[dpdk-dev] [PATCH 2/3] rte_sched: introduce reciprocal divide

[Stephen Hemminger <stephen@networkplumber.org>]

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This adds (with permission of the original author)
reciprocal divide based on algorithm in Linux.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
---
 lib/librte_sched/Makefile         |  6 ++--
 lib/librte_sched/rte_reciprocal.c | 72 +++++++++++++++++++++++++++++++++++++++
 lib/librte_sched/rte_reciprocal.h | 39 +++++++++++++++++++++
 3 files changed, 115 insertions(+), 2 deletions(-)
 create mode 100644 lib/librte_sched/rte_reciprocal.c
 create mode 100644 lib/librte_sched/rte_reciprocal.h

diff --git a/lib/librte_sched/Makefile b/lib/librte_sched/Makefile
index b1cb285..e0a2c6d 100644
--- a/lib/librte_sched/Makefile
+++ b/lib/librte_sched/Makefile
@@ -48,10 +48,12 @@ LIBABIVER := 1
 #
 # all source are stored in SRCS-y
 #
-SRCS-$(CONFIG_RTE_LIBRTE_SCHED) += rte_sched.c rte_red.c rte_approx.c
+SRCS-$(CONFIG_RTE_LIBRTE_SCHED) += rte_sched.c rte_red.c rte_approx.c \
+	rte_reciprocal.c
 
 # install includes
-SYMLINK-$(CONFIG_RTE_LIBRTE_SCHED)-include := rte_sched.h rte_bitmap.h rte_sched_common.h rte_red.h rte_approx.h
+SYMLINK-$(CONFIG_RTE_LIBRTE_SCHED)-include := rte_sched.h rte_bitmap.h \
+	rte_sched_common.h rte_red.h rte_approx.h rte_reciprocal.h
 
 # this lib depends upon:
 DEPDIRS-$(CONFIG_RTE_LIBRTE_SCHED) += lib/librte_mempool lib/librte_mbuf
diff --git a/lib/librte_sched/rte_reciprocal.c b/lib/librte_sched/rte_reciprocal.c
new file mode 100644
index 0000000..652f023
--- /dev/null
+++ b/lib/librte_sched/rte_reciprocal.c
@@ -0,0 +1,72 @@
+/*-
+ *   BSD LICENSE
+ *
+ *   Copyright(c) Hannes Frederic Sowa
+ *   All rights reserved.
+ *
+ *   Redistribution and use in source and binary forms, with or without
+ *   modification, are permitted provided that the following conditions
+ *   are met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in
+ *       the documentation and/or other materials provided with the
+ *       distribution.
+ *     * Neither the name of Intel Corporation nor the names of its
+ *       contributors may be used to endorse or promote products derived
+ *       from this software without specific prior written permission.
+ *
+ *   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ *   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ *   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ *   A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ *   OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ *   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ *   LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ *   DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ *   THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ *   (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ *   OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <stdio.h>
+#include <stdint.h>
+
+#include <rte_common.h>
+
+#include "rte_reciprocal.h"
+
+/* find largest set bit.
+ * portable and slow but does not matter for this usage.
+ */
+static inline int fls(uint32_t x)
+{
+	int b;
+
+	for (b = 31; b >= 0; --b) {
+		if (x & (1u << b))
+			return b + 1;
+	}
+
+	return 0;
+}
+
+struct rte_reciprocal rte_reciprocal_value(uint32_t d)
+{
+	struct rte_reciprocal R;
+	uint64_t m;
+	int l;
+
+	l = fls(d - 1);
+	m = ((1ULL << 32) * ((1ULL << l) - d));
+	m /= d;
+
+	++m;
+	R.m = m;
+	R.sh1 = RTE_MIN(l, 1);
+	R.sh2 = RTE_MAX(l - 1, 0);
+
+	return R;
+}
diff --git a/lib/librte_sched/rte_reciprocal.h b/lib/librte_sched/rte_reciprocal.h
new file mode 100644
index 0000000..abd1525
--- /dev/null
+++ b/lib/librte_sched/rte_reciprocal.h
@@ -0,0 +1,39 @@
+/*
+ * Reciprocal divide
+ *
+ * Used with permission from original authors
+ *  Hannes Frederic Sowa and Daniel Borkmann
+ *
+ * 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.
+ */
+
+#ifndef _RTE_RECIPROCAL_H_
+#define _RTE_RECIPROCAL_H_
+
+struct rte_reciprocal {
+	uint32_t m;
+	uint8_t sh1, sh2;
+};
+
+static inline uint32_t rte_reciprocal_divide(uint32_t a, struct rte_reciprocal R)
+{
+	uint32_t t = (uint32_t)(((uint64_t)a * R.m) >> 32);
+
+	return (t + ((a - t) >> R.sh1)) >> R.sh2;
+}
+
+struct rte_reciprocal rte_reciprocal_value(uint32_t d);
+
+#endif /* _RTE_RECIPROCAL_H_ */
-- 
2.1.4