linux-kernel - [PATCH] net: Remove branch in csum

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Date:   Fri, 11 Feb 2022 09:48:16 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        netdev@...r.kernel.org
Subject: [PATCH] net: Remove branch in csum_shift()

Today's implementation of csum_shift() leads to branching based on
parity of 'offset'

	000002f8 <csum_block_add>:
	     2f8:	70 a5 00 01 	andi.   r5,r5,1
	     2fc:	41 a2 00 08 	beq     304 <csum_block_add+0xc>
	     300:	54 84 c0 3e 	rotlwi  r4,r4,24
	     304:	7c 63 20 14 	addc    r3,r3,r4
	     308:	7c 63 01 94 	addze   r3,r3
	     30c:	4e 80 00 20 	blr

Use first bit of 'offset' directly as input of the rotation instead of
branching.

	000002f8 <csum_block_add>:
	     2f8:	54 a5 1f 38 	rlwinm  r5,r5,3,28,28
	     2fc:	20 a5 00 20 	subfic  r5,r5,32
	     300:	5c 84 28 3e 	rotlw   r4,r4,r5
	     304:	7c 63 20 14 	addc    r3,r3,r4
	     308:	7c 63 01 94 	addze   r3,r3
	     30c:	4e 80 00 20 	blr

And change to left shift instead of right shift to skip one more
instruction. This has no impact on the final sum.

	000002f8 <csum_block_add>:
	     2f8:	54 a5 1f 38 	rlwinm  r5,r5,3,28,28
	     2fc:	5c 84 28 3e 	rotlw   r4,r4,r5
	     300:	7c 63 20 14 	addc    r3,r3,r4
	     304:	7c 63 01 94 	addze   r3,r3
	     308:	4e 80 00 20 	blr

Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
---
 include/net/checksum.h | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/include/net/checksum.h b/include/net/checksum.h
index 5218041e5c8f..9badcd5532ef 100644
--- a/include/net/checksum.h
+++ b/include/net/checksum.h
@@ -83,9 +83,7 @@ static inline __sum16 csum16_sub(__sum16 csum, __be16 addend)
 static inline __wsum csum_shift(__wsum sum, int offset)
 {
 	/* rotate sum to align it with a 16b boundary */
-	if (offset & 1)
-		return (__force __wsum)ror32((__force u32)sum, 8);
-	return sum;
+	return (__force __wsum)rol32((__force u32)sum, (offset & 1) << 3);
 }
 
 static inline __wsum
-- 
2.34.1