[PATCH v4 1/3] random: add rte_drand() function

[Stephen Hemminger <stephen@networkplumber.org>]

The PIE code and other applications can benefit from having a
fast way to get a random floating point value. This new function
is equivalent to drand() in the standard library.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 app/test/test_rand_perf.c              |  7 +++++
 doc/guides/rel_notes/release_22_07.rst |  5 ++++
 lib/eal/common/rte_random.c            | 41 ++++++++++++++++++++++++++
 lib/eal/include/rte_random.h           | 18 +++++++++++
 lib/eal/meson.build                    |  3 ++
 lib/eal/version.map                    |  1 +
 6 files changed, 75 insertions(+)

diff --git a/app/test/test_rand_perf.c b/app/test/test_rand_perf.c
index fe797ebfa1ca..26fb1d9a586e 100644
--- a/app/test/test_rand_perf.c
+++ b/app/test/test_rand_perf.c
@@ -20,6 +20,7 @@ static volatile uint64_t vsum;
 
 enum rand_type {
 	rand_type_64,
+	rand_type_float,
 	rand_type_bounded_best_case,
 	rand_type_bounded_worst_case
 };
@@ -30,6 +31,8 @@ rand_type_desc(enum rand_type rand_type)
 	switch (rand_type) {
 	case rand_type_64:
 		return "Full 64-bit [rte_rand()]";
+	case rand_type_float:
+		return "Floating point [rte_drand()]";
 	case rand_type_bounded_best_case:
 		return "Bounded average best-case [rte_rand_max()]";
 	case rand_type_bounded_worst_case:
@@ -55,6 +58,9 @@ test_rand_perf_type(enum rand_type rand_type)
 		case rand_type_64:
 			sum += rte_rand();
 			break;
+		case rand_type_float:
+			sum += 1000. * rte_drand();
+			break;
 		case rand_type_bounded_best_case:
 			sum += rte_rand_max(BEST_CASE_BOUND);
 			break;
@@ -83,6 +89,7 @@ test_rand_perf(void)
 	printf("Pseudo-random number generation latencies:\n");
 
 	test_rand_perf_type(rand_type_64);
+	test_rand_perf_type(rand_type_float);
 	test_rand_perf_type(rand_type_bounded_best_case);
 	test_rand_perf_type(rand_type_bounded_worst_case);
 
diff --git a/doc/guides/rel_notes/release_22_07.rst b/doc/guides/rel_notes/release_22_07.rst
index e49cacecefd4..b131ea577226 100644
--- a/doc/guides/rel_notes/release_22_07.rst
+++ b/doc/guides/rel_notes/release_22_07.rst
@@ -104,6 +104,11 @@ New Features
   * ``RTE_EVENT_QUEUE_ATTR_WEIGHT``
   * ``RTE_EVENT_QUEUE_ATTR_AFFINITY``
 
+* ** Added function get random floating point number.**
+
+  Added the function ``rte_drand()`` to provide a pseudo-random
+  floating point number.
+
 
 Removed Items
 -------------
diff --git a/lib/eal/common/rte_random.c b/lib/eal/common/rte_random.c
index 4535cc980cec..3dc3484ee655 100644
--- a/lib/eal/common/rte_random.c
+++ b/lib/eal/common/rte_random.c
@@ -6,6 +6,9 @@
 #include <x86intrin.h>
 #endif
 #include <unistd.h>
+#ifdef RTE_LIBEAL_USE_IEEE754
+#include <ieee754.h>
+#endif
 
 #include <rte_branch_prediction.h>
 #include <rte_cycles.h>
@@ -173,6 +176,44 @@ rte_rand_max(uint64_t upper_bound)
 	return res;
 }
 
+double
+rte_drand(void)
+{
+	struct rte_rand_state *state = __rte_rand_get_state();
+	uint64_t rand64 = __rte_rand_lfsr258(state);
+#ifdef RTE_LIBEAL_USE_IEEE754
+	union ieee754_double u = {
+		.ieee = {
+			.negative = 0,
+			.exponent = IEEE754_DOUBLE_BIAS,
+		},
+	};
+
+	/* Take 64 bit random value and put it into the mantissa
+	 * This uses direct access to IEEE format to avoid doing
+	 * any direct floating point math here.
+	 */
+	u.ieee.mantissa0 = rand64 >> 32;
+	u.ieee.mantissa1 = rand64;
+
+	return u.d - 1.0;
+#else
+	/* Slower method requiring floating point divide
+	 *
+	 * The double mantissa only has 53 bits, so we uniformly mask off the
+	 * high 11 bits and then floating-point divide by 2^53 to achieve a
+	 * result in [0, 1).
+	 *
+	 * We are not allowed to emit 1.0, so denom must be one greater than
+	 * the possible range of the preceeding step.
+	 */
+	static const uint64_t denom = (uint64_t)1 << 53;
+
+	rand64 &= denom - 1;
+	return (double)rand64 / denom;
+#endif
+}
+
 static uint64_t
 __rte_random_initial_seed(void)
 {
diff --git a/lib/eal/include/rte_random.h b/lib/eal/include/rte_random.h
index 29f5f1325a30..f6541c2b0f08 100644
--- a/lib/eal/include/rte_random.h
+++ b/lib/eal/include/rte_random.h
@@ -65,6 +65,24 @@ rte_rand(void);
 uint64_t
 rte_rand_max(uint64_t upper_bound);
 
+/**
+ * @warning
+ * @b EXPERIMENTAL: this API may change without prior notice
+ *
+ * Generates a pseudo-random floating point number.
+ *
+ * This function returns a nonnegative double-precision floating random
+ * number uniformly distributed over the interval [0.0, 1.0).
+ *
+ * The generator is not cryptographically secure.
+ * If called from lcore threads, this function is thread-safe.
+ *
+ * @return
+ *   A pseudo-random value between 0 and 1.0.
+ */
+__rte_experimental
+double rte_drand(void);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/lib/eal/meson.build b/lib/eal/meson.build
index 056beb946119..e50524901c98 100644
--- a/lib/eal/meson.build
+++ b/lib/eal/meson.build
@@ -32,3 +32,6 @@ endif
 if cc.has_function('getentropy', prefix : '#include <unistd.h>')
     cflags += '-DRTE_LIBEAL_USE_GETENTROPY'
 endif
+if cc.has_header_symbol('ieee754.h', 'union ieee754_double')
+    cflags += '-DRTE_LIBEAL_USE_IEEE754'
+endif
diff --git a/lib/eal/version.map b/lib/eal/version.map
index d49e30bd042f..cfbade9a33e9 100644
--- a/lib/eal/version.map
+++ b/lib/eal/version.map
@@ -422,6 +422,7 @@ EXPERIMENTAL {
 	rte_intr_type_set;
 
 	# added in 22.07
+	rte_drand;
 	rte_thread_get_affinity_by_id;
 	rte_thread_self;
 	rte_thread_set_affinity_by_id;
-- 
2.35.1