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From:   Charlie Jenkins <charlie@rivosinc.com>
Date:   Fri, 15 Sep 2023 10:01:17 -0700
Subject: [PATCH v6 1/4] asm-generic: Improve csum_fold
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Message-Id: <20230915-optimize_checksum-v6-1-14a6cf61c618@rivosinc.com>
References: <20230915-optimize_checksum-v6-0-14a6cf61c618@rivosinc.com>
In-Reply-To: <20230915-optimize_checksum-v6-0-14a6cf61c618@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,
        linux-arch@vger.kernel.org
Cc:     Paul Walmsley <paul.walmsley@sifive.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Arnd Bergmann <arnd@arndb.de>,
        David Laight <david.laight@aculab.com>
Precedence: bulk

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@rivosinc.com>
Reviewed-by: David Laight <david.laight@aculab.com>
---
 include/asm-generic/checksum.h | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/include/asm-generic/checksum.h b/include/asm-generic/checksum.h
index 43e18db89c14..37f5ec70ac93 100644
--- a/include/asm-generic/checksum.h
+++ b/include/asm-generic/checksum.h
@@ -31,9 +31,7 @@ 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
 

-- 
2.42.0