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

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.

Cheers
---Dave