> From: Konstantin Ananyev [mailto:[email protected]] > Sent: Saturday, 25 October 2025 12.25 > > > > > > > > > > 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]> > > > > > > --- > > > > > > v7: > > > > > > * Go back to long names instead of numerical value in > > > > > RTE_MBUF_DIRECT() > > > > > > macro. > > > > > > (Konstantin Ananyev) > > > > > > * Updated static_assert() accordingly. > > > > > > > > [...] > > > > > > > > > > * > > > > > > * 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) \ > > > > > > + !(((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))) > > > > > > +#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] & \ > > > > > > + (char)((RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) >> (7 * > > > > > > CHAR_BIT))) > > > > > > +#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) \ > > > > > > - (!((mb)->ol_flags & (RTE_MBUF_F_INDIRECT | > > > > > > RTE_MBUF_F_EXTERNAL))) > > > > > > + !(((const char *)(&(mb)->ol_flags))[0] & \ > > > > > > + (char)((RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) >> (7 * > > > > > > CHAR_BIT))) > > > > > > +#endif /* RTE_BYTE_ORDER */ > > > > > > +#endif /* !__DOXYGEN__ */ > > > > > > +/* Verify the optimization above. */ > > > > > > +static_assert(((RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) & > > > > > > (UINT64_C(0xFF) << (7 * CHAR_BIT))) == > > > > > > + (RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL), > > > > > > + "(RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) is not at > > MSB"); > > > > > > > > > > > > /** Uninitialized or unspecified port. */ > > > > > > #define RTE_MBUF_PORT_INVALID UINT16_MAX > > > > > > -- > > > > > > > > > > LGTM, thanks for refactoring. > > > > > > > > Thank you for reviewing, Konstantin. > > > > > > > > I had no preference for v7 or v6, but Bruce and Thomas preferred > v6, so v6 was > > > > applied. > > > > > > Yes, I saw Thomas email, after I sent my reply already. > > > Looks like I was late with my vote. > > > My preference still would be to avoid hard-coded constants in the > code, > > > but seems that it is just me. > > > > Me too I want to avoid hardcoded constants. > > But in this case, it is very well documented, > > and there is a trade-off with length and reading. > > If we allow hard-coded constants in one place, > then it would be harder for us to disallow them in other places. > If we allow them everywhere - code will become a mess pretty soon.
Vector code has plenty of hardcoded constants. E.g.: https://elixir.bootlin.com/dpdk/v25.07/source/drivers/net/intel/i40e/i40e_rxtx_vec_avx512.c#L154 So we already do allow it. However, I think uses of numerical constants (like 0x60 here) should always be accompanied by a static_assert(), so a change of the underlying value will be caught at build time. > Anyway, the changes are already merged, so probably not point > to keep arguing on that subject. I normally have a strong preference for descriptive names over numbers, but in this case, I was in doubt. A majority of reviewers voted that using 0x60 made the code easier to read, so let's stick with that. > > > > > The comment starts with > > * The plain macro would be: > > * \code{.c} > > * #define RTE_MBUF_DIRECT(mb) \ > > * (!((mb)->ol_flags & (RTE_MBUF_F_INDIRECT | > RTE_MBUF_F_EXTERNAL))) > > * \endcode > > > > so I believe it is very clear already. > >
