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Re: [PATCH v3 1/6] xen/arm: Skip initializing the BSS section when it is empty
- To: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>, Ayan Kumar Halder <ayan.kumar.halder@xxxxxxx>
- From: Ayan Kumar Halder <ayankuma@xxxxxxx>
- Date: Thu, 10 Oct 2024 18:29:16 +0100
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
- Delivery-date: Thu, 10 Oct 2024 17:29:33 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
Hi Frediano,
Appreciate your feedback.
On 10/10/2024 15:43, Frediano Ziglio wrote:
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
I am not really sure if this implementation is better than the previous
one. The drawback of my implementation is that we have an extra 'cmp'
instruction. The drawback of this implementation is that we need an
extra label and there is an un-conditional branch after ldr (readability
is difficult). May be I am biased. :)
How does this look ?
FUNC_LOCAL(zero_bss)
/* Zero BSS only when requested */
cbnz x26, skip_bss
PRINT("- Zero BSS -\r\n")
ldr x0, =__bss_start /* x0 := vaddr(__bss_start) */
ldr x1, =__bss_end /* x1 := vaddr(__bss_end) */
1: cmp x1, x0
beq skip_bss
str xzr, [x0], #8
b 1b
skip_bss:
ret
END(zero_bss)
- Ayan
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