[dpdk-dev] [PATCH v2] eal: add new prefetch write variants

[Harry van Haaren]

This commit adds a new rte_prefetch0_write() variants, suggesting to the
compiler to use a prefetch instruction with intention to write. As a
compiler builtin, the compiler can choose based on compilation target
what the best implementation for this instruction is.

Signed-off-by: Harry van Haaren <harry.van.haaren at intel.com>

---

v2:
- Add L1, L2, and L3 variants as ARM64 uarch supports them (Pavan)

The integer constants passed to the builtin are not available as
a #define value, and doing #defines just for this write variant
does not seems a nice solution to me... particularly for those using
IDEs where any #define value is auto-hinted for code-completion.
---
 lib/librte_eal/include/generic/rte_prefetch.h | 49 +++++++++++++++++++
 1 file changed, 49 insertions(+)

diff --git a/lib/librte_eal/include/generic/rte_prefetch.h b/lib/librte_eal/include/generic/rte_prefetch.h
index 6e47bdfbad..3dfca77a74 100644
--- a/lib/librte_eal/include/generic/rte_prefetch.h
+++ b/lib/librte_eal/include/generic/rte_prefetch.h
@@ -51,4 +51,53 @@ static inline void rte_prefetch2(const volatile void *p);
  */
 static inline void rte_prefetch_non_temporal(const volatile void *p);
 
+/**
+ * Prefetch a cache line into all cache levels, with intention to write. This
+ * prefetch variant hints to the CPU that the program is expecting to write to
+ * the cache line being prefetched.
+ *
+ * @param p Address to prefetch
+ */
+static inline void rte_prefetch0_write(const void *p)
+{
+	/* 1 indicates intention to write, 3 sets target cache level to L1. See
+	 * GCC docs where these integer constants are described in more detail:
+	 *  https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
+	 */
+	__builtin_prefetch(p, 1, 3);
+}
+
+/**
+ * Prefetch a cache line into all cache levels, except the 0th, with intention
+ * to write. This prefetch variant hints to the CPU that the program is
+ * expecting to write to the cache line being prefetched.
+ *
+ * @param p Address to prefetch
+ */
+static inline void rte_prefetch1_write(const void *p)
+{
+	/* 1 indicates intention to write, 2 sets target cache level to L2. See
+	 * GCC docs where these integer constants are described in more detail:
+	 *  https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
+	 */
+	__builtin_prefetch(p, 1, 2);
+}
+
+/**
+ * Prefetch a cache line into all cache levels, except the 0th and 1st, with
+ * intention to write. This prefetch variant hints to the CPU that the program
+ * is expecting to write to the cache line being prefetched.
+ *
+ * @param p Address to prefetch
+ */
+static inline void rte_prefetch2_write(const void *p)
+{
+	/* 1 indicates intention to write, 1 sets target cache level to L3. See
+	 * GCC docs where these integer constants are described in more detail:
+	 *  https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
+	 */
+	__builtin_prefetch(p, 1, 1);
+}
+
+
 #endif /* _RTE_PREFETCH_H_ */
-- 
2.17.1