[PATCH v3 1/3] lib/math/gcd: Use static key to select implementation at runtime

[Kuan-Wei Chiu]

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.

Co-developed-by: Yu-Chun Lin <eleanor15x at gmail.com>
Signed-off-by: Yu-Chun Lin <eleanor15x at gmail.com>
Signed-off-by: Kuan-Wei Chiu <visitorckw at gmail.com>
---
 include/linux/gcd.h |  3 +++
 lib/math/gcd.c      | 27 +++++++++++++++------------
 2 files changed, 18 insertions(+), 12 deletions(-)

diff --git a/include/linux/gcd.h b/include/linux/gcd.h
index cb572677fd7f..616e81a7f7e3 100644
--- a/include/linux/gcd.h
+++ b/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__;
 
diff --git a/lib/math/gcd.c b/lib/math/gcd.c
index e3b042214d1b..62efca6787ae 100644
--- a/lib/math/gcd.c
+++ b/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, unsigned long b)
 	}
 }
 
-#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, unsigned long b)
 	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, unsigned long b)
 	}
 }
 
-#endif
-
 EXPORT_SYMBOL_GPL(gcd);
-- 
2.34.1