linux-kernel - [PATCH v14 1/5] asm-generic: Improve csum

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Message-Id: <20231227-optimize_checksum-v14-1-ddfd48016566@rivosinc.com>
Date: Wed, 27 Dec 2023 09:38:00 -0800
From: Charlie Jenkins <charlie@...osinc.com>
To: Charlie Jenkins <charlie@...osinc.com>, 
 Palmer Dabbelt <palmer@...belt.com>, Conor Dooley <conor@...nel.org>, 
 Samuel Holland <samuel.holland@...ive.com>, 
 David Laight <David.Laight@...lab.com>, Xiao Wang <xiao.w.wang@...el.com>, 
 Evan Green <evan@...osinc.com>, Guo Ren <guoren@...nel.org>, 
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
 linux-arch@...r.kernel.org
Cc: Paul Walmsley <paul.walmsley@...ive.com>, 
 Albert Ou <aou@...s.berkeley.edu>, Arnd Bergmann <arnd@...db.de>, 
 David Laight <david.laight@...lab.com>
Subject: [PATCH v14 1/5] asm-generic: Improve csum_fold

This csum_fold implementation introduced into arch/arc by Vineet Gupta
is better than the default implementation on at least arc, x86, and
riscv. Using GCC trunk and compiling non-inlined version, this
implementation has 41.6667%, 25% fewer instructions on riscv64, x86-64
respectively with -O3 optimization. Most implmentations override this
default in asm, but this should be more performant than all of those
other implementations except for arm which has barrel shifting and
sparc32 which has a carry flag.

Signed-off-by: Charlie Jenkins <charlie@...osinc.com>
Reviewed-by: David Laight <david.laight@...lab.com>
---
 include/asm-generic/checksum.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/asm-generic/checksum.h b/include/asm-generic/checksum.h
index 43e18db89c14..ad928cce268b 100644
--- a/include/asm-generic/checksum.h
+++ b/include/asm-generic/checksum.h
@@ -2,6 +2,8 @@
 #ifndef __ASM_GENERIC_CHECKSUM_H
 #define __ASM_GENERIC_CHECKSUM_H
 
+#include <linux/bitops.h>
+
 /*
  * computes the checksum of a memory block at buff, length len,
  * and adds in "sum" (32-bit)
@@ -31,9 +33,7 @@ extern __sum16 ip_fast_csum(const void *iph, unsigned int ihl);
 static inline __sum16 csum_fold(__wsum csum)
 {
 	u32 sum = (__force u32)csum;
-	sum = (sum & 0xffff) + (sum >> 16);
-	sum = (sum & 0xffff) + (sum >> 16);
-	return (__force __sum16)~sum;
+	return (__force __sum16)((~sum - ror32(sum, 16)) >> 16);
 }
 #endif
 

-- 
2.43.0