Re: [PATCH v15 04/10] arm64: Kprobes with single stepping support

[Daniel Thompson]

On 27/07/16 12:38, Dave Martin wrote:
> On Wed, Jul 27, 2016 at 12:19:59PM +0100, Daniel Thompson wrote:
>
> [...]
>
> > It is interesting to note that you force large_func() to stack its arguments
> > (by providing 8 dummy int arguments first) then the implicit
> > pass-by-reference behavior is still preserved even for a stacked argument;
> > large_func() ends up as:
> >
> > ~~~
> > large_func:
> > 	ldr	x0, [sp]
> > 	ldr	x0, [x0]
> > 	ret
> > ~~~
> >
> > Only thing is... I *still* haven't found anything in the AArch64 PCS which
> > describes this behavior.
> >
> > I'm coming to believe that this is a mistake and this information (and the
> > threshold at which implicit pass-by-reference kicks in) should be documented
> > in section 7.
>
> Is that answered by this?
>
>     B.3. If the argument type is a Composite Type that is larger than
>     16 bytes, then the argument is copied to memory allocated by the
>     caller and the argument is replaced by a pointer to the copy.
>
> Experimenting with gcc's behaviour seems to back this up.

Absolutely answered by that. Thanks (and sorry for the noise)!


Daniel.