linux-kernel - [PATCH 11/12] bitmap: defer calculating weight of 'new' in bitmap

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Message-Id: <20230828184353.5145-12-yury.norov@gmail.com>
Date:   Mon, 28 Aug 2023 11:43:51 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: [PATCH 11/12] bitmap: defer calculating weight of 'new' in bitmap_remap()

When 'old' map is empty, we don't need to calculate weight of 'new' map,
because all 'src' bits are simply copied to dst.

Signed-off-by: Yury Norov <yury.norov@...il.com>
---
 lib/bitmap.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/lib/bitmap.c b/lib/bitmap.c
index f62ea97e942c..1fca60d54cb4 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -996,23 +996,24 @@ void __bitmap_remap(unsigned long *dst, const unsigned long *src,
 		const unsigned long *old, const unsigned long *new,
 		unsigned int nbits)
 {
-	unsigned int oldbit, w, n;
+	unsigned int oldbit, w = 0, n;
 
 	if (dst == src)		/* following doesn't handle inplace remaps */
 		return;
 
-	w = bitmap_weight(new, nbits);
-	if (w == 0) {
-		bitmap_copy(dst, src, nbits);
-		return;
-	}
-
 	/* Identity part */
 	bitmap_andnot(dst, src, old, nbits);
 
 	/* Remapping part */
 	for_each_and_bit(oldbit, src, old, nbits) {
 		n = bitmap_weight(old, oldbit);
+		if (w == 0) { /* if not initialized */
+			w = bitmap_weight(new, nbits);
+			if (w == 0) { /* if empty */
+				bitmap_copy(dst, src, nbits);
+				return;
+			}
+		}
 		__set_bit(find_nth_bit(new, nbits, n % w), dst);
 	}
 }
-- 
2.39.2