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

From: Dave Martin
Date: Wed Jul 27 2016 - 07:38:21 EST

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.