[RFC v2 3/6] eal: add exactly-once bit access functions

["Mattias Rönnblom" <mattias.ronnblom@ericsson.com>]

Add bit test/set/clear/assign functions which prevents certain
compiler optimizations and guarantees that program-level memory loads
and/or stores will actually occur.

These functions are useful when interacting with memory-mapped
hardware devices.

The "once" family of functions does not promise atomicity and provides
no memory ordering guarantees beyond the C11 relaxed memory model.

Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
---
 lib/eal/include/rte_bitops.h | 170 +++++++++++++++++++++++++++++++++++
 1 file changed, 170 insertions(+)

diff --git a/lib/eal/include/rte_bitops.h b/lib/eal/include/rte_bitops.h
index 75a29fdfe0..a2746e657f 100644
--- a/lib/eal/include/rte_bitops.h
+++ b/lib/eal/include/rte_bitops.h
@@ -201,6 +201,147 @@ extern "C" {
 		 uint32_t *: __rte_bit_assign32,			\
 		 uint64_t *: __rte_bit_assign64)(addr, nr, value)
 
+/**
+ * @warning
+ * @b EXPERIMENTAL: this API may change without prior notice.
+ *
+ * Generic selection macro to test exactly once the value of a bit in
+ * a 32-bit or 64-bit word. The type of operation depends on the type
+ * of the @c addr parameter.
+ *
+ * This function is guaranteed to result in exactly one memory load
+ * (e.g., it may not be eliminate or merged by the compiler).
+ *
+ * \code{.c}
+ * rte_bit_once_set(addr, 17);
+ * if (rte_bit_once_test(addr, 17)) {
+ *     ...
+ * }
+ * \endcode
+ *
+ * In the above example, rte_bit_once_set() may not be removed by
+ * the compiler, which would be allowed in case rte_bit_set() and
+ * rte_bit_test() was used.
+ *
+ * \code{.c}
+ * while (rte_bit_once_test(addr, 17);
+ *     ;
+ * \endcode
+ *
+ * In case rte_bit_test(addr, 17) was used instead, the resulting
+ * object code could (and in many cases would be) replaced with
+ * the equivalent to
+ * \code{.c}
+ * if (rte_bit_test(addr, 17)) {
+ *   for (;;) // spin forever
+ *       ;
+ * }
+ * \endcode
+ *
+ * rte_bit_once_test() does not give any guarantees in regards to
+ * memory ordering or atomicity.
+ *
+ * The regular bit set operations (e.g., rte_bit_test()) should be
+ * preferred over the "once" family of operations (e.g.,
+ * rte_bit_once_test()) if possible, since the latter may prevent
+ * optimizations crucial for run-time performance.
+ *
+ * @param addr
+ *   A pointer to the word to query.
+ * @param nr
+ *   The index of the bit.
+ * @return
+ *   Returns true if the bit is set, and false otherwise.
+ */
+
+#define rte_bit_once_test(addr, nr)				\
+	_Generic((addr),					\
+		 uint32_t *: __rte_bit_once_test32,		\
+		 uint64_t *: __rte_bit_once_test64)(addr, nr)
+
+/**
+ * @warning
+ * @b EXPERIMENTAL: this API may change without prior notice.
+ *
+ * Set bit in word exactly once.
+ *
+ * Set bit specified by @c nr in the word pointed to by @c addr to '1'
+ * exactly once.
+ *
+ * This function is guaranteed to result in exactly one memory load
+ * and exactly one memory store, *or* an atomic bit set operation.
+ *
+ * See rte_bit_test_once32() for more information and uses cases for
+ * the "once" class of functions.
+ *
+ * This macro does not give any guarantees in regards to memory
+ * ordering or atomicity.
+ *
+ * @param addr
+ *   A pointer to the word to modify.
+ * @param nr
+ *   The index of the bit.
+ */
+#define rte_bit_once_set(addr, nr)				\
+	_Generic((addr),					\
+		 uint32_t *: __rte_bit_once_set32,		\
+		 uint64_t *: __rte_bit_once_set64)(addr, nr)
+
+/**
+ * @warning
+ * @b EXPERIMENTAL: this API may change without prior notice.
+ *
+ * Clear bit in word exactly once.
+ *
+ * Set bit specified by @c nr in the word pointed to by @c addr to '0'
+ * exactly once.
+ *
+ * This function is guaranteed to result in exactly one memory load
+ * and exactly one memory store, *or* an atomic bit clear operation.
+ *
+ * See rte_bit_test_once32() for more information and uses cases for
+ * the "once" class of functions.
+ *
+ * This macro does not give any guarantees in regards to memory
+ * ordering or atomicity.
+ *
+ * @param addr
+ *   A pointer to the word to modify.
+ * @param nr
+ *   The index of the bit.
+ */
+#define rte_bit_once_clear(addr, nr)				\
+	_Generic((addr),					\
+		 uint32_t *: __rte_bit_once_clear32,		\
+		 uint64_t *: __rte_bit_once_clear64)(addr, nr)
+
+/**
+ * @warning
+ * @b EXPERIMENTAL: this API may change without prior notice.
+ *
+ * Assign a value to bit in a word exactly once.
+ *
+ * Set bit specified by @c nr in the word pointed to by @c addr to the
+ * value indicated by @c value exactly once.
+ *
+ * This function is guaranteed to result in exactly one memory load
+ * and exactly one memory store, *or* an atomic bit clear operation.
+ *
+ * This function does not give any guarantees in regards to memory
+ * ordering or atomicity.
+ *
+ * @param addr
+ *   A pointer to the word to modify.
+ * @param nr
+ *   The index of the bit.
+ * @param value
+ *   The new value of the bit - true for '1', or false for '0'.
+ */
+#define rte_bit_once_assign(addr, nr, value)				\
+	_Generic((addr),						\
+		 uint32_t *: __rte_bit_once_assign32,			\
+		 uint64_t *: __rte_bit_once_assign64)(addr, nr, value)
+
 #define __RTE_GEN_BIT_TEST(name, size, qualifier)			\
 	static inline bool						\
 	name(const qualifier uint ## size ## _t *addr, unsigned int nr)	\
@@ -239,6 +380,14 @@ __RTE_GEN_BIT_TEST(__rte_bit_test64, 64,)
 __RTE_GEN_BIT_SET(__rte_bit_set64, 64,)
 __RTE_GEN_BIT_CLEAR(__rte_bit_clear64, 64,)
 
+__RTE_GEN_BIT_TEST(__rte_bit_once_test32, 32, volatile)
+__RTE_GEN_BIT_SET(__rte_bit_once_set32, 32, volatile)
+__RTE_GEN_BIT_CLEAR(__rte_bit_once_clear32, 32, volatile)
+
+__RTE_GEN_BIT_TEST(__rte_bit_once_test64, 64, volatile)
+__RTE_GEN_BIT_SET(__rte_bit_once_set64, 64, volatile)
+__RTE_GEN_BIT_CLEAR(__rte_bit_once_clear64, 64, volatile)
+
 __rte_experimental
 static inline void
 __rte_bit_assign32(uint32_t *addr, unsigned int nr, bool value)
@@ -259,6 +408,27 @@ __rte_bit_assign64(uint64_t *addr, unsigned int nr, bool value)
 		__rte_bit_clear64(addr, nr);
 }
 
+
+__rte_experimental
+static inline void
+__rte_bit_once_assign32(uint32_t *addr, unsigned int nr, bool value)
+{
+	if (value)
+		__rte_bit_once_set32(addr, nr);
+	else
+		__rte_bit_once_clear32(addr, nr);
+}
+
+__rte_experimental
+static inline void
+__rte_bit_once_assign64(volatile uint64_t *addr, unsigned int nr, bool value)
+{
+	if (value)
+		__rte_bit_once_set64(addr, nr);
+	else
+		__rte_bit_once_clear64(addr, nr);
+}
+
 /*------------------------ 32-bit relaxed operations ------------------------*/
 
 /**
-- 
2.34.1