* Minor arm __syscall_cp_asm optimisation
@ 2019-08-01 6:00 Patrick Oppenlander
2019-08-06 13:07 ` Szabolcs Nagy
0 siblings, 1 reply; 2+ messages in thread
From: Patrick Oppenlander @ 2019-08-01 6:00 UTC (permalink / raw)
To: musl
[-- Attachment #1: Type: text/plain, Size: 431 bytes --]
Hi all,
while debugging something unrelated I noticed a minor optimisation we
can make to __syscall_cp_asm for arm.
Unless I'm missing something the only reason we needed to preserve the
link register was because we were using a branch-link instruction to
branch to __cp_cancel. Replacing this with a branch means we can avoid
the save/restore as the link register is no longer modified.
Patch attached.
Kind regards,
Patrick
[-- Attachment #2: 0001-don-t-unnecessarily-preserve-link-register.patch --]
[-- Type: text/x-patch, Size: 950 bytes --]
From dc8d33fdf483e65ecaa688be6fa2888f914db8cb Mon Sep 17 00:00:00 2001
From: Patrick Oppenlander <patrick.oppenlander@gmail.com>
Date: Thu, 1 Aug 2019 14:34:59 +1000
Subject: [PATCH] don't unnecessarily preserve link register
---
src/thread/arm/syscall_cp.s | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)
diff --git a/src/thread/arm/syscall_cp.s b/src/thread/arm/syscall_cp.s
index a5730c08..e607dd42 100644
--- a/src/thread/arm/syscall_cp.s
+++ b/src/thread/arm/syscall_cp.s
@@ -11,19 +11,19 @@
.type __syscall_cp_asm,%function
__syscall_cp_asm:
mov ip,sp
- stmfd sp!,{r4,r5,r6,r7,lr}
+ stmfd sp!,{r4,r5,r6,r7}
__cp_begin:
ldr r0,[r0]
cmp r0,#0
- blne __cp_cancel
+ bne __cp_cancel
mov r7,r1
mov r0,r2
mov r1,r3
ldmfd ip,{r2,r3,r4,r5,r6}
svc 0
__cp_end:
- ldmfd sp!,{r4,r5,r6,r7,lr}
+ ldmfd sp!,{r4,r5,r6,r7}
bx lr
__cp_cancel:
- ldmfd sp!,{r4,r5,r6,r7,lr}
+ ldmfd sp!,{r4,r5,r6,r7}
b __cancel
--
2.22.0
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: Minor arm __syscall_cp_asm optimisation
2019-08-01 6:00 Minor arm __syscall_cp_asm optimisation Patrick Oppenlander
@ 2019-08-06 13:07 ` Szabolcs Nagy
0 siblings, 0 replies; 2+ messages in thread
From: Szabolcs Nagy @ 2019-08-06 13:07 UTC (permalink / raw)
To: musl
* Patrick Oppenlander <patrick.oppenlander@gmail.com> [2019-08-01 16:00:16 +1000]:
> Hi all,
>
> while debugging something unrelated I noticed a minor optimisation we
> can make to __syscall_cp_asm for arm.
>
> Unless I'm missing something the only reason we needed to preserve the
> link register was because we were using a branch-link instruction to
> branch to __cp_cancel. Replacing this with a branch means we can avoid
> the save/restore as the link register is no longer modified.
the change makes sense to me.
^ permalink raw reply [flat|nested] 2+ messages in thread
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