[PATCH v2 06/15] arm: Replace 'adrl' in EFI crt0

[Ilias Apalodimas]

On Sun, 16 Mar 2025 at 00:18, Sam Edwards <cfsworks at gmail.com> wrote:
>
> LLVM's IAS does not (and cannot easily) support the 'adrl'
> pseudoinstruction, and ARM developers generally do not consider it
> portable across assembler implementations either.
>
> Instead, expand it into the two subtract instructions it would emit
> anyway. An explanation of the math follows:
>
> The .+8 and .+4 refer to the same memory location; this is because the
> .+4 expression occurs in a subsequent instruction, 4 bytes after the
> first. This memory location is the value of the PC register when it is
> read by the first sub instruction. Thus, both inner parenthesized
> expressions evaluate to the same result: PC's offset relative to
> image_base. The subtract instructions then remove one byte each
> (low, then high) of the total offset, thereby getting the absolute
> address of image_base loaded in r0.
>
> Signed-off-by: Sam Edwards <CFSworks at gmail.com>

Reviewed-by: Ilias Apalodimas <ilias.apalodimas at linaro.org>
> ---
>  arch/arm/lib/crt0_arm_efi.S | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm/lib/crt0_arm_efi.S b/arch/arm/lib/crt0_arm_efi.S
> index 91b0fe12c51..235b3a0c48f 100644
> --- a/arch/arm/lib/crt0_arm_efi.S
> +++ b/arch/arm/lib/crt0_arm_efi.S
> @@ -149,7 +149,8 @@ _start:
>         adr             r1, .L_DYNAMIC
>         ldr             r0, [r1]
>         add             r1, r0, r1
> -       adrl            r0, image_base
> +       sub             r0, pc, #((.+8-image_base) & 0xff)
> +       sub             r0, r0, #((.+4-image_base) & 0xff00)
>         bl              _relocate
>         teq             r0, #0
>         bne             0f
> --
> 2.48.1
>