datapath: compat: Downstream the reciprocal_div.{c,h}.

[sliver-openvswitch.git] / datapath / linux / compat / include / linux / reciprocal_div.h

diff --git a/datapath/linux/compat/include/linux/reciprocal_div.h b/datapath/linux/compat/include/linux/reciprocal_div.h
new file mode 100644 (file)
index 0000000..2def5c6
--- /dev/null
+++ b/datapath/linux/compat/include/linux/reciprocal_div.h
@@ -0,0 +1,37 @@
+#ifndef _LINUX_RECIPROCAL_DIV_WRAPPER_H
+#define _LINUX_RECIPROCAL_DIV_WRAPPER_H 1
+
+#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.
+ */
+
+#define reciprocal_value rpl_reciprocal_value
+struct reciprocal_value {
+       u32 m;
+       u8 sh1, sh2;
+};
+
+struct reciprocal_value reciprocal_value(u32 d);
+
+#define reciprocal_divide rpl_reciprocal_divide
+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_WRAPPER_H */