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Message-ID: <47AAB935.8040507@cosmosbay.com>
Date: Thu, 07 Feb 2008 08:54:29 +0100
From: Eric Dumazet <dada1@cosmosbay.com>
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To: Andrew Morton <akpm@linux-foundation.org>
CC: Linux kernel <linux-kernel@vger.kernel.org>
Subject: [PATCH] Avoid divides in BITS_TO_LONGS
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BITS_PER_LONG is a signed value (32 or 64)

DIV_ROUND_UP(nr, BITS_PER_LONG) performs signed arithmetic if "nr" is signed too.

Converting BITS_TO_LONGS(nr) to DIV_ROUND_UP(nr, BITS_PER_BYTE * sizeof(long))
makes sure compiler can perform a right shift, even if "nr" is a signed value, 
instead of an expensive integer divide.

Applying this patch saves 141 bytes on x86 when CONFIG_CC_OPTIMIZE_FOR_SIZE=y
and speedup bitmap operations.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>


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diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index 69c1edb..be5c27c 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -6,8 +6,8 @@
 #define BIT(nr)			(1UL << (nr))
 #define BIT_MASK(nr)		(1UL << ((nr) % BITS_PER_LONG))
 #define BIT_WORD(nr)		((nr) / BITS_PER_LONG)
-#define BITS_TO_LONGS(nr)	DIV_ROUND_UP(nr, BITS_PER_LONG)
 #define BITS_PER_BYTE		8
+#define BITS_TO_LONGS(nr)	DIV_ROUND_UP(nr, BITS_PER_BYTE * sizeof(long))
 #endif
 
 /*

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