytes of local storage
>| 
>| is equivalent to:
>| 
>|         push %ebp
>|         mov %esp, %ebp
>|         sub $8, %esp
>| 
>| but most compilers that I've seen unroll 'enter' rather than using it
>| directly.
>| *shrug*
> 
> From what I have discovered, 'enter' uses more cycles than the corresponding
> instructions, and yet 'leave' does not.

That is what I seem to recall as well.  On the 386, enter $8, $0 would take 10
cycles, whereas the separate instructions would take 2 + 2 + 2 = 6 cycles. 
Granted these instructions all have RAW dependencies, and I'm not sure if that
would wreak havoc in the form of register stalls on 686+ or not which might
drown out the 1 or 2 cycles saved on 686+.

-- 

John Baldwin <jhb@FreeBSD.org> -- http://www.FreeBSD.org/~jhb/
PGP Key: http://www.baldwin.cx/~john/pgpkey.asc
"Power Users Use the Power to Serve!"  -  http://www.FreeBSD.org/

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