Re: Possible incorrect usage of STACKALIGN in kern_exec

[Matthew Mondor <mm_lists%pulsar-zone.net@localhost>]

On Tue, 24 Jan 2012 21:01:49 +0100
Martin Husemann <martin%duskware.de@localhost> wrote:

> On Tue, Jan 24, 2012 at 08:21:42PM +0100, Paul Fleischer wrote:
> > Is the usage of STACKALIGN indeed incorrect in this situation, or am I
> > missing the big picture?
> 
> I stumbled across this when revamping execve1 for posix_spawn recently.
> 
> The intention seems to be to align the stack on a 8 byte boundary
> (where arm usualy only requires 4 byte alignment). I did not dig in the
> ARM ABI docs deep enough to see why this would be needed.
> 
> However, the current implementation seems to be broken - the macro works
> on the stack pointer but not on a length variable, as you noted.
> 
> Can anyone explain why arm would need 8 byte alignment?

Do some architectures (i.e. x86) have better performance if the stack
is 16-bytes aligned?  If so, perhaps that this could be MI, satisfying
both 8-bytes (or 4-bytes) alignment, by aligning stacks at 16-bytes?
Would this be considered wasteful?  Of course, x86-64 MD code could
also be used...

There is also a related PR but which is for threads stack alignment:
lib/39465

Thanks,
-- 
Matt