Re: [Xen-devel] [PATCH v2 19/35] xen/arm32: head: Add a macro to move an immediate constant into a 32-bit register

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 22 Jul 2019, Julien Grall wrote:
> The current boot code is using the pattern ldr rX, =... to move an
> immediate constant into a 32-bit register.
> 
> This pattern implies to load the immediate constant from a literal pool,
> meaning a memory access will be performed.
> 
> The memory access can be avoided by using movw/movt instructions.
> 
> A new macro is introduced to move an immediate constant into a 32-bit
> register without a memory load. Follow-up patches will make use of it.
> 
> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>

That's interesting!

Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
>     Changes in v2:
>         - Patch added
> ---
>  xen/arch/arm/arm32/head.S | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/xen/arch/arm/arm32/head.S b/xen/arch/arm/arm32/head.S
> index 18ded49a04..99f4af18d8 100644
> --- a/xen/arch/arm/arm32/head.S
> +++ b/xen/arch/arm/arm32/head.S
> @@ -37,6 +37,15 @@
>  #endif
>  
>  /*
> + * Move an immediate constant into a 32-bit register using movw/movt
> + * instructions.
> + */
> +.macro mov_w reg, word
> +        movw  \reg, #:lower16:\word
> +        movt  \reg, #:upper16:\word
> +.endm
> +
> +/*
>   * Common register usage in this file:
>   *   r0  -
>   *   r1  -
> -- 
> 2.11.0
> 

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