mbuf: optimize segment prefree

Refactored rte_pktmbuf_prefree_seg() for both performance and readability.

With the optimized RTE_MBUF_DIRECT() macro, the common likely code path
now fits within one instruction cache line on x86-64 when built with GCC.

Signed-off-by: Morten Brørup <[email protected]>
Acked-by: Konstantin Ananyev <[email protected]>
Acked-by: Chengwen Feng <[email protected]>
Reviewed-by: Bruce Richardson <[email protected]>

Author: MortenBroerup

lib/mbuf/rte_mbuf.h

@@ -31,6 +31,7 @@
  * http://www.kohala.com/start/tcpipiv2.html
  */
 
+#include <stdbool.h>
 #include <stdint.h>
 
 #include <rte_common.h>
@@ -1458,44 +1459,30 @@ static inline int __rte_pktmbuf_pinned_extbuf_decref(struct rte_mbuf *m)
 static __rte_always_inline struct rte_mbuf *
 rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
 {
-	__rte_mbuf_sanity_check(m, 0);
-
-	if (likely(rte_mbuf_refcnt_read(m) == 1)) {
-
-		if (!RTE_MBUF_DIRECT(m)) {
-			rte_pktmbuf_detach(m);
-			if (RTE_MBUF_HAS_EXTBUF(m) &&
-			    RTE_MBUF_HAS_PINNED_EXTBUF(m) &&
-			    __rte_pktmbuf_pinned_extbuf_decref(m))
-				return NULL;
-		}
-
-		if (m->next != NULL)
-			m->next = NULL;
-		if (m->nb_segs != 1)
-			m->nb_segs = 1;
+	bool refcnt_not_one;
 
-		return m;
+	__rte_mbuf_sanity_check(m, 0);
 
-	} else if (__rte_mbuf_refcnt_update(m, -1) == 0) {
+	refcnt_not_one = unlikely(rte_mbuf_refcnt_read(m) != 1);
+	if (refcnt_not_one && __rte_mbuf_refcnt_update(m, -1) != 0)
+		return NULL;
 
-		if (!RTE_MBUF_DIRECT(m)) {
-			rte_pktmbuf_detach(m);
-			if (RTE_MBUF_HAS_EXTBUF(m) &&
-			    RTE_MBUF_HAS_PINNED_EXTBUF(m) &&
-			    __rte_pktmbuf_pinned_extbuf_decref(m))
-				return NULL;
-		}
+	if (unlikely(!RTE_MBUF_DIRECT(m))) {
+		rte_pktmbuf_detach(m);
+		if (RTE_MBUF_HAS_EXTBUF(m) &&
+				RTE_MBUF_HAS_PINNED_EXTBUF(m) &&
+				__rte_pktmbuf_pinned_extbuf_decref(m))
+			return NULL;
+	}
 
-		if (m->next != NULL)
-			m->next = NULL;
-		if (m->nb_segs != 1)
-			m->nb_segs = 1;
+	if (refcnt_not_one)
 		rte_mbuf_refcnt_set(m, 1);
+	if (m->nb_segs != 1)
+		m->nb_segs = 1;
+	if (m->next != NULL)
+		m->next = NULL;
 
-		return m;
-	}
-	return NULL;
+	return m;
 }
 
 /**

lib/mbuf/rte_mbuf_core.h

@@ -711,9 +711,49 @@ struct rte_mbuf_ext_shared_info {
  *
  * If a mbuf embeds its own data after the rte_mbuf structure, this mbuf
  * can be defined as a direct mbuf.
- */
+ *
+ * Note: Macro optimized for code size.
+ *
+ * The plain macro would be:
+ * \code{.c}
+ *      #define RTE_MBUF_DIRECT(mb) \
+ *          (!((mb)->ol_flags & (RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL)))
+ * \endcode
+ *
+ * The flags RTE_MBUF_F_INDIRECT and RTE_MBUF_F_EXTERNAL are both in the MSB
+ * (most significant byte) of the 64-bit ol_flags field,
+ * so we only compare this one byte instead of all 64 bits.
+ *
+ * E.g., GCC version 16.0.0 20251019 (experimental) generates the following code for x86-64.
+ *
+ * With the plain macro, 17 bytes of instructions:
+ * \code
+ *      movabs rax,0x6000000000000000       // 10 bytes
+ *      and    rax,QWORD PTR [rdi+0x18]     // 4 bytes
+ *      sete   al                           // 3 bytes
+ * \endcode
+ * With this optimized macro, only 7 bytes of instructions:
+ * \code
+ *      test   BYTE PTR [rdi+0x1f],0x60     // 4 bytes
+ *      sete   al                           // 3 bytes
+ * \endcode
+ */
+#ifdef __DOXYGEN__
 #define RTE_MBUF_DIRECT(mb) \
-	(!((mb)->ol_flags & (RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL)))
+	!(((const char *)(&(mb)->ol_flags))[MSB_OFFSET /* 7 or 0, depending on endianness */] & \
+	(char)((RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) >> (7 * CHAR_BIT)) /* 0x60 */)
+#else /* !__DOXYGEN__ */
+#if RTE_BYTE_ORDER == RTE_LITTLE_ENDIAN
+/* On little endian architecture, the MSB of a 64-bit integer is at byte offset 7. */
+#define RTE_MBUF_DIRECT(mb) !(((const char *)(&(mb)->ol_flags))[7] & 0x60)
+#elif RTE_BYTE_ORDER == RTE_BIG_ENDIAN
+/* On big endian architecture, the MSB of a 64-bit integer is at byte offset 0. */
+#define RTE_MBUF_DIRECT(mb) !(((const char *)(&(mb)->ol_flags))[0] & 0x60)
+#endif /* RTE_BYTE_ORDER */
+#endif /* !__DOXYGEN__ */
+/* Verify the optimization above. */
+static_assert((RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) == UINT64_C(0x60) << (7 * CHAR_BIT),
+		"(RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) is not 0x60 at MSB");
 
 /** Uninitialized or unspecified port. */
 #define RTE_MBUF_PORT_INVALID UINT16_MAX