+ lib-math-gcd-use-static-key-to-select-implementation-at-runtime.patch added to mm-nonmm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: lib/math/gcd: use static key to select implementation at runtime
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     lib-math-gcd-use-static-key-to-select-implementation-at-runtime.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/lib-math-gcd-use-static-key-to-select-implementation-at-runtime.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Kuan-Wei Chiu <visitorckw@gmail.com>
Subject: lib/math/gcd: use static key to select implementation at runtime
Date: Fri, 6 Jun 2025 21:47:56 +0800

Patch series "Optimize GCD performance on RISC-V by selecting
implementation at runtime", v3.

On platforms like RISC-V, the compiler may generate hardware FFS
instructions even if the underlying CPU does not actually support them. 
Currently, the GCD implementation is chosen at compile time based on
CONFIG_CPU_NO_EFFICIENT_FFS, which can result in suboptimal behavior on
such systems.

Introduce a static key, efficient_ffs_key, to enable runtime selection
between the binary GCD (using ffs) and the odd-even GCD implementation. 
This allows the kernel to default to the faster binary GCD when FFS is
efficient, while retaining the ability to fall back when needed.

Link: https://lkml.kernel.org/r/20250606134758.1308400-1-visitorckw@gmail.com
Link: https://lkml.kernel.org/r/20250606134758.1308400-2-visitorckw@gmail.com
Co-developed-by: Yu-Chun Lin <eleanor15x@gmail.com>
Signed-off-by: Yu-Chun Lin <eleanor15x@gmail.com>
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Ching-Chun (Jim) Huang <jserv@ccns.ncku.edu.tw>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Yu-Chun Lin <eleanor15x@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/gcd.h |    3 +++
 lib/math/gcd.c      |   27 +++++++++++++++------------
 2 files changed, 18 insertions(+), 12 deletions(-)

--- a/include/linux/gcd.h~lib-math-gcd-use-static-key-to-select-implementation-at-runtime
+++ a/include/linux/gcd.h
@@ -3,6 +3,9 @@
 #define _GCD_H
 
 #include <linux/compiler.h>
+#include <linux/jump_label.h>
+
+DECLARE_STATIC_KEY_TRUE(efficient_ffs_key);
 
 unsigned long gcd(unsigned long a, unsigned long b) __attribute_const__;
 
--- a/lib/math/gcd.c~lib-math-gcd-use-static-key-to-select-implementation-at-runtime
+++ a/lib/math/gcd.c
@@ -11,22 +11,16 @@
  * has decent hardware division.
  */
 
+DEFINE_STATIC_KEY_TRUE(efficient_ffs_key);
+
 #if !defined(CONFIG_CPU_NO_EFFICIENT_FFS)
 
 /* If __ffs is available, the even/odd algorithm benchmarks slower. */
 
-/**
- * gcd - calculate and return the greatest common divisor of 2 unsigned longs
- * @a: first value
- * @b: second value
- */
-unsigned long gcd(unsigned long a, unsigned long b)
+static unsigned long binary_gcd(unsigned long a, unsigned long b)
 {
 	unsigned long r = a | b;
 
-	if (!a || !b)
-		return r;
-
 	b >>= __ffs(b);
 	if (b == 1)
 		return r & -r;
@@ -44,9 +38,15 @@ unsigned long gcd(unsigned long a, unsig
 	}
 }
 
-#else
+#endif
 
 /* If normalization is done by loops, the even/odd algorithm is a win. */
+
+/**
+ * gcd - calculate and return the greatest common divisor of 2 unsigned longs
+ * @a: first value
+ * @b: second value
+ */
 unsigned long gcd(unsigned long a, unsigned long b)
 {
 	unsigned long r = a | b;
@@ -54,6 +54,11 @@ unsigned long gcd(unsigned long a, unsig
 	if (!a || !b)
 		return r;
 
+#if !defined(CONFIG_CPU_NO_EFFICIENT_FFS)
+	if (static_branch_likely(&efficient_ffs_key))
+		return binary_gcd(a, b);
+#endif
+
 	/* Isolate lsbit of r */
 	r &= -r;
 
@@ -80,6 +85,4 @@ unsigned long gcd(unsigned long a, unsig
 	}
 }
 
-#endif
-
 EXPORT_SYMBOL_GPL(gcd);
_

Patches currently in -mm which might be from visitorckw@gmail.com are

lib-math-gcd-use-static-key-to-select-implementation-at-runtime.patch
riscv-optimize-gcd-code-size-when-config_riscv_isa_zbb-is-disabled.patch
riscv-optimize-gcd-performance-on-risc-v-without-zbb-extension.patch