linux-kernel - [RFC PATCH 1/2] bits: introduce ffs

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Message-ID: <9767487fcab7dbe7a7282a48a492171629eb935b.1767975412.git.ptesarik@suse.com>
Date: Fri,  9 Jan 2026 17:37:56 +0100
From: Petr Tesarik <ptesarik@...e.com>
To: Yury Norov <yury.norov@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: Richard Henderson <richard.henderson@...aro.org>,
	Matt Turner <mattst88@...il.com>,
	Magnus Lindholm <linmag7@...il.com>,
	Vineet Gupta <vgupta@...nel.org>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	"Maciej W. Rozycki" <macro@...am.me.uk>,
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
	Madhavan Srinivasan <maddy@...ux.ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Heiko Carstens <hca@...ux.ibm.com>,
	Vasily Gorbik <gor@...ux.ibm.com>,
	Alexander Gordeev <agordeev@...ux.ibm.com>,
	Chris Zankel <chris@...kel.net>,
	Max Filippov <jcmvbkbc@...il.com>,
	Patrik Jakobsson <patrik.r.jakobsson@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>,
	Simona Vetter <simona@...ll.ch>,
	Robin Murphy <robin.murphy@....com>,
	Joerg Roedel <joro@...tes.org>,
	Will Deacon <will@...nel.org>,
	Jakub Kicinski <kuba@...nel.org>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Paolo Abeni <pabeni@...hat.com>,
	Oliver Neukum <oliver@...kum.org>,
	Arnd Bergmann <arnd@...db.de>,
	Kuan-Wei Chiu <visitorckw@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marcel Holtmann <marcel@...tmann.org>,
	Johan Hedberg <johan.hedberg@...il.com>,
	Luiz Augusto von Dentz <luiz.dentz@...il.com>,
	Pablo Neira Ayuso <pablo@...filter.org>,
	Florian Westphal <fw@...len.de>,
	linux-kernel@...r.kernel.org,
	Petr Tesarik <ptesarik@...e.com>
Subject: [RFC PATCH 1/2] bits: introduce ffs_val()

Introduce a macro that can efficiently extract the least significant
non-zero bit from a value.

Interestingly, this bit-twiddling trick is open-coded in some places, but
it also appears to be little known, leading to various inefficient
implementations in other places. Let's make it part of the standard bitops
arsenal.

Define the macro in a separate header file included from <linux/bitops.h>,
to allow using it in very low-level header files that may not want to
include all of <linux/bitops.h>.

Signed-off-by: Petr Tesarik <ptesarik@...e.com>
---
 MAINTAINERS             |  1 +
 include/linux/bitops.h  |  1 +
 include/linux/ffs_val.h | 21 +++++++++++++++++++++
 3 files changed, 23 insertions(+)
 create mode 100644 include/linux/ffs_val.h

diff --git a/MAINTAINERS b/MAINTAINERS
index a0dd762f5648b..8f15c76a67ea2 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4466,6 +4466,7 @@ F:	arch/*/lib/bitops.c
 F:	include/asm-generic/bitops
 F:	include/asm-generic/bitops.h
 F:	include/linux/bitops.h
+F:	include/linux/ffs_val.h
 F:	lib/hweight.c
 F:	lib/test_bitops.c
 F:	tools/*/bitops*
diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index ea7898cc59039..209f0c3e07b9e 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -4,6 +4,7 @@
 
 #include <asm/types.h>
 #include <linux/bits.h>
+#include <linux/ffs_val.h>
 #include <linux/typecheck.h>
 
 #include <uapi/linux/kernel.h>
diff --git a/include/linux/ffs_val.h b/include/linux/ffs_val.h
new file mode 100644
index 0000000000000..193ec86d2b53b
--- /dev/null
+++ b/include/linux/ffs_val.h
@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_LINUX_FFS_VAL_H_
+#define _ASM_LINUX_FFS_VAL_H_
+
+/**
+ * ffs_val - find the value of the first set bit
+ * @x: the value to search
+ *
+ * Unlike ffs(), which returns a bit position, ffs_val() returns the bit
+ * value itself.
+ *
+ * Returns:
+ * least significant non-zero bit, 0 if all bits are zero
+ */
+#define ffs_val(x)			\
+({					\
+	const typeof(x) val__ = (x);	\
+	val__ & -val__;			\
+})
+
+#endif /* _ASM_LINUX_FFS_VAL_H_ */
-- 
2.52.0