X-Recipient: archive-cygwin AT delorie DOT com
X-SWARE-Spam-Status: No, hits=-2.5 required=5.0	tests=AWL,BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,FREEMAIL_FROM,RCVD_IN_DNSWL_LOW
X-Spam-Check-By: sourceware.org
Message-ID: <4EF1F937.9040107@gmail.com>
Date: Wed, 21 Dec 2011 15:20:23 +0000
From: Dave Korn <dave DOT korn DOT cygwin AT gmail DOT com>
User-Agent: Thunderbird 2.0.0.17 (Windows/20080914)
MIME-Version: 1.0
To: cygwin AT cygwin DOT com
Subject: Re: 16 byte pthread stack alignments
References: <Pine DOT CYG DOT 4 DOT 58 DOT 1112201743030 DOT 3964 AT PC1163-8460-XP DOT flightsafety DOT com> <20111221094211 DOT GH23547 AT calimero DOT vinschen DOT de>
In-Reply-To: <20111221094211.GH23547@calimero.vinschen.de>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
X-IsSubscribed: yes
Mailing-List: contact cygwin-help AT cygwin DOT com; run by ezmlm
List-Id: <cygwin.cygwin.com>
List-Subscribe: <mailto:cygwin-subscribe AT cygwin DOT com>
List-Archive: <http://sourceware.org/ml/cygwin/>
List-Post: <mailto:cygwin AT cygwin DOT com>
List-Help: <mailto:cygwin-help AT cygwin DOT com>, <http://sourceware.org/ml/#faqs>
Sender: cygwin-owner AT cygwin DOT com
Mail-Followup-To: cygwin AT cygwin DOT com
Delivered-To: mailing list cygwin AT cygwin DOT com

On 21/12/2011 09:42, Corinna Vinschen wrote:

> But OTOH I have to admit that I don't see how this alignment business
> worked at all.  Aligning the stack to 16 byte in mainCRTStartup doesn't
> guarantee that the stack is still 16 byte aligned in main().  If that
> worked so far, it seems like a miracle.  The call stack looks like this:
> 
>   mainCRTStartup
>   -> cygwin_crt0
>      -> _dll_crt0
>         -> _main_tls->call
> 	   -> _main_tls->call2
> 	      -> dll_crt0_1
> 	         -> main
> 
> Every function on the stack changes the stack pointer.  How did that
> work?  Coincidence?
> 
> And then again, isn't it gcc's job to make sure that the generated code
> makes sure the stack is correctly aligned for certain opcodes?
> 
> What am I missing?

  GCC assumes that the stack starts off 16-aligned when the OS hands over to
the exe's entrypoint, and then makes sure it stays that way by always rounding
stack frame sizes up to the nearest multiple of 16.  Or at any rate that's how
it's supposed to work.

    cheers,
      DaveK


--
Problem reports:       http://cygwin.com/problems.html
FAQ:                   http://cygwin.com/faq/
Documentation:         http://cygwin.com/docs.html
Unsubscribe info:      http://cygwin.com/ml/#unsubscribe-simple