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Date: Mon, 9 Jan 2012 09:41:07 -0600
From: Brian Ford <Brian DOT Ford AT FlightSafety DOT com>
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Subject: Re: 16 byte pthread stack alignments
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On Mon, 9 Jan 2012, Corinna Vinschen wrote:

> On Dec 27 18:06, Brian Ford wrote:
> > On Fri, 23 Dec 2011, Corinna Vinschen wrote:
> >
> > > Sorry, but what I don't get from your reply is if the andl worked or
> > > not.
> >
> > No; by itself, it does not.  Adding a "subl $12, %%esp" following it so
> > that the stack is 16 byte aligned after the thread arg is pushed does
> > work.  There are probably more efficient and/or cleaner ways of doing it
> > though.
> >
> > STC attached, but note that it seems to always pass with gcc-4.  Only gcc
> > 3.4.4 appears to require the extra alignment.
>
> Ok, this is even more puzzeling.  The thread function called from the
> thread_wrapper function is NOT the application thread function, but the
> Cygwin internal function thread_init_wrapper.  Given that this function
> is built with the same gcc 4.x compiler as the rest of Cygwin, how on
> earth can this fail at all? Shouldn't the alignment be always correct on
> the subsequent call to the application function, given that gcc-4 is
> supposed to care?

I'm speculating, but I believe gcc-4 only re-aligns the stack in case an
instruction in that function requires more strict alignment than the
default ABI to save overhead.

-- 
Brian Ford
Staff Realtime Software Engineer
VITAL - Visual Simulation Systems
FlightSafety International
the best safety device in any aircraft is a well-trained crew...

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