linux-kernel - [PATCH] tcp_cubic: use 32 bit math

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Date:	Wed, 7 Mar 2007 17:07:31 -0800
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Willy Tarreau <w@....eu>, David Miller <davem@...emloft.net>
Cc:	rkuhn@....physik.tu-muenchen.de, andi@...stfloor.org,
	dada1@...mosbay.com, jengelh@...ux01.gwdg.de,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: [PATCH] tcp_cubic: use 32 bit math

The basic calculation has to be done in 32 bits to avoid
doing 64 bit divide by 3. The value x is only 22bits max
so only need full 64 bits only for x^2.

Signed-off-by: Stephen Hemminger <shemminger@...ux-foundation.org>

---
 net/ipv4/tcp_cubic.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- net-2.6.22.orig/net/ipv4/tcp_cubic.c	2007-03-07 15:51:37.000000000 -0800
+++ net-2.6.22/net/ipv4/tcp_cubic.c	2007-03-07 17:06:02.000000000 -0800
@@ -96,7 +96,7 @@
  */
 static u32 cubic_root(u64 a)
 {
-	u64 x;
+	u32 x;
 
 	/* Initial estimate is based on:
 	 * cbrt(x) = exp(log(x) / 3)
@@ -104,9 +104,9 @@
 	x = 1u << (fls64(a)/3);
 
 	/* converges to 32 bits in 3 iterations */
-	x = (2 * x + div64_64(a, x*x)) / 3;
-	x = (2 * x + div64_64(a, x*x)) / 3;
-	x = (2 * x + div64_64(a, x*x)) / 3;
+	x = (2 * x + (u32)div64_64(a, (u64)x*(u64)x)) / 3;
+	x = (2 * x + (u32)div64_64(a, (u64)x*(u64)x)) / 3;
+	x = (2 * x + (u32)div64_64(a, (u64)x*(u64)x)) / 3;
 
 	return x;
 }
-
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