[PATCH] ARCv2: spinlock/rwlock/atomics: reduce 1 instruction in exponential backoff

[PATCH] ARCv2: spinlock/rwlock/atomics: reduce 1 instruction in exponential backoff

[Vineet Gupta]

The increment of delay counter was 2 instructions:
Arithmatic Shfit Left (ASL) + set to 1 on overflow

This can be done in 1 using ROtate Left (ROL)

Suggested-by: Nigel Topham <ntopham@synopsys.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
---
 arch/arc/include/asm/atomic.h   | 3 +--
 arch/arc/include/asm/spinlock.h | 3 +--
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/arch/arc/include/asm/atomic.h b/arch/arc/include/asm/atomic.h
index 629dfd0a0c6b..87d18ae53115 100644
--- a/arch/arc/include/asm/atomic.h
+++ b/arch/arc/include/asm/atomic.h
@@ -34,8 +34,7 @@
 	"	mov	%[tmp], %[delay]	\n"	/* tmp = delay */	\
 	"2: 	brne.d	%[tmp], 0, 2b		\n"	/* while (tmp != 0) */	\
 	"	sub	%[tmp], %[tmp], 1	\n"	/* tmp-- */		\
-	"	asl.f	%[delay], %[delay], 1	\n"	/* delay *= 2 */	\
-	"	mov.z	%[delay], 1		\n"	/* handle overflow */	\
+	"	rol	%[delay], %[delay]	\n"	/* delay *= 2 */	\
 	"	b	1b			\n"	/* start over */	\
 	"4: ; --- success ---			\n"				\
 
diff --git a/arch/arc/include/asm/spinlock.h b/arch/arc/include/asm/spinlock.h
index 7071fc0da56a..db8c59d1eaeb 100644
--- a/arch/arc/include/asm/spinlock.h
+++ b/arch/arc/include/asm/spinlock.h
@@ -260,8 +260,7 @@ static inline void arch_write_unlock(arch_rwlock_t *rw)
 	"	mov	%[tmp], %[delay]	\n"	/* tmp = delay */	\
 	"2: 	brne.d	%[tmp], 0, 2b		\n"	/* while (tmp != 0) */	\
 	"	sub	%[tmp], %[tmp], 1	\n"	/* tmp-- */		\
-	"	asl.f	%[delay], %[delay], 1	\n"	/* delay *= 2 */	\
-	"	mov.z	%[delay], 1		\n"	/* handle overflow */	\
+	"	rol	%[delay], %[delay]	\n"	/* delay *= 2 */	\
 	"	b	1b			\n"	/* start over */	\
 	"					\n"				\
 	"4: ; --- done ---			\n"				\
-- 
1.9.1

Re: [PATCH] ARCv2: spinlock/rwlock/atomics: reduce 1 instruction in exponential backoff

[Peter Zijlstra]

On Fri, Aug 07, 2015 at 01:52:19PM +0530, Vineet Gupta wrote:
> The increment of delay counter was 2 instructions:
> Arithmatic Shfit Left (ASL) + set to 1 on overflow
> 
> This can be done in 1 using ROtate Left (ROL)

Cute :-)