1 files changed, 35 insertions, 0 deletions
diff --git a/include/linux/reciprocal_div.h b/include/linux/reciprocal_div.h
new file mode 100644
index 000000000..8c5a3fb6c
--- /dev/null
+++ b/include/linux/reciprocal_div.h
@@ -0,0 +1,35 @@
+#ifndef _LINUX_RECIPROCAL_DIV_H
+#define _LINUX_RECIPROCAL_DIV_H
+
+#include <linux/types.h>
+
+/*
+ * This algorithm is based on the paper "Division by Invariant
+ * Integers Using Multiplication" by Torbjörn Granlund and Peter
+ * L. Montgomery.
+ *
+ * The assembler implementation from Agner Fog, which this code is
+ * based on, can be found here:
+ * http://www.agner.org/optimize/asmlib.zip
+ *
+ * This optimization for A/B is helpful if the divisor B is mostly
+ * runtime invariant. The reciprocal of B is calculated in the
+ * slow-path with reciprocal_value(). The fast-path can then just use
+ * a much faster multiplication operation with a variable dividend A
+ * to calculate the division A/B.
+ */
+
+struct reciprocal_value {
+	u32 m;
+	u8 sh1, sh2;
+};
+
+struct reciprocal_value reciprocal_value(u32 d);
+
+static inline u32 reciprocal_divide(u32 a, struct reciprocal_value R)
+{
+	u32 t = (u32)(((u64)a * R.m) >> 32);
+	return (t + ((a - t) >> R.sh1)) >> R.sh2;
+}
+
+#endif /* _LINUX_RECIPROCAL_DIV_H */