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From:   Charlie Jenkins <charlie@rivosinc.com>
Date:   Thu, 14 Sep 2023 20:49:37 -0700
Subject: [PATCH v5 1/4] asm-generic: Improve csum_fold
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Message-Id: <20230914-optimize_checksum-v5-1-c95b82a2757e@rivosinc.com>
References: <20230914-optimize_checksum-v5-0-c95b82a2757e@rivosinc.com>
In-Reply-To: <20230914-optimize_checksum-v5-0-c95b82a2757e@rivosinc.com>
To:     Charlie Jenkins <charlie@rivosinc.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Conor Dooley <conor@kernel.org>,
        Samuel Holland <samuel.holland@sifive.com>,
        David Laight <David.Laight@aculab.com>,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Cc:     Paul Walmsley <paul.walmsley@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>
Precedence: bulk

This csum_fold implementation introduced into arch/arc by Vineet Gupta
is better than the default implementation on at least arc, x86, arm, and
riscv. Using GCC trunk and compiling non-inlined version, this
implementation has 41.6667%, 25%, 16.6667% fewer instructions on
riscv64, x86-64, and arm64 respectively with -O3 optimization.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
---
 include/asm-generic/checksum.h | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)

diff --git a/include/asm-generic/checksum.h b/include/asm-generic/checksum.h
index 43e18db89c14..adab9ac4312c 100644
--- a/include/asm-generic/checksum.h
+++ b/include/asm-generic/checksum.h
@@ -30,10 +30,7 @@ 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)((~csum - ror32(csum, 16)) >> 16);
 }
 #endif
 

-- 
2.42.0