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[dm-devel] [PATCH] dm-space-map-disk: improve bit testing



dm-space-map-disk: improve bit testing

This patch improves bit testing in bitmap_word_used:
* replace three tests with just one condition.
* make the test accurate --- the function returns 0 only if there exists
  a pair of two zeroed bits. Previously, the function returned 0 in some other
  cases too.

Signed-off-by: Mikulas Patocka <mpatocka redhat com>

---
 drivers/md/persistent-data/dm-space-map-disk.c |    6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

Index: linux-3.0-fast/drivers/md/persistent-data/dm-space-map-disk.c
===================================================================
--- linux-3.0-fast.orig/drivers/md/persistent-data/dm-space-map-disk.c	2011-08-15 20:39:38.000000000 +0200
+++ linux-3.0-fast/drivers/md/persistent-data/dm-space-map-disk.c	2011-08-15 20:44:34.000000000 +0200
@@ -70,7 +70,6 @@ void *dm_bitmap_data(struct dm_block *b)
 
 #define WORD_MASK_LOW 0x5555555555555555ULL
 #define WORD_MASK_HIGH 0xAAAAAAAAAAAAAAAAULL
-#define WORD_MASK_ALL 0xFFFFFFFFFFFFFFFFULL
 
 static unsigned bitmap_word_used(void *addr, unsigned b)
 {
@@ -78,10 +77,9 @@ static unsigned bitmap_word_used(void *a
 	__le64 *w_le = words_le + (b >> ENTRIES_SHIFT);
 
 	uint64_t bits = le64_to_cpu(*w_le);
+	uint64_t mask = (bits - WORD_MASK_LOW) & WORD_MASK_HIGH;
 
-	return ((bits & WORD_MASK_LOW) == WORD_MASK_LOW ||
-		(bits & WORD_MASK_HIGH) == WORD_MASK_HIGH ||
-		(bits & WORD_MASK_ALL) == WORD_MASK_ALL);
+	return !(~bits & mask);
 }
 
 unsigned sm_lookup_bitmap(void *addr, unsigned b)


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