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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
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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

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