Subject: [PATCH 1/2] arm64: add macro to handle large immediates

From: [email-masked] (Mark Rutland)
Date: Wed, 6 Jan 2016 12:21:04 +0000

On Wed, Jan 06, 2016 at 12:15:14PM +0100, Ard Biesheuvel wrote:
> On 6 January 2016 at 12:05, Mark Rutland <[email-masked]> wrote:
> > Sometimes we want to be able to load values greater than 0xff into a
> > register, without placing said values in a literal pool. Arranging for
> > the value to be split up across a number of movz and movk instructions
> > is tedious and error-prone.
> >
> > Following the example of {adr,str,ldr}_l, this patch adds a new mov_l
> > macro which can be used to load immediate values of up to 64 bits into a
> > register.
> >
> > Signed-off-by: Mark Rutland <[email-masked]>
> > Cc: Ard Biesheuvel <[email-masked]>
> > Cc: Catalin Marinas <[email-masked]>
> > Cc: Marc Zyngier <[email-masked]>
> > Cc: Will Deacon <[email-masked]>
> > ---
> >  arch/arm64/include/asm/assembler.h | 13 +++++++++++++
> >  1 file changed, 13 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/assembler.h b/arch/arm64/include/asm/assembler.h
> > index 12eff92..64fd0a2 100644
> > --- a/arch/arm64/include/asm/assembler.h
> > +++ b/arch/arm64/include/asm/assembler.h
> > @@ -193,6 +193,19 @@ lr .req    x30             // link register
> >         str     \src, [\tmp, :lo12:\sym]
> >         .endm
> >
> > +       /*
> > +        * Move a large immediate up to 64-bits.
> > +        *
> > +        * @dst: destination register (64 bit wide)
> > +        * @val: value
> > +        */
> > +       .macro  mov_l, dst, val
> > +       movz    \dst, :abs_g0_nc:\val
> > +       movk    \dst, :abs_g1_nc:\val
> > +       movk    \dst, :abs_g2_nc:\val
> > +       movk    \dst, :abs_g3:\val
> > +       .endm
> > +
> 
> Ack for the general idea, but for correctness, you should pair the
> movk instructions with the _nc relocations (i.e., keep movz first, but
> invert the order of the relocs)

Ah, I hadn't spotted the restriction. I'll change that to:

	movz	\dst, :abs_g3:\val
	movk	\dst, :abs_g2:\val
	movk	\dst, :abs_g1:\val
	movk	\dst, :abs_g0:\val

That raises a related question. Is it the linker's responsibility to
fill in the shift encoding in the hw field as part of the g{3,2,1}
relocs?

Mine seems to, but I don't know if that's strictly required or correct
as the AArrch64 ELF spec only mentions the immediate field for *ABS_G*,
and the shift is encoded in hw rather than imm16.

Thanks,
Mark.