linux-kernel - RE: [PATCH 3.12 065/122] lib/checksum.c: fix carry in csum_tcpudp

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6D1CAE4D1E@AcuExch.aculab.com>
Date:	Tue, 17 Feb 2015 12:04:22 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Jiri Slaby' <jslaby@...e.cz>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	karl beldan <karl.beldan@...il.com>,
	Karl Beldan <karl.beldan@...ierawaves.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"Arnd Bergmann" <arnd@...db.de>,
	Mike Frysinger <vapier@...too.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: RE: [PATCH 3.12 065/122] lib/checksum.c: fix carry in
 csum_tcpudp_nofold

> +static inline u32 from64to32(u64 x)
> +{
> +	/* add up 32-bit and 32-bit for 32+c bit */
> +	x = (x & 0xffffffff) + (x >> 32);
> +	/* add up carry.. */
> +	x = (x & 0xffffffff) + (x >> 32);
> +	return (u32)x;
> +}

As a matter of interest, does the compiler optimise away the
second (x & 0xffffffff) ?
The code could just be:
	x = (x & 0xffffffff) + (x >> 32);
	return x + (x >> 32);

	David

	
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