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

From: Daniel Thompson
Date: Wed Jul 27 2016 - 07:43:03 EST

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:

ldr x0, [sp]
ldr x0, [x0]

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)!