llvm dev - Sep 2012 - [LLVMdev] ARM aapcs calling convention for small vectors

Hi all, 


I was wondering if ARM aapcs calling convention defines how to pass small
vectors as parameter to a routine.
By small vectors, I mean with size less than a 32-bit integer. For instance if
we consider following code:

; ModuleID = 'smallvect.ll'
define arm_aapcscc void  @foo(<2 x i8>* %p) {
L.entry:
    %0 = load <2 x i8>* %p
    call arm_aapcscc void @bar(<2 x i8> %0) 
    ret void
}

declare arm_aapcscc void  @bar(<2 x i8> %a) 

and we compile it using llc -march=arm -mcpu=cortex-a9 we got following assembly
generated:

...
foo:                                    @ @foo
@ BB#0:                                 @ %L.entry
	push	{lr}
	vld1.16	{d16[0]}, [r0, :16]
	vmovl.u8	q8, d16
	vmovl.u16	q8, d16
	vmov	r0, r1, d16
	bl	bar
	pop	{lr}
	bx	lr
.Ltmp0:
	.size	foo, .Ltmp0-foo
...

Hi and Low part of vector is passed to bar using r0, r1 register. I was thinking
that only r0 could have been used since size of vector is 16-bits.
So is this behavior occuring because vector argument passing might not have been
defined in ARM aapcs ?

Thanks for your answers
Seb

> I was wondering if ARM aapcs calling convention defines how to pass small
vectors as parameter to a routine.
> By small vectors, I mean with size less than a 32-bit integer.
The AAPCS is silent on the matter. Vectors with size <= 32 aren't
recognised by the PCS so a language wanting to support them with
well-defined semantics (including LLVM) is expected to choose and
specify which machine types they map to (and how!). This is much like
how the C++ ABI specifies how its types map onto PCS-level ones.

I doubt LLVM has made an active choice on this though; it's probably
just relying on default behaviour being good enough to make it work.

Tim.

On 21 September 2012 16:37, Tim Northover <t.p.northover at gmail.com>
wrote:
> I doubt LLVM has made an active choice on this though; it's probably
> just relying on default behaviour being good enough to make it work.
Though, this piece of code could be smarter than that. Patches welcome! ;)

-- 
cheers,
--renato

http://systemcall.org/