Re: GCC 4.3.0 Status Report (2008-02-14)

[Mark Mitchell <mark at codesourcery dot com>]

Daniel Jacobowitz wrote:
> On Mon, Feb 18, 2008 at 08:16:39PM -0500, David Edelsohn wrote:
> > 	The Altivec ABI does change the stack alignment.  This should not
> > create an incompatibility for old code, but new code may not receive the
> > stack properly aligned without additional dynamic alignment.
>
> I thought so, too, but Geoff explained to me that this is wrong.  It
> only affects powerpc-eabi.  powerpc-linux already has a sufficiently
> aligned stack; it's just not reflected in STACK_BOUNDARY.
>
> On powerpc-linux, if you do not pass or return vector arguments, the
> only affect of -mabi=altivec is that AltiVec registers are saved and
> restored correctly.

So, if I understand correctly, on Power GNU/Linux, this is actually not 
a scary change at all as things will work monotonically better than they 
did before.  Is that right?

> On powerpc-eabi, we would need to dynamically align the stack for
> -maltivec to behave.

There's certainly somewhat less need for long-term binary compatibility 
on bare-metal targets.  If someone were to need compatibility there, we 
could implement the dynamic stack-alignment.

So, it doesn't sound like there's much to worry about.  Do you agree?

Thanks,

--
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713