[PATCH v5] graph: expose node context as pointers

[Robin Jarry <rjarry@redhat.com>]

In some cases, the node context data is used to store two pointers
because the data is larger than the reserved 16 bytes. Having to define
intermediate structures just to be able to cast is tedious. And without
intermediate structures, casting to opaque pointers is hard without
violating strict aliasing rules.

Add an unnamed union to allow storing opaque pointers in the node
context. Unfortunately, aligning an unnamed union that contains an array
produces inconsistent results between C and C++. To preserve ABI/API
compatibility in both C and C++, move all fast-path area fields into an
unnamed struct which is cache aligned. Use __rte_cache_min_aligned to
preserve existing alignment on architectures where cache lines are 128
bytes.

Add a static assert to ensure that the unnamed union is not larger than
the context array (RTE_NODE_CTX_SZ).

Signed-off-by: Robin Jarry <rjarry@redhat.com>
---

Notes:
    v5:
    
    * Helper functions to hide casting proved to be harder than expected.
      Naive casting may even be impossible without breaking strict aliasing
      rules. The only other option would be to use explicit memcpy calls.
    * Unnamed union tentative again. As suggested by Tyler (thank you!),
      using an intermediate unnamed struct to carry the alignment produces
      consistent ABI in C and C++.
    * Also, Tyler (thank you!) suggested that the fast path area alignment
      size may be incorrect for architectures where the cache line is not 64
      bytes. There will be a 64 bytes hole in the structure at the end of
      the unnamed struct before the zero length next nodes array. Use
      __rte_cache_min_aligned to preserve existing alignment.
    
    v4:
    
    * Replaced the unnamed union with helper inline functions.
    
    v3:
    
    * Added __extension__ to the unnamed struct inside the union.
    * Fixed C++ header checks.
    * Replaced alignas() with an explicit static_assert.

 lib/graph/rte_graph_worker_common.h | 27 ++++++++++++++++++++-------
 1 file changed, 20 insertions(+), 7 deletions(-)

diff --git a/lib/graph/rte_graph_worker_common.h b/lib/graph/rte_graph_worker_common.h
index 36d864e2c14e..84d4997bbbf6 100644
--- a/lib/graph/rte_graph_worker_common.h
+++ b/lib/graph/rte_graph_worker_common.h
@@ -12,7 +12,9 @@
  * process, enqueue and move streams of objects to the next nodes.
  */
 
+#include <assert.h>
 #include <stdalign.h>
+#include <stddef.h>
 
 #include <rte_common.h>
 #include <rte_cycles.h>
@@ -111,14 +113,21 @@ struct __rte_cache_aligned rte_node {
 		} dispatch;
 	};
 	/* Fast path area  */
+	__extension__ struct __rte_cache_min_aligned {
 #define RTE_NODE_CTX_SZ 16
-	alignas(RTE_CACHE_LINE_SIZE) uint8_t ctx[RTE_NODE_CTX_SZ]; /**< Node Context. */
-	uint16_t size;		/**< Total number of objects available. */
-	uint16_t idx;		/**< Number of objects used. */
-	rte_graph_off_t off;	/**< Offset of node in the graph reel. */
-	uint64_t total_cycles;	/**< Cycles spent in this node. */
-	uint64_t total_calls;	/**< Calls done to this node. */
-	uint64_t total_objs;	/**< Objects processed by this node. */
+		union {
+			uint8_t ctx[RTE_NODE_CTX_SZ];
+			__extension__ struct {
+				void *ctx_ptr;
+				void *ctx_ptr2;
+			};
+		}; /**< Node Context. */
+		uint16_t size;		/**< Total number of objects available. */
+		uint16_t idx;		/**< Number of objects used. */
+		rte_graph_off_t off;	/**< Offset of node in the graph reel. */
+		uint64_t total_cycles;	/**< Cycles spent in this node. */
+		uint64_t total_calls;	/**< Calls done to this node. */
+		uint64_t total_objs;	/**< Objects processed by this node. */
 		union {
 			void **objs;	   /**< Array of object pointers. */
 			uint64_t objs_u64;
@@ -127,9 +136,13 @@ struct __rte_cache_aligned rte_node {
 			rte_node_process_t process; /**< Process function. */
 			uint64_t process_u64;
 		};
+	};
 	alignas(RTE_CACHE_LINE_MIN_SIZE) struct rte_node *nodes[]; /**< Next nodes. */
 };
 
+static_assert(offsetof(struct rte_node, size) - offsetof(struct rte_node, ctx) == RTE_NODE_CTX_SZ,
+	"rte_node context must be RTE_NODE_CTX_SZ bytes exactly");
+
 /**
  * @internal
  *
-- 
2.44.0