linux-kernel - [PATCH 02/13] lib/generic-radix-tree: Optimise out ilog2(variable).

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Message-ID: <5a4d1a42f61d4fd28e7ef177efd958af@AcuMS.aculab.com>
Date:   Tue, 25 Aug 2020 14:53:07 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "'linux-sctp@...r.kernel.org'" <linux-sctp@...r.kernel.org>,
        Eric Biggers <ebiggers@...nel.org>,
        'Marcelo Ricardo Leitner' <marcelo.leitner@...il.com>,
        'Catalin Marinas' <catalin.marinas@....com>,
        "'kent.overstreet@...il.com'" <kent.overstreet@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "'Neil Horman'" <nhorman@...driver.com>
Subject: [PATCH 02/13] lib/generic-radix-tree: Optimise out ilog2(variable).

ilog2() is relatively expensive (especially without bit-scan instructions).
Replace the comparisons 'ilog2(x) >= y' with '(x >> y) != 0'.

Signed-off-by: David Laight <david.laight@...lab.com>
---
 lib/generic-radix-tree.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/lib/generic-radix-tree.c b/lib/generic-radix-tree.c
index 5695fe547f9d..d5e97b51308d 100644
--- a/lib/generic-radix-tree.c
+++ b/lib/generic-radix-tree.c
@@ -57,7 +57,7 @@ void *__genradix_ptr(struct __genradix *radix, size_t offset)
 	struct genradix_node *n = genradix_root_to_node(r);
 	unsigned level		= genradix_root_to_depth(r);
 
-	if (ilog2(offset) >= genradix_depth_shift(level))
+	if (offset >> genradix_depth_shift(level))
 		return NULL;
 
 	while (1) {
@@ -115,7 +115,7 @@ void *__genradix_ptr_alloc(struct __genradix *radix, size_t offset,
 		n	= genradix_root_to_node(r);
 		level	= genradix_root_to_depth(r);
 
-		if (n && ilog2(offset) < genradix_depth_shift(level))
+		if (n && !(offset >> genradix_depth_shift(level)))
 			break;
 
 		new_node = genradix_alloc_node(gfp_mask);
@@ -171,7 +171,7 @@ void *__genradix_iter_peek(struct genradix_iter *iter,
 	n	= genradix_root_to_node(r);
 	level	= genradix_root_to_depth(r);
 
-	if (ilog2(iter->offset) >= genradix_depth_shift(level))
+	if (iter->offset >> genradix_depth_shift(level))
 		return NULL;
 
 	while (level) {
-- 
2.25.1

-
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