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Date: Thu, 22 Dec 2011 12:51:30 -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 Wed, 21 Dec 2011, Corinna Vinschen wrote:

> On second thought I'm a bit puzzled that the pthread stack isn't
> correctly aligned as well.  Ignoring the pthread_attr_setstack case
> which wasn't supported so far anyway, the OS stack set up by
> CreateThread is 64K aligned.  From that 64K aligned StackBase value, I
> subtract 12704 == 0x31a0 bytes.  So the result should be 16 byte
> aligned even without the andl $-16, %%esp.  Why isn't it?!?

It appears it is, here.

> Does anybody care to tell me what's wrong with the assembler code in
> thread_wrapper?

I don't pretend to understand why, but it appears gcc is expecting the
stack to be 16 byte aligned on entry to the called function, which
includes the 4 byte argument and the 4 byte return address in this case.
I could be wrong, but it appears that would do it.

For a long discussion by those who apparently agree with you about how
crazy this is see:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40838

It looks like gcc 4.6 may finally "do the right thing" itself.

-- 
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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