X-Recipient: archive-cygwin AT delorie DOT com
X-Spam-Check-By: sourceware.org
Date: Wed, 21 Dec 2011 16:41:04 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: 16 byte pthread stack alignments
Message-ID: <20111221154104.GB11841@calimero.vinschen.de>
Reply-To: cygwin AT cygwin DOT com
Mail-Followup-To: cygwin AT cygwin DOT com
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> <4EF1F937 DOT 9040107 AT gmail DOT com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <4EF1F937.9040107@gmail.com>
User-Agent: Mutt/1.5.21 (2010-09-15)
Mailing-List: contact cygwin-help AT cygwin DOT com; run by ezmlm
Precedence: bulk
List-Id: <cygwin.cygwin.com>
List-Unsubscribe: <mailto:cygwin-unsubscribe-archive-cygwin=delorie DOT com AT cygwin DOT 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 Dec 21 15:20, Dave Korn wrote:
> 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.

Ok.  Does that mean my patch from
http://cygwin.com/ml/cygwin/2011-12/msg00435.html should be the right
thing to do for pthreads?  I guess I will have to do the same in
_dll_crt0 then...


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

--
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