linux-kernel - [PATCH 4/4] parisc: checksum: Optimize from32to16

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Date: Wed, 21 Feb 2024 18:37:14 -0800
From: Charlie Jenkins <charlie@...osinc.com>
To: "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>, 
 Helge Deller <deller@....de>, Arnd Bergmann <arnd@...db.de>, 
 Andrew Morton <akpm@...ux-foundation.org>, 
 Guenter Roeck <linux@...ck-us.net>
Cc: linux-parisc@...r.kernel.org, linux-kernel@...r.kernel.org, 
 linux-arch@...r.kernel.org, Charlie Jenkins <charlie@...osinc.com>
Subject: [PATCH 4/4] parisc: checksum: Optimize from32to16

Replace the shifting and masking of x with a rotation. This generates
better assembly.

Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
---
 arch/parisc/lib/checksum.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/arch/parisc/lib/checksum.c b/arch/parisc/lib/checksum.c
index eaa660491e24..1ae8cc730d13 100644
--- a/arch/parisc/lib/checksum.c
+++ b/arch/parisc/lib/checksum.c
@@ -27,11 +27,8 @@

 static inline unsigned short from32to16(unsigned int x)
 {
-	/* 32 bits --> 16 bits + carry */
-	x = (x & 0xffff) + (x >> 16);
-	/* 16 bits + carry --> 16 bits including carry */
-	x = (x & 0xffff) + (x >> 16);
-	return (unsigned short)x;
+	x += ror32(x, 16);
+	return (unsigned short)(x >> 16);
 }

 unsigned int do_csum(const unsigned char *buff, int len)

-- 
2.34.1