[U-Boot] [PATCH] arm: New stack pointer is already aligned

[Albert ARIBAUD]

Hi Wolfgang,

On Fri, 29 Nov 2013 07:14:13 +0100, Wolfgang Denk <wd at denx.de> wrote:

> Dear Tom Rini,
> 
> In message <1385580930-9830-1-git-send-email-trini at ti.com> you wrote:
> > The code in arch/arm/lib/board.c::board_init_f that sets
> > gd->start_addr_sp has already make sure we're 8-byte aligned, so we
> > don't need to do that again.
> > 
> > Cc: Albert ARIBAUD <albert.u.boot at aribaud.net>
> > Signed-off-by: Tom Rini <trini at ti.com>
> > ---
> >  arch/arm/lib/crt0.S |    1 -
> >  1 file changed, 1 deletion(-)
> > 
> > diff --git a/arch/arm/lib/crt0.S b/arch/arm/lib/crt0.S
> > index ac54b93..6b5ec01 100644
> > --- a/arch/arm/lib/crt0.S
> > +++ b/arch/arm/lib/crt0.S
> > @@ -82,7 +82,6 @@ ENTRY(_main)
> >   */
> >  
> >  	ldr	sp, [r9, #GD_START_ADDR_SP]	/* sp = gd->start_addr_sp */
> > -	bic	sp, sp, #7	/* 8-byte alignment for ABI compliance */
> >  	ldr	r9, [r9, #GD_BD]		/* r9 = gd->bd */
> >  	sub	r9, r9, #GD_SIZE		/* new GD is below bd */
> 
> I recommend to keep this instruction. It's just a bit of defensive
> programming, and removing it does not save any measurable amount of
> memory footprint nor execution time.

I would even go further: it is the setting of SP in C code which
should not be kept. I doubt the C specification mentions what should /
might happen when changing the stack pointer on the fly in code which
might need the stack at any point.
 
> Best regards,
> 
> Wolfgang Denk

Amicalement,
-- 
Albert.