Re: [PATCH v3 1/6] xen/arm: Skip initializing the BSS section when it is empty

[Frediano Ziglio <frediano.ziglio@xxxxxxxxx>]

On Thu, Oct 10, 2024 at 3:05 PM Ayan Kumar Halder
<ayan.kumar.halder@xxxxxxx> wrote:
>
> If the BSS section is empty, then the function can just return.
>
> Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
> ---
> Changes from :-
>
> v1..v2 - New patch introduced in v3.
>
>  xen/arch/arm/arm64/head.S | 2 ++
>  1 file changed, 2 insertions(+)
>
> diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> index 14c3720d80..72c7b24498 100644
> --- a/xen/arch/arm/arm64/head.S
> +++ b/xen/arch/arm/arm64/head.S
> @@ -346,6 +346,8 @@ FUNC_LOCAL(zero_bss)
>          PRINT("- Zero BSS -\r\n")
>          ldr   x0, =__bss_start       /* x0 := vaddr(__bss_start) */
>          ldr   x1, =__bss_end         /* x1 := vaddr(__bss_end)   */
> +        cmp   x1, x0
> +        beq   skip_bss
>
>  1:      str   xzr, [x0], #8
>          cmp   x0, x1

Why not just transforming the "do while" loop into a "while" one and
just jump to cmp?

Something like (not tested)

diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
index 14c3720d80..987f243578 100644
--- a/xen/arch/arm/arm64/head.S
+++ b/xen/arch/arm/arm64/head.S
@@ -346,9 +346,10 @@ FUNC_LOCAL(zero_bss)
        PRINT("- Zero BSS -\r\n")
        ldr   x0, =__bss_start       /* x0 := vaddr(__bss_start) */
        ldr   x1, =__bss_end         /* x1 := vaddr(__bss_end)   */
+        b     2f

1:      str   xzr, [x0], #8
-        cmp   x0, x1
+2:      cmp   x0, x1
        b.lo  1b

skip_bss:

Frediano