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]> --- v2: * Fixed typo in commit description. * Fixed indentation. * Added detailed description to the optimized RTE_MBUF_DIRECT() macro. (Stephen Hemminger) * Added static_assert() to verify that the optimized RTE_MBUF_DIRECT() macro is valid, specifically that the tested bits are in the MSB of the 64-bit field. --- lib/mbuf/rte_mbuf.h | 51 +++++++++++++++------------------------- lib/mbuf/rte_mbuf_core.h | 27 +++++++++++++++++++++ 2 files changed, 46 insertions(+), 32 deletions(-) diff --git a/lib/mbuf/rte_mbuf.h b/lib/mbuf/rte_mbuf.h index 3df22125de..2004391f57 100644 --- a/lib/mbuf/rte_mbuf.h +++ b/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; } /** diff --git a/lib/mbuf/rte_mbuf_core.h b/lib/mbuf/rte_mbuf_core.h index a0df265b5d..41f40e1967 100644 --- a/lib/mbuf/rte_mbuf_core.h +++ b/lib/mbuf/rte_mbuf_core.h @@ -715,6 +715,33 @@ struct rte_mbuf_ext_shared_info { #define RTE_MBUF_DIRECT(mb) \ (!((mb)->ol_flags & (RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL))) +#if defined(RTE_TOOLCHAIN_GCC) && defined(RTE_ARCH_X86) +/* Optimization for code size. + * GCC only optimizes single-bit MSB tests this way, so we do it by hand with multi-bit. + * + * The flags RTE_MBUF_F_INDIRECT and RTE_MBUF_F_EXTERNAL are both in the MSB of the + * 64-bit ol_flags field, so we only compare this one byte instead of all 64 bits. + * On little endian architecture, the MSB of a 64-bit integer is at byte offest 7. + * + * Note: Tested using GCC version 16.0.0 20251019 (experimental). + * + * Without this optimization, GCC generates 17 bytes of instructions: + * movabs rax,0x6000000000000000 // 10 bytes + * and rax,QWORD PTR [rdi+0x18] // 4 bytes + * sete al // 3 bytes + * With this optimization, GCC generates only 7 bytes of instructions: + * test BYTE PTR [rdi+0x1f],0x60 // 4 bytes + * sete al // 3 bytes + */ +#undef RTE_MBUF_DIRECT +#define RTE_MBUF_DIRECT(mb) \ + (!(((const uint8_t *)(mb))[offsetof(struct rte_mbuf, ol_flags) + 7] & \ + (uint8_t)((RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) >> (7 * 8)))) +static_assert(((RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) >> (7 * 8)) << (7 * 8) == + (RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL), + "RTE_MBUF_F_INDIRECT and/or RTE_MBUF_F_EXTERNAL are not in MSB."); +#endif + /** Uninitialized or unspecified port. */ #define RTE_MBUF_PORT_INVALID UINT16_MAX /** For backwards compatibility. */ -- 2.43.0
